Title of Invention: LETTUCE VARIETIES 'BRENTWOOD', 'GREENWOOD', AND 'BOYNTON' Abstract: New lettuce varieties designated ‘Brentwood’, ‘Greenwood’, and ‘Boynton’ are described. ‘Brentwood’, ‘Greenwood’, and ‘Boynton’ are lettuce varieties exhibiting stability and uniformity. Detailed Description: CROSS-REFERENCE TO RELATED APPLICATION This application claims the benefit of U.S. Provisional Application No. 62/464,117, filed Feb. 27, 2017, which is hereby incorporated by reference in its entirety. FIELD OF THE INVENTION The present invention relates to the field of plant breeding. In particular, this invention relates to new lettuce, Lactuca sativa varieties designated ‘Brentwood’, ‘Greenwood’, and ‘Boynton’. BACKGROUND OF THE INVENTION Cultivated forms of lettuce belong to the highly polymorphic species Lactuca sativa that is grown for its edible head and leaves. As a crop, lettuce is grown commercially wherever environmental conditions permit the production of an economically viable yield. For planting purposes, the lettuce season is typically divided into three categories (i.e., early, mid, and late), with coastal areas planting from January to August, and desert regions planting from August to December. Fresh lettuce is consumed nearly exclusively as fresh, raw product and occasionally as a cooked vegetable. Lactuca sativa is in the Cichorieae tribe of the Asteraceae (Compositae) family. Lettuce is related to chicory, sunflower, aster, dandelion, artichoke, and chrysanthemum. Sativa is one of about 300 species in the genus Lactuca. There are seven different morphological types of lettuce. The crisphead group includes the iceberg and batavian types. Iceberg lettuce has a large, firm head with a crisp texture and a white or creamy yellow interior. The batavian lettuce predates the iceberg type and has a smaller and less firm head. The butterhead group has a small, soft head with an almost oily texture. The romaine, also known as cos lettuce, has elongated upright leaves forming a loose, loaf-shaped head and the outer leaves are usually dark green. Leaf lettuce, also known as cutting lettuce, comes in many varieties, none of which form a head, and include the green oak leaf variety. Latin lettuce, also known as grasse-type lettuce, looks like a cross between romaine and butterhead. Stem lettuce has long, narrow leaves and thick, edible stems. Oilseed lettuce is a type grown for its large seeds that are pressed to obtain oil. Lettuce is an increasingly popular crop. Worldwide lettuce consumption continues to increase. As a result of this demand, there is a continued need for new lettuce varieties. In particular, there is a need for improved lettuce varieties that are stable, high yielding, and agronomically sound. SUMMARY OF THE INVENTION In order to meet these needs, the present invention is directed to improved lettuce varieties. As used herein lettuce variety ‘Brentwood’ is the same lettuce variety as lettuce variety ‘E01L30219’ having NCIMB Accession Number X1 and disclosed in U.S. Provisional Application No. 62/464,117, filed Feb. 27, 2017. While the name has changed, lettuce variety ‘Brentwood’ has all the defining characteristics of lettuce variety ‘E01L30219’. In one embodiment, the present invention is directed to lettuce, Lactuca sativa, seed designated as ‘Brentwood’ having NCIMB Accession Number X1. In one embodiment, the present invention is directed to a Lactuca sativa lettuce plant and parts isolated therefrom produced by growing ‘Brentwood’ lettuce seed. In another embodiment, the present invention is directed to a Lactuca sativa plant and parts isolated therefrom having all the physiological and morphological characteristics of a Lactuca sativa plant produced by growing ‘Brentwood’ lettuce seed having NCIMB Accession Number X1. In still another embodiment, the present invention is directed to an F1 hybrid Lactuca sativa lettuce seed, plants grown from the seed, and a head isolated therefrom having ‘Brentwood’ as a parent, where ‘Brentwood’ is grown from ‘Brentwood’ lettuce seed having NCIMB Accession Number X1. Lettuce plant parts include lettuce heads, lettuce leaves, parts of lettuce leaves, pollen, ovules, flowers, and the like. In another embodiment, the present invention is further directed to lettuce heads, lettuce leaves, parts of lettuce leaves, flowers, pollen, and ovules isolated from ‘Brentwood’ lettuce plants. In another embodiment, the present invention is further directed to tissue culture of ‘Brentwood’ lettuce plants, and to lettuce plants regenerated from the tissue culture, where the plant has all of the morphological and physiological characteristics of ‘Brentwood’ lettuce plants. In still another embodiment, the present invention is further directed to packaging material containing ‘Brentwood’ plant parts. Such packaging material includes but is not limited to boxes, plastic bags, etc. The ‘Brentwood’ plant parts may be combined with other plant parts of other plant varieties. In yet another embodiment, the present invention is further directed to a method of selecting lettuce plants, by a) growing ‘Brentwood’ lettuce plants where the ‘Brentwood’ plants are grown from lettuce seed having NCIMB Accession Number X1 and b) selecting a plant from step a). In another embodiment, the present invention is further directed to lettuce plants, plant parts and seeds produced by the lettuce plants where the lettuce plants are isolated by the selection method of the invention. In another embodiment, the present invention is further directed to a method of breeding lettuce plants by crossing a lettuce plant with a plant grown from ‘Brentwood’ lettuce seed having NCIMB Accession Number X1. In still another embodiment, the present invention is further directed to lettuce plants, lettuce parts from the lettuce plants, and seeds produced therefrom where the lettuce plant is isolated by the breeding method of the invention. In another embodiment, the present invention is directed to methods for producing a male sterile lettuce plant by introducing a nucleic acid molecule that confers male sterility into a lettuce plant produced by growing ‘Brentwood’ lettuce seed, and to male sterile lettuce plants produced by such methods. In another embodiment, the present invention is directed to methods of producing an herbicide resistant lettuce plant by introducing a gene conferring herbicide resistance into a lettuce plant produced by growing ‘Brentwood’ lettuce seed, where the gene is selected from glyphosate, sulfonylurea, imidazolinone, dicamba, glufosinate, phenoxy proprionic acid, L-phosphinothricin, cyclohexone, cyclohexanedione, triazine, and benzonitrile. Certain embodiments are also directed to herbicide resistant lettuce plants produced by such methods. In another embodiment, the present invention is directed to methods of producing a pest or insect resistant lettuce plan by introducing a gene conferring pest or insect resistance into a lettuce plant produced by growing ‘Brentwood’ lettuce seed, and to pest or insect resistant lettuce plants produced by such methods. In certain embodiments, the gene conferring pest or insect resistance encodes a Bacillus thuringiensis endotoxin. In another embodiment, the present invention is directed to methods of producing a disease resistant lettuce plant by introducing a gene conferring disease resistance into a lettuce plant produced by growing ‘Brentwood’ lettuce seed, and to disease resistant lettuce plants produced by such methods. In another embodiment, the present invention is directed to methods of producing a lettuce plant with a value-added trait by introducing a gene conferring a value-added trait into a lettuce plant produced by growing ‘Brentwood’ lettuce seed, where the gene encodes a protein selected from a ferritin, a nitrate reductase, and a monellin. Certain embodiments are also directed to lettuce plants having a value-added trait produced by such methods. In another embodiment, the present invention is directed to methods of introducing a desired trait into lettuce variety ‘Brentwood’, by: (a) crossing a ‘Brentwood’ plant, where a sample of ‘Brentwood’ lettuce seed was deposited under NCIMB Accession Number X1, with a plant of another lettuce variety that contains a desired trait to produce progeny plants, where the desired trait is selected from male sterility; herbicide resistance; insect or pest resistance; modified bolting; and resistance to bacterial disease, fungal disease or viral disease; (b) selecting one or more progeny plants that have the desired trait; (c) backcrossing the selected progeny plants with a ‘Brentwood’ plant to produce backcross progeny plants; (d) selecting for backcross progeny plants that have the desired trait and all of the physiological and morphological characteristics of lettuce variety ‘Brentwood’; and (e) repeating steps (c) and (d) two or more times in succession to produce selected third or higher backcross progeny plants that comprise the desired trait. Certain embodiments are also directed to lettuce plants produced by such methods, where the plants have the desired trait and all of the physiological and morphological characteristics of lettuce variety ‘Brentwood’. In certain embodiments, the desired trait is herbicide resistance and the resistance is conferred to an herbicide selected from glyphosate, sulfonylurea, imidazolinone, dicamba, glufosinate, phenoxy proprionic acid, L-phosphinothricin, cyclohexone, cyclohexanedione, triazine, and benzonitrile. In another embodiment, the present invention provides for single gene converted plants of ‘Brentwood’. The single transferred gene may preferably be a dominant or recessive allele. Preferably, the single transferred gene will confer such traits as male sterility, herbicide resistance, insect or pest resistance, modified fatty acid metabolism, modified carbohydrate metabolism, resistance for bacterial, fungal, or viral disease, male fertility, enhanced nutritional quality, and industrial usage. As used herein lettuce variety ‘Greenwood’ is the same lettuce variety as lettuce variety ‘E01L30240’ having NCIMB Accession Number X2 and disclosed in U.S. Provisional Application No. 62/464,117, filed Feb. 27, 2017. While the name has changed, lettuce variety ‘Greenwood’ has all the defining characteristics of lettuce variety ‘E01L30240’. In one embodiment, the present invention is directed to lettuce, Lactuca sativa, seed designated as ‘Greenwood’ having NCIMB Accession Number X2. In one embodiment, the present invention is directed to a Lactuca sativa lettuce plant and parts isolated therefrom produced by growing ‘Greenwood’ lettuce seed. In another embodiment, the present invention is directed to a Lactuca sativa plant and parts isolated therefrom having all the physiological and morphological characteristics of a Lactuca sativa plant produced by growing ‘Greenwood’ lettuce seed having NCIMB Accession Number X2. In still another embodiment, the present invention is directed to an F1 hybrid Lactuca sativa lettuce seed, plants grown from the seed, and a head isolated therefrom having ‘Greenwood’ as a parent, where ‘Greenwood’ is grown from ‘Greenwood’ lettuce seed having NCIMB Accession Number X2. Lettuce plant parts include lettuce heads, lettuce leaves, parts of lettuce leaves, pollen, ovules, flowers, and the like. In another embodiment, the present invention is further directed to lettuce heads, lettuce leaves, parts of lettuce leaves, flowers, pollen, and ovules isolated from ‘Greenwood’ lettuce plants. In another embodiment, the present invention is further directed to tissue culture of ‘Greenwood’ lettuce plants, and to lettuce plants regenerated from the tissue culture, where the plant has all of the morphological and physiological characteristics of ‘Greenwood’ lettuce plants. In still another embodiment, the present invention is further directed to packaging material containing ‘Greenwood’ plant parts. Such packaging material includes but is not limited to boxes, plastic bags, etc. The ‘Greenwood’ plant parts may be combined with other plant parts of other plant varieties. In yet another embodiment, the present invention is further directed to a method of selecting lettuce plants, by a) growing ‘Greenwood’ lettuce plants where the ‘Greenwood’ plants are grown from lettuce seed having NCIMB Accession Number X2 and b) selecting a plant from step a). In another embodiment, the present invention is further directed to lettuce plants, plant parts and seeds produced by the lettuce plants where the lettuce plants are isolated by the selection method of the invention. In another embodiment, the present invention is further directed to a method of breeding lettuce plants by crossing a lettuce plant with a plant grown from ‘Greenwood’ lettuce seed having NCIMB Accession Number X2. In still another embodiment, the present invention is further directed to lettuce plants, lettuce parts from the lettuce plants, and seeds produced therefrom where the lettuce plant is isolated by the breeding method of the invention. In another embodiment, the present invention is directed to methods for producing a male sterile lettuce plant by introducing a nucleic acid molecule that confers male sterility into a lettuce plant produced by growing ‘Greenwood’ lettuce seed, and to male sterile lettuce plants produced by such methods. In another embodiment, the present invention is directed to methods of producing an herbicide resistant lettuce plant by introducing a gene conferring herbicide resistance into a lettuce plant produced by growing ‘Greenwood’ lettuce seed, where the gene is selected from glyphosate, sulfonylurea, imidazolinone, dicamba, glufosinate, phenoxy proprionic acid, L-phosphinothricin, cyclohexone, cyclohexanedione, triazine, and benzonitrile. Certain embodiments are also directed to herbicide resistant lettuce plants produced by such methods. In another embodiment, the present invention is directed to methods of producing a pest or insect resistant lettuce plan by introducing a gene conferring pest or insect resistance into a lettuce plant produced by growing ‘Greenwood’ lettuce seed, and to pest or insect resistant lettuce plants produced by such methods. In certain embodiments, the gene conferring pest or insect resistance encodes a Bacillus thuringiensis endotoxin. In another embodiment, the present invention is directed to methods of producing a disease resistant lettuce plant by introducing a gene conferring disease resistance into a lettuce plant produced by growing ‘Greenwood’ lettuce seed, and to disease resistant lettuce plants produced by such methods. In another embodiment, the present invention is directed to methods of producing a lettuce plant with a value-added trait by introducing a gene conferring a value-added trait into a lettuce plant produced by growing ‘Greenwood’ lettuce seed, where the gene encodes a protein selected from a ferritin, a nitrate reductase, and a monellin. Certain embodiments are also directed to lettuce plants having a value-added trait produced by such methods. In another embodiment, the present invention is directed to methods of introducing a desired trait into lettuce variety ‘Greenwood’, by: (a) crossing a ‘Greenwood’ plant, where a sample of ‘Greenwood’ lettuce seed was deposited under NCIMB Accession Number X2, with a plant of another lettuce variety that contains a desired trait to produce progeny plants, where the desired trait is selected from male sterility; herbicide resistance; insect or pest resistance; modified bolting; and resistance to bacterial disease, fungal disease or viral disease; (b) selecting one or more progeny plants that have the desired trait; (c) backcrossing the selected progeny plants with a ‘Greenwood’ plant to produce backcross progeny plants; (d) selecting for backcross progeny plants that have the desired trait and all of the physiological and morphological characteristics of lettuce variety ‘Greenwood’; and (e) repeating steps (c) and (d) two or more times in succession to produce selected third or higher backcross progeny plants that comprise the desired trait. Certain embodiments are also directed to lettuce plants produced by such methods, where the plants have the desired trait and all of the physiological and morphological characteristics of lettuce variety ‘Greenwood’. In certain embodiments, the desired trait is herbicide resistance and the resistance is conferred to an herbicide selected from glyphosate, sulfonylurea, imidazolinone, dicamba, glufosinate, phenoxy proprionic acid, L-phosphinothricin, cyclohexone, cyclohexanedione, triazine, and benzonitrile. In another embodiment, the present invention provides for single gene converted plants of ‘Greenwood’. The single transferred gene may preferably be a dominant or recessive allele. Preferably, the single transferred gene will confer such traits as male sterility, herbicide resistance, insect or pest resistance, modified fatty acid metabolism, modified carbohydrate metabolism, resistance for bacterial, fungal, or viral disease, male fertility, enhanced nutritional quality, and industrial usage. As used herein lettuce variety ‘Boynton’ is the same lettuce variety as lettuce variety ‘E01L30452’ having NCIMB Accession Number X3 and disclosed in U.S. Provisional Application No. 62/464,117, filed Feb. 27, 2017. While the name has changed, lettuce variety ‘Boynton’ has all the defining characteristics of lettuce variety ‘E01L30452’. In one embodiment, the present invention is directed to lettuce, Lactuca sativa, seed designated as ‘Boynton’ having NCIMB Accession Number X3. In one embodiment, the present invention is directed to a Lactuca sativa lettuce plant and parts isolated therefrom produced by growing ‘Boynton’ lettuce seed. In another embodiment, the present invention is directed to a Lactuca sativa plant and parts isolated therefrom having all the physiological and morphological characteristics of a Lactuca sativa plant produced by growing ‘Boynton’ lettuce seed having NCIMB Accession Number X3. In still another embodiment, the present invention is directed to an F1 hybrid Lactuca sativa lettuce seed, plants grown from the seed, and a head isolated therefrom having ‘Boynton’ as a parent, where ‘Boynton’ is grown from ‘Boynton’ lettuce seed having NCIMB Accession Number X3. Lettuce plant parts include lettuce heads, lettuce leaves, parts of lettuce leaves, pollen, ovules, flowers, and the like. In another embodiment, the present invention is further directed to lettuce heads, lettuce leaves, parts of lettuce leaves, flowers, pollen, and ovules isolated from ‘Boynton’ lettuce plants. In another embodiment, the present invention is further directed to tissue culture of ‘Boynton’ lettuce plants, and to lettuce plants regenerated from the tissue culture, where the plant has all of the morphological and physiological characteristics of ‘Boynton’ lettuce plants. In still another embodiment, the present invention is further directed to packaging material containing ‘Boynton’ plant parts. Such packaging material includes but is not limited to boxes, plastic bags, etc. The ‘Boynton’ plant parts may be combined with other plant parts of other plant varieties. In yet another embodiment, the present invention is further directed to a method of selecting lettuce plants, by a) growing ‘Boynton’ lettuce plants where the ‘Boynton’ plants are grown from lettuce seed having NCIMB Accession Number X3 and b) selecting a plant from step a). In another embodiment, the present invention is further directed to lettuce plants, plant parts and seeds produced by the lettuce plants where the lettuce plants are isolated by the selection method of the invention. In another embodiment, the present invention is further directed to a method of breeding lettuce plants by crossing a lettuce plant with a plant grown from ‘Boynton’ lettuce seed having NCIMB Accession Number X3. In still another embodiment, the present invention is further directed to lettuce plants, lettuce parts from the lettuce plants, and seeds produced therefrom where the lettuce plant is isolated by the breeding method of the invention. In another embodiment, the present invention is directed to methods for producing a male sterile lettuce plant by introducing a nucleic acid molecule that confers male sterility into a lettuce plant produced by growing ‘Boynton’ lettuce seed, and to male sterile lettuce plants produced by such methods. In another embodiment, the present invention is directed to methods of producing an herbicide resistant lettuce plant by introducing a gene conferring herbicide resistance into a lettuce plant produced by growing ‘Boynton’ lettuce seed, where the gene is selected from glyphosate, sulfonylurea, imidazolinone, dicamba, glufosinate, phenoxy proprionic acid, L-phosphinothricin, cyclohexone, cyclohexanedione, triazine, and benzonitrile. Certain embodiments are also directed to herbicide resistant lettuce plants produced by such methods. In another embodiment, the present invention is directed to methods of producing a pest or insect resistant lettuce plan by introducing a gene conferring pest or insect resistance into a lettuce plant produced by growing ‘Boynton’ lettuce seed, and to pest or insect resistant lettuce plants produced by such methods. In certain embodiments, the gene conferring pest or insect resistance encodes a Bacillus thuringiensis endotoxin. In another embodiment, the present invention is directed to methods of producing a disease resistant lettuce plant by introducing a gene conferring disease resistance into a lettuce plant produced by growing ‘Boynton’ lettuce seed, and to disease resistant lettuce plants produced by such methods. In another embodiment, the present invention is directed to methods of producing a lettuce plant with a value-added trait by introducing a gene conferring a value-added trait into a lettuce plant produced by growing ‘Boynton’ lettuce seed, where the gene encodes a protein selected from a ferritin, a nitrate reductase, and a monellin. Certain embodiments are also directed to lettuce plants having a value-added trait produced by such methods. In another embodiment, the present invention is directed to methods of introducing a desired trait into lettuce variety ‘Boynton’, by: (a) crossing a ‘Boynton’ plant, where a sample of ‘Boynton’ lettuce seed was deposited under NCIMB Accession Number X3, with a plant of another lettuce variety that contains a desired trait to produce progeny plants, where the desired trait is selected from male sterility; herbicide resistance; insect or pest resistance; modified bolting; and resistance to bacterial disease, fungal disease or viral disease; (b) selecting one or more progeny plants that have the desired trait; (c) backcrossing the selected progeny plants with a ‘Boynton’ plant to produce backcross progeny plants; (d) selecting for backcross progeny plants that have the desired trait and all of the physiological and morphological characteristics of lettuce variety ‘Boynton’; and (e) repeating steps (c) and (d) two or more times in succession to produce selected third or higher backcross progeny plants that comprise the desired trait. Certain embodiments are also directed to lettuce plants produced by such methods, where the plants have the desired trait and all of the physiological and morphological characteristics of lettuce variety ‘Boynton’. In certain embodiments, the desired trait is herbicide resistance and the resistance is conferred to an herbicide selected from glyphosate, sulfonylurea, imidazolinone, dicamba, glufosinate, phenoxy proprionic acid, L-phosphinothricin, cyclohexone, cyclohexanedione, triazine, and benzonitrile. In another embodiment, the present invention provides for single gene converted plants of ‘Boynton’. The single transferred gene may preferably be a dominant or recessive allele. Preferably, the single transferred gene will confer such traits as male sterility, herbicide resistance, insect or pest resistance, modified fatty acid metabolism, modified carbohydrate metabolism, resistance for bacterial, fungal, or viral disease, male fertility, enhanced nutritional quality, and industrial usage. In a further embodiment, the present invention relates to methods for developing lettuce plants in a lettuce plant breeding program using plant breeding techniques including recurrent selection, backcrossing, pedigree breeding, restriction fragment length polymorphism enhanced selection, genetic marker enhanced selection, and transformation. Seeds, lettuce plants, and parts thereof, produced by such breeding methods are also part of the invention. In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference by study of the following description. BRIEF DESCRIPTION OF THE DRAWINGS The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the office upon request and payment of the necessary fee. FIG. 1 shows top view and side view of leaves of lettuce variety ‘Brentwood’. FIGS. 2A, 2B, and 2C show a comparison between lettuce varieties ‘Brentwood’ and ‘Eztela’. FIG. 2A shows top view and side view of leaves of lettuce variety ‘Brentwood’. FIG. 2B shows a top view of leaves of lettuce variety ‘Eztela’. FIG. 2C shows a side view of leaves of lettuce variety ‘Eztela’. FIGS. 3A and 3B show leaves of lettuce variety ‘Greenwood’. FIG. 3A shows a top view of leaves of lettuce variety ‘Greenwood’. FIG. 3B shows a side view of leaves of lettuce variety ‘Greenwood’. FIGS. 4A, 4B, and 4C show a comparison between lettuce varieties ‘Greenwood’ and ‘Eztron’. FIG. 4A shows a top view of leaves of lettuce variety ‘Greenwood’. FIG. 4B shows a side view of leaves of lettuce variety ‘Greenwood’. FIG. 4C shows top view and side view of leaves of lettuce variety ‘Eztron’. FIG. 5 shows leaves of lettuce variety ‘Boynton’. FIGS. 6A and 6B show a comparison between lettuce varieties ‘Boynton’ and ‘Mathix’. FIG. 6A shows leaves of lettuce variety ‘Boynton’. FIG. 6B shows leaves of lettuce variety ‘Mathix’. DETAILED DESCRIPTION OF THE INVENTION There are numerous steps in the development of novel, desirable lettuce germplasm. Plant breeding begins with the analysis of problems and weaknesses of current lettuce germplasms, the establishment of program goals, and the definition of specific breeding objectives. The next step is selection of germplasm that possess the traits to meet the program goals. The goal is to combine in a single variety or hybrid an improved combination of desirable traits from the parental germplasm. These important traits may include increased head size and weight, higher seed yield, improved color, resistance to diseases and insects, tolerance to drought and heat, and better agronomic quality. Choice of breeding or selection methods can depend on the mode of plant reproduction, the heritability of the trait(s) being improved, and the type of variety used commercially (e.g., F1 hybrid variety, pureline variety, etc.). For highly heritable traits, a choice of superior individual plants evaluated at a single location will be effective, whereas for traits with low heritability, selection should be based on mean values obtained from replicated evaluations of families of related plants. Popular selection methods commonly include pedigree selection, modified pedigree selection, mass selection, and recurrent selection. The complexity of inheritance influences choice of the breeding method. Backcross breeding is used to transfer one or a few favorable genes for a highly heritable trait into a desirable variety. This approach has been used extensively for breeding disease-resistant varieties. Various recurrent selection techniques are used to improve quantitatively inherited traits controlled by numerous genes. The use of recurrent selection in self-pollinating crops depends on the ease of pollination, the frequency of successful hybrids from each pollination, and the number of hybrid offspring from each successful cross. Each breeding program may include a periodic, objective evaluation of the efficiency of the breeding procedure. Evaluation criteria vary depending on the goal and objectives, and can include gain from selection per year based on comparisons to an appropriate standard, the overall value of the advanced breeding lines, and the number of successful varieties produced per unit of input (e.g., per year, per dollar expended, etc.). Promising advanced breeding lines may be thoroughly tested and compared to appropriate standards in environments representative of the commercial target area(s) for at least three years. The best lines can then be candidates for new commercial varieties. Those still deficient in a few traits may be used as parents to produce new populations for further selection. These processes, which lead to the final step of marketing and distribution, may take from ten to twenty years from the time the first cross or selection is made. One goal of lettuce plant breeding is to develop new, unique, and genetically superior lettuce varieties. A breeder can initially select and crosses two or more parental lines, followed by repeated selfing and selection, producing many new genetic combinations. Moreover, a breeder can generate multiple different genetic combinations by crossing, selfing, and mutations. A plant breeder can then select which germplasms to advance to the next generation. These germplasms may then be grown under different geographical, climatic, and soil conditions, and further selections can be made during, and at the end of, the growing season. The development of commercial lettuce varieties thus requires the development of parental lettuce varieties, the crossing of these varieties, and the evaluation of the crosses. Pedigree breeding and recurrent selection breeding methods may be used to develop varieties from breeding populations. Breeding programs can be used to combine desirable traits from two or more varieties or various broad-based sources into breeding pools from which new varieties are developed by selfing and selection of desired phenotypes. The new varieties are crossed with other varieties and the hybrids from these crosses are evaluated to determine which have commercial potential. Pedigree breeding is generally used for the improvement of self-pollinating crops or inbred lines of cross-pollinating crops. Two parents which possess favorable, complementary traits are crossed to produce an F1. An F2 population is produced by selfing one or several F1's or by intercrossing two F1's (sib mating). Selection of the best individuals is usually begun in the F2 population. Then, beginning in the F3, the best individuals in the best families are selected. Replicated testing of families, or hybrid combinations involving individuals of these families, often follows in the F4 generation to improve the effectiveness of selection for traits with low heritability. At an advanced stage of inbreeding (i.e., F6 and F7), the best lines or mixtures of phenotypically similar lines are tested for potential release as new varieties. Mass and recurrent selections can be used to improve populations of either self- or cross-pollinating crops. A genetically variable population of heterozygous individuals is either identified or created by intercrossing several different parents. The best plants are selected based on individual superiority, outstanding progeny, or excellent combining ability. The selected plants are intercrossed to produce a new population in which further cycles of selection are continued. Backcross breeding may be used to transfer genes for a simply inherited, highly heritable trait into a desirable homozygous cultivar or line that is the recurrent parent. The source of the trait to be transferred is called the donor parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) and the desirable trait transferred from the donor parent. After the initial cross, individuals possessing the phenotype of the donor parent are selected and repeatedly crossed (backcrossed) to the recurrent parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) and the desirable trait transferred from the donor parent. The single-seed descent procedure in the strict sense refers to planting a segregating population, harvesting a sample of one seed per plant, and using the one-seed sample to plant the next generation. When the population has been advanced from the F2 to the desired level of inbreeding, the plants from which lines are derived will each trace to different F2 individuals. The number of plants in a population declines with each generation due to failure of some seeds to germinate or some plants to produce at least one seed. As a result, not all of the F2 plants originally sampled in the population will be represented by a progeny when generation advance is completed. In addition to phenotypic observations, the genotype of a plant can also be examined. There are many laboratory-based techniques known in the art that are available for the analysis, comparison and characterization of plant genotype. Such techniques include, without limitation, Isozyme Electrophoresis, Restriction Fragment Length Polymorphisms (RFLPs), Randomly Amplified Polymorphic DNAs (RAPDs), Arbitrarily Primed Polymerase Chain Reaction (AP-PCR), DNA Amplification Fingerprinting (DAF), Sequence Characterized Amplified Regions (SCARs), Amplified Fragment Length polymorphisms (AFLPs), Simple Sequence Repeats (SSRs, which are also referred to as Microsatellites), and Single Nucleotide Polymorphisms (SNPs). Molecular markers can also be used during the breeding process for the selection of qualitative traits. For example, markers closely linked to alleles or markers containing sequences within the actual alleles of interest can be used to select plants that contain the alleles of interest during a backcrossing breeding program. The markers can also be used to select toward the genome of the recurrent parent and against the markers of the donor parent. This procedure attempts to minimize the amount of genome from the donor parent that remains in the selected plants. It can also be used to reduce the number of crosses back to the recurrent parent needed in a backcrossing program. The use of molecular markers in the selection process is often called genetic marker enhanced selection or marker-assisted selection. Molecular markers may also be used to identify and exclude certain sources of germplasm as parental varieties or ancestors of a plant by providing a means of tracking genetic profiles through crosses. Mutation breeding may also be used to introduce new traits into lettuce varieties. Mutations that occur spontaneously or are artificially induced can be useful sources of variability for a plant breeder. The goal of artificial mutagenesis is to increase the rate of mutation for a desired characteristic. Mutation rates can be increased by many different means including temperature, long-term seed storage, tissue culture conditions, radiation (such as X-rays, Gamma rays, neutrons, Beta radiation, or ultraviolet radiation), chemical mutagens (such as base analogs like 5-bromo-uracil), antibiotics, alkylating agents (such as sulfur mustards, nitrogen mustards, epoxides, ethyleneamines, sulfates, sulfonates, sulfones, or lactones), azide, hydroxylamine, nitrous acid, or acridines. Once a desired trait is observed through mutagenesis the trait may then be incorporated into existing germplasm by traditional breeding techniques. Details of mutation breeding can be found in Principles of Cultivar Development by Fehr, Macmillan Publishing Company (1993). The production of double haploids can also be used for the development of homozygous varieties in a breeding program. Double haploids are produced by the doubling of a set of chromosomes from a heterozygous plant to produce a completely homozygous individual. For example, see Wan, et al., Theor. Appl. Genet., 77:889-892 (1989). Additional non-limiting examples of breeding methods that may be used include, without limitation, those found in Principles of Plant Breeding, John Wiley and Son, pp. 115-161 (1960); Allard (1960); Simmonds (1979); Sneep, et al. (1979); Fehr (1987); and “Carrots and Related Vegetable Umbelliferae,” Rubatzky, V. E., et al. (1999). Definitions In the description that follows, a number of terms are used. In order to provide a clear and consistent understanding of the specification and claims, including the scope to be given such terms, the following definitions are provided: Allele. The allele is any of one or more alternative forms of a gene, all of which relate to one trait or characteristic. In a diploid cell or organism, the two alleles of a given gene occupy corresponding loci on a pair of homologous chromosomes. Backcrossing. Backcrossing is a process in which a breeder repeatedly crosses hybrid progeny back to one of the parents, for example, a first generation hybrid F1 with one of the parental genotype of the F1 hybrid. Big Vein virus. Big vein is a disease of lettuce caused by Lettuce Mirafiori Big Vein Virus which is transmitted by the fungus Olpidium virulentus, with vein clearing and leaf shrinkage resulting in plants of poor quality and reduced marketable value. Bolting. The premature development of a flowering stalk, and subsequent seed, before a plant produces a food crop. Bolting is typically caused by late planting. Bremia lactucae. An oomycete that causes downy mildew in lettuce in cooler growing regions. Core length. Length of the internal lettuce stem measured from the base of the cut and trimmed head to the tip of the stem. Corky root. A disease caused by the bacterium Sphingomonas suberifaciens, which causes the entire taproot to become brown, severely cracked, and non-functional. Cotyledon. One of the first leaves of the embryo of a seed plant; typically one or more in monocotyledons, two in dicotyledons, and two or more in gymnosperms. Essentially all the physiological and morphological characteristics. A plant having essentially all the physiological and morphological characteristics means a plant having the physiological and morphological characteristics of the recurrent parent, except for the characteristics derived from the converted gene. First water date. The date the seed first receives adequate moisture to germinate. This can and often does equal the planting date. Gene. As used herein, “gene” refers to a segment of nucleic acid. A gene can be introduced into a genome of a species, whether from a different species or from the same species, using transformation or various breeding methods. Head diameter. Diameter of the cut and trimmed head, sliced vertically, and measured at the widest point perpendicular to the stem. Head height. Height of the cut and trimmed head, sliced vertically, and measured from the base of the cut stem to the cap leaf. Head weight. Weight of saleable lettuce head, cut and trimmed to market specifications. Lettuce Mosaic virus. A disease that can cause a stunted, deformed, or mottled pattern in young lettuce and yellow, twisted, and deformed leaves in older lettuce. Maturity date. Maturity refers to the stage when the plants are of full size or optimum weight, in marketable form or shape to be of commercial or economic value. Nasonovia ribisnigri. A lettuce aphid that colonizes the innermost leaves of the lettuce plant, contaminating areas that cannot be treated easily with insecticides. Quantitative Trait Loci. Quantitative Trait Loci (QTL) refers to genetic loci that control to some degree, numerically representable traits that are usually continuously distributed. Ratio of head height/diameter. Head height divided by the head diameter is an indication of the head shape; <1 is flattened, 1=round, and >1 is pointed. Regeneration. Regeneration refers to the development of a plant from tissue culture. RHS. RHS refers to the Royal Horticultural Society of England which publishes an official botanical color chart quantitatively identifying colors according to a defined numbering system. The chart may be purchased from Royal Horticulture Society Enterprise Ltd., RHS Garden; Wisley, Woking; Surrey GU236QB, UK. Single gene converted. Single gene converted or conversion plant refers to plants which are developed by a plant breeding technique called backcrossing or via genetic engineering where essentially all of the desired morphological and physiological characteristics of a line are recovered in addition to the single gene transferred into the line via the backcrossing technique or via genetic engineering. Tip burn. Means a browning of the edges or tips of lettuce leaves that is a physiological response to a lack of calcium. Wet date. The wet date corresponds to the first planting date of lettuce. Overview of the Variety ‘Brentwood’ Lettuce variety ‘Brentwood’ is a dark reddish-green colored cutting (i.e., leaf) lettuce variety that is medium bolting with medium leaf thickness and a medium-large plant diameter. ‘Brentwood’ has a growing season that includes spring, summer, autumn, and winter, and is suitable for cultivation in the open. Additionally, lettuce variety ‘Brentwood’ is resistant to Nasonovia ribisnigri biotype 0 (Nr:0); resistant to Bremia lactucae (downy mildew) strains Bl: 16-Bl:33; intermediate resistance to Fusarium oxysporum f sp. Lactucae race 1 (Fol: 1); and susceptible to lettuce mosaic virus (LMV) strain Ls-1. FIG. 1 depicts leaves of lettuce variety ‘Brentwood’. Lettuce variety ‘Brentwood’ is the result of numerous generations of plant selections chosen for its leaf thickness, plant diameter, and its resistance to Nasonovia ribisnigri, and Bremia lactucae (downy mildew). Lettuce variety ‘Brentwood’ is distinguished from similar lettuce variety ‘Eztela’ in that ‘Brentwood’ is resistant to Bremia lactucae (downy mildew) strains Bl:16-Bl:33, whereas ‘Eztela’ is only resistant to Bremia lactucae strains Bl:16-27, Bl:29, and Bl:32. Moreover, lettuce variety ‘Brentwood’ has a smaller plant diameter, thicker leaves, and a stronger intensity of leaf anthocyanin coloration than lettuce variety ‘Eztela’. Further distinguishing features are apparent from the comparison of the two varieties depicted in FIGS. 2A, 2B and 2C. The variety has shown uniformity and stability for the traits, within the limits of environmental influence for the traits. It has been self-pollinated a sufficient number of generations with careful attention to uniformity of plant type. The line has been increased with continued observation for uniformity. No variant traits have been observed or are expected in variety ‘Brentwood’. Objective Description of the Variety ‘Brentwood’ Lettuce variety ‘Brentwood’ has the following morphologic and other characteristics: Plant type: Cutting (i.e., leaf lettuce) Seed: Color: Black Mature Leaves: Margin: Hue of green color of outer leaves: Reddish Intensity of color of outer leaves: Dark Anthocyanin coloration: Present Bolting: Class: Medium (e.g., comparable to ‘Eztela’) Disease/Pest Resistance: Lettuce mosaic virus (LMV) strain Ls-1: Susceptible Downy Mildew (Bremia lactucae) (Bl): Resistant to Bl:16-Bl:33 Fusarium oxysporum fsp. lactucae race 1 (Fol:1): Intermediate resistance Pests: Nasonovia ribisnigri biotype 0 (Nr:0): Resistant Comparisons to Commercial Lettuce Variety Table 1 below compares characteristics of lettuce variety ‘Brentwood’ with lettuce variety ‘Eztela’. Column 1 lists the characteristics, column 2 shows the characteristics for lettuce variety ‘Brentwood’, and column 3 shows the characteristics for lettuce variety ‘Eztela’. TABLE 1 Characteristic ‘Brentwood’ ‘Eztela’ Plant diameter Medium-large Large Leaf thickness Medium Thin Intensity of leaf Medium-strong Medium anthocyanin coloration Bremia lactucae Resistant to Bl: 16-33 Resistant to Bl: 16-27, resistance Bl: 29, and Bl: 32 Overview of the Variety ‘Greenwood’ Lettuce variety ‘Greenwood’ is a light green colored cutting (i.e., leaf) lettuce variety that is late bolting with a very strong leaf blade and medium leaf thickness. ‘Greenwood’ has a growing season that includes spring, summer, autumn, and winter, and is suitable for cultivation in the open. Additionally, lettuce variety ‘Greenwood’ is resistant to Nasonovia ribisnigri biotype 0 (Nr:0); resistant to Bremia lactucae (downy mildew) strains Bl:16-Bl:33; and resistant to lettuce mosaic virus (LMV) strain Ls-1. FIGS. 3A and 3B depict leaves of lettuce variety ‘Greenwood’. Lettuce variety ‘Greenwood’ is the result of numerous generations of plant selections chosen for its late bolting, very strong leaf blade, medium leaf thickness, and its resistance to Nasonovia ribisnigri, Bremia lactucae (downy mildew), and lettuce mosaic virus (LMV). Lettuce variety ‘Greenwood’ is distinguished from similar lettuce variety ‘Eztron’ in that ‘Greenwood’ has a stronger degree of undulation of margin of leaf blade, later time of beginning of bolting under long day conditions, thinner leaf texture, and higher leaf blade density than lettuce variety ‘Eztron’. Moreover, outer leaves of ‘Greenwood’ have a horizontal attitude at harvest maturity, whereas outer leaves of ‘Eztron’ have a semi-erect attitude at harvest maturity. Further distinguishing features are apparent from the comparison of the two varieties depicted in FIGS. 4A, 4B, and 4C. The variety has shown uniformity and stability for the traits, within the limits of environmental influence for the traits. It has been self-pollinated a sufficient number of generations with careful attention to uniformity of plant type. The line has been increased with continued observation for uniformity. No variant traits have been observed or are expected in variety ‘Greenwood’. Objective Description of the Variety ‘Greenwood’ Lettuce variety ‘Greenwood’ has the following morphologic and other characteristics: Plant type: Cutting (i.e., leaf lettuce) Seed: Color: Black Mature Leaves: Margin: Hue of green color of outer leaves: Absent Intensity of color of outer leaves: Light Anthocyanin coloration: Absent Bolting: Class: Late (e.g., comparable to ‘Eztron’) Disease/Pest Resistance: Lettuce mosaic virus (LMV) strain Ls-1: Resistant Downy Mildew (Bremia lactucae) (Bl): Resistant to Bl:16-Bl:33 Pests: Nasonovia ribisnigri biotype 0 (Nr:0): Resistant Comparisons to Commercial Lettuce Varieties Table 2 below compares characteristics of lettuce variety ‘Greenwood’ with lettuce variety ‘Eztron’. Column 1 lists the characteristics, column 2 shows the characteristics for lettuce variety ‘Greenwood’, and column 3 shows the characteristics for lettuce variety ‘Eztron’. TABLE 2 Characteristic ‘Greenwood’ ‘Eztron’ Leaf blade: degree of undulation of Very strong Medium-strong margin Time to beginning of bolting under long Late Medium day conditions Leaf: texture Thin Medium Leaf: attitude at harvest maturity outer Horizontal Semi-erect leaves Leaf blade density Dense Medium Table 3 below compares characteristics of lettuce variety ‘Greenwood’ with lettuce variety ‘Experience’. Column 1 lists the characteristics, column 2 shows the characteristics for lettuce variety ‘Greenwood’, and column 3 shows the characteristics for lettuce variety ‘Experience’. TABLE 3 Characteristics ‘Greenwood’ ‘Experience’ Leaf color Light green Yellowish blond Leaf thickness Medium Thick Overview of the Variety ‘Boynton’ Lettuce variety ‘Boynton’ is a very dark reddish green colored red oakleaf (i.e., cutting) lettuce variety that is late bolting with a small plant diameter, and strong intensity of leaf anthocyanin. ‘Boynton’ has a growing season that includes spring, summer, and autumn, and is suitable for cultivation in the open. Additionally, lettuce variety ‘Boynton’ is resistant to Nasonovia ribisnigri biotype 0 (Nr:0); resistant to Tomato bushy stunt virus (lettuce die-back); resistant to root aphids (Pb); resistant to Bremia lactucae (downy mildew) strains Bl:16-Bl:33; and susceptible to lettuce mosaic virus (LMV) strain Ls-1. FIG. 5 depicts leaves of lettuce variety ‘Boynton’. Lettuce variety ‘Boynton’ is the result of numerous generations of plant selections chosen for its plant diameter, color, intensity of leaf anthocyanin, and its resistance to Nasonovia ribisnigri, Bremia lactucae (downy mildew), and Tomato bushy stunt virus (lettuce die-back). Lettuce variety ‘Boynton’ is distinguished from similar lettuce variety ‘Mathix’ in that ‘Boynton’ has a smaller plant diameter and stronger intensity of leaf anthocyanin coloration than lettuce variety ‘Mathix’. Moreover, ‘Boynton’ has very loose head density and very weak leaf blistering, whereas ‘Mathix’ has medium head density and medium leaf blistering. Further distinguishing features are apparent from the comparison of the two varieties depicted in FIGS. 6A and 6B. The variety has shown uniformity and stability for the traits, within the limits of environmental influence for the traits. It has been self-pollinated a sufficient number of generations with careful attention to uniformity of plant type. The line has been increased with continued observation for uniformity. No variant traits have been observed or are expected in variety ‘Boynton’. Objective Description of the Variety ‘Boynton’ Lettuce variety ‘Boynton’ has the following morphologic and other characteristics: Plant type: Cutting (i.e., leaf lettuce) Seed: Color: Black Mature Leaves: Margin: Hue of green color of outer leaves: Reddish Intensity of color of outer leaves: Very dark Anthocyanin coloration: Present Bolting: Class: Late (e.g., comparable to ‘Mathix’) Disease/Pest Resistance: Lettuce mosaic virus (LMV) strain Ls-1: Susceptible Downy Mildew (Bremia lactucae) (Bl): Resistant to Bl:16-Bl:33 Tomato bushy stunt virus (lettuce die-back) TB SV: Resistant Pests: Nasonovia ribisnigri biotype 0 (Nr:0): Resistant Root aphids (Pb): Resistant Comparisons to Commercial Lettuce Varieties Table 4 below compares characteristics of lettuce variety ‘Boynton’ with lettuce variety ‘Mathix’. Column 1 lists the characteristics, column 2 shows the characteristics for lettuce variety ‘Boynton’, and column 3 shows the characteristics for lettuce variety ‘Mathix’. TABLE 4 Characteristic ‘Boynton’ ‘Mathix’ Plant diameter Small Medium Intensity of leaf anthocyanin coloration Strong Medium Head density Very loose Medium Leaf blistering Very weak Medium Attitude of outer leaves at harvest maturity Semi-erect Medium Table 5 below compares characteristics of lettuce variety ‘Boynton’ with lettuce variety ‘Cornouaï’. Column 1 lists the characteristics, column 2 shows the characteristics for lettuce variety ‘Boynton’, and column 3 shows the characteristics for lettuce ‘Cornouaï’. TABLE 5 Characteristic ‘Boynton’ ‘Cornouaï’ Plant diameter Small Medium Intensity of leaf anthocyanin coloration Strong Medium Attitude of outer leaves at harvest maturity Semi-erect Medium Further Embodiments Gene Conversions When the term “lettuce plant” is used in the context of the present invention, this also includes any gene conversions of that variety. The term “gene converted plant” as used herein refers to those lettuce plants which are developed by backcrossing, genetic engineering, or mutation, where essentially all of the desired morphological and physiological characteristics of a variety are recovered in addition to the one or more genes transferred into the variety via the backcrossing technique, genetic engineering, or mutation. Backcrossing methods can be used with the present invention to improve or introduce a characteristic into the variety. The term “backcrossing” as used herein refers to the repeated crossing of a hybrid progeny back to the recurrent parent, i.e., backcrossing 1, 2, 3, 4, 5, 6, 7, 8, 9, or more times to the recurrent parent. The parental lettuce plant which contributes the gene for the desired characteristic is termed the “nonrecurrent” or “donor parent.” This terminology refers to the fact that the nonrecurrent parent is used one time in the backcross protocol and therefore does not recur. The parental lettuce plant to which the gene or genes from the nonrecurrent parent are transferred is known as the recurrent parent as it is used for several rounds in the backcrossing protocol. Poehlman & Sleper (1994) and Fehr (1993). In a typical backcross protocol, the original variety of interest (recurrent parent) is crossed to a second variety (nonrecurrent parent) that carries the gene of interest to be transferred. The resulting progeny from this cross are then crossed again to the recurrent parent and the process is repeated until a lettuce plant is obtained where essentially all of the desired morphological and physiological characteristics of the recurrent parent are recovered in the converted plant, in addition to the transferred gene from the nonrecurrent parent. The selection of a suitable recurrent parent is an important step for a successful backcrossing procedure. The goal of a backcross protocol is to alter or substitute a trait or characteristic in the original line. To accomplish this, a gene of the recurrent variety is modified or substituted with the desired gene from the nonrecurrent parent, while retaining essentially all of the rest of the desired genetic, and therefore the desired physiological and morphological, constitution of the original line. The choice of the particular nonrecurrent parent will depend on the purpose of the backcross. One of the major purposes is to add some commercially desirable, agronomically important trait to the plant. The exact backcrossing protocol will depend on the characteristic or trait being altered to determine an appropriate testing protocol. Although backcrossing methods are simplified when the characteristic being transferred is a dominant allele, a recessive allele may also be transferred. In this instance it may be necessary to introduce a test of the progeny to determine if the desired characteristic has been successfully transferred. Many gene traits have been identified that are not regularly selected in the development of a new line but that can be improved by backcrossing techniques. Examples of these traits include, but are not limited to, male sterility, modified fatty acid metabolism, modified carbohydrate metabolism, herbicide resistance, resistance for bacterial, fungal, or viral disease, insect resistance, enhanced nutritional quality, industrial usage, yield stability, and yield enhancement. These genes are generally inherited through the nucleus. Several of these gene traits are described in U.S. Pat. Nos. 5,777,196, 5,948,957, and 5,969,212, the disclosures of which are specifically hereby incorporated by reference. Tissue Culture Further reproduction of the variety can occur by tissue culture and regeneration. Tissue culture of various tissues of lettuce and regeneration of plants therefrom is well known and widely published. For example, reference may be had to Teng, et al., HortScience, 27:9, 1030-1032 (1992); Teng, et al., HortScience, 28:6, 669-1671 (1993); Zhang, et al., Journal of Genetics and Breeding, 46:3,
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