Butters: seeds and seedling of caulophyllum. 



17 



become somewhat uniformly scattered throughout, with a group of 

 several near the antipodal end of the embryo-sac (Fig. 2, PI. IV). 

 For some time, at least, the divisions of all the endosperm nuclei 

 are simultaneous. 



Until the seeds are one-half their ultimate size — about one month 

 after flowering — the endosperm remains as a parietal layer not 

 over 25 mic. thick surrounding the large central vacuole of the em- 

 bryo-sac, and containing rather numerous large nuclei (8 mic. x 16 

 mic). These nuclei are somewhat flattened in form, and lie in a sin- 

 gle layer, without any intervening cell walls. 



About four weeks after flowering, the endosperm layer begins to 

 thicken, and anticlinal cell walls appear between the nuclei. As 

 the cytoplasm thickens, some of the nuclei migrate in towards the 

 center of the embr}^o-sac, and the development of cell walls proceeds 

 centripetally. In this manner in less than ten days, the whole 

 cavity of the seed becomes filled with parenchymatous endosperm. 



eTft 



Text Figure 2. Development of seed. A. Ten days after flowering. 

 X ID. B. Six weeks after flowering. X 5. C. Ripe seed with the fleshy 

 testa removed except the invaginated portion. X 5. En, endosperm, etn, 

 embryo, a, chalazal vesicle, other letters as in text fig i. 



This period coincides in time with the invagination of the microp- 

 ylar end of the embryo-sac, and is the period of most rapid growth 

 in the seed. 



For some time after the endosperm becomes cellular, growth con- 

 tinues throughout, with frequent cell divisions, and the tissue re- 

 mains very thin-walled. Finally, when the seed is almost ripe, 

 the walls become much thickened, and in the dry, ripe seed the 

 endosperm is exceedingly hard. It contains no starch at any time, 

 the reserve materials of the seed being stored in the form of oil, 

 and of the thick cellulose walls of the endosperm cells. 



