LACTUCA SATIVA 631 



of cell h produce the suspensor, which is variable in length and 

 in the number of its cells. (Fig. 318, G.) 



Endosperm Formation. — The divisions of the primary endo- 

 sperm nucleus and the nuclei subsequently derived from it occur 

 slightly prior to those of the embryo; and by the time the latter 

 is four-celled, there usually have been four successive divisions 

 of the endosperm nuclei. About to hours after pollination when 

 the embryo is eight-celled, wall formation is initiated in the 

 endosperm; and its cells completely fill the embryo sac. They 

 continue rapid growth and division, keeping pace with the enlarg- 

 ing cavity; but the developing embryo grows more rapidly than 

 the endosperm which it digests, and a week after pollination the 

 endosperm is almost entirely resorbed. When the seed is mature, 

 only the two outermost layers of endosperm remain. These are 

 closely appressed to the cell walls of the innermost layer of the 

 integument, forming a membrane which completely surrounds 

 the embryo. 



ANATOMY 



Fruit and Seed. — During the preceding developments which 

 take place within the young akene, the bracts of the involucre 

 are tightly compressed about the developing fruits. The beak of 

 each fruit elongates and rapidly lifts the pappus upward, so that 

 within four or five days after anthesis it appears through the 

 bracts. In the New York variety, Jones found that approximately 

 li days elapsed from anthesis to fruit maturity, subject to variation 

 due to temperature and other factors. In this variety, the heads 

 contain about 18 flowers and produce a slightly smaller number of 

 normal akenes. 



The beaked akene is spindle- or lance-shaped with a surface 

 that has a number of longitudinal ribs. Kondo (14) found a 

 variation of from 13 to 13 in 1.1. varieties. In the New York 

 variety, the average length of the akene is 4.1 mm.; the breadth 

 I.I mm. and thickness 0.4 mm. Cummings (8) has determined 

 that size of seed is a factor in production, stating that 



"the merits of large seed in lettuce culture were shown in the production 

 of larger seedlings, an increased weight of edibly matured plants, 

 which displayed better heading-up capabilities, earliness and uniformity 

 in filling the heads; in short, augmented earliness and quality." 



