a "nutlet" (or nucule, a term otherwise referring to the oogonium of stoneworts). In botany, the term "nutlet" specifically refers to a pyrena or pyrene, which is a seed covered by a stony layer, such as the kernel of a drupe. Walnuts and hickories (Juglandaceae) have fruits that are difficult to classify. They are considered to be nuts under some definitions but are also referred to as drupaceous nuts. In common use, a "tree nut" is, as the name implies, any nut coming from a tree. This most often comes up regarding food allergies; a person may be allergic specifically to peanuts (which are not tree nuts but legumes), whereas others may be allergic to the wider range of nuts that grow on trees. Production In the 21st century, some dozen species constitute most of the worldwide production of nuts, shown in the table below for major commercial nuts. Food and health effects Nuts contain the diverse nutrients that are needed for the growth of a new plant. Composition varies, but they tend to have a low water and carbohydrate content, with high levels of fats, protein, dietary minerals, and vitamins. The digestibility of the protein at about 90% is slightly lower than that of meat and fish, but can be improved by thorough chewing. The fats are largely unsaturated and nuts are a source of essential omega-3 fatty acids. As part of a healthy human diet, long-term consumption of diverse nutrients in nuts may contribute to a lower risk of cardiovascular diseases, reduced levels of blood cholesterol, and lower all-cause mortality. For vegetarians and vegans, nuts provide many of the essential nutrients which may be in short supply in other plant foods. Nuts supply nutrients for humans and wildlife. Because nuts generally have a high oil content, they are a significant energy source. Many seeds are edible by humans and used in cooking, eaten raw, sprouted, or roasted as a snack food, ground to make nut butters, or pressed for oil that is used in cooking and cosmetics. Moderate nut consumption about per week may benefit weight control and contribute to lowering body weight in humans. Nuts used for food are a common source of food allergens. Reactions can range from mild symptoms to severe ones, a condition known as anaphylaxis, which can be life-threatening. The reaction is due to the release of histamine by the body in response