 Depending on the variety and time of year , lettuce generally lives 65 – 130 days from planting to harvesting . Because lettuce that flowers ( through the process known as " bolting " ) becomes bitter and unsaleable , plants grown for consumption are rarely allowed to grow to maturity . Lettuce flowers more quickly in hot temperatures , while freezing temperatures cause slower growth and sometimes damage to outer leaves . Once plants move past the edible stage , they develop flower stalks up to 3 feet ( 0 @.@ 9 m ) high with small yellow blossoms . Like other members of the tribe Cichorieae , lettuce inflorescences ( also known as flower heads or capitula ) are composed of multiple florets , each with a modified calyx called a pappus ( which becomes the feathery " parachute " of the fruit ) , a corolla of five petals fused into a ligule or strap , and the reproductive parts . These include fused anthers that form a tube which surrounds a style and bipartite stigma . As the anthers shed pollen , the style elongates to allow the stigmas , now coated with pollen , to emerge from the tube . The ovaries form compressed , obovate ( teardrop @-@ shaped ) dry fruits that do not open at maturity , measuring 3 to 4 mm long . The fruits have 5 – 7 ribs on each side and are tipped by two rows of small white hairs . The pappus remains at the top of each fruit as a dispersal structure . Each fruit contains one seed , which can be white , yellow , gray or brown depending on the variety of lettuce . 
