374 THE ANATOMY OF WOODY PLANTS 



latter, unlike the similar structure in the microspores of the higher 

 Coniferales, does not give rise to stalk and body cells, but originates 

 directly by division the two generative nuclei which function in 

 fertilization. The microspores are sheltered in sporangia, which 

 are typically four in number, on each sporophyll. The sporangia 

 owe their dehiscence to a mechanical layer within the wall, resem- 

 bling in structure that found in Ginkgo and certain Coniferales, but 

 no longer related, as in these, to the fibrovascular system of the 

 sporophyll. The megasporangium is much modified and, as in 

 other known seed plants, is without any opening mechanism. 

 In certain of the lower and amentiferous dicotyledons, however, 

 tracheary structures have been discovered in the inferior region 

 of the megasporangium (Casuarina, Corylus, Castanea). The 

 germinated megaspore normally gives rise to a gametophyte 

 containing originally eight naked nuclei which result from three 

 successive divisions. Of these, six become organized as cells by 

 the development of inclosing protoplasmic bodies, while the two 

 remaining nuclei, one from each pole of the elongated sac, unite 

 to constitute the endosperm nucleus, a structure characteristic 

 for the angiosperms and not occurring in any lower forms. Three 

 cells in the micropylar region of the gametophyte become organized 

 as the single egg and the two synergidae. Three others constitute 

 the antipodals in the base of the gametophyte. After the egg has 

 been fertilized by one of the sperm nuclei of the pollen tube and 

 the endosperm nucleus has contracted a union with the remaining 

 male element, the egg develops as the embryo and the endosperm 

 nucleus gives rise to a mass of tissue which usually quite supplants 

 the original gametophyte or embryo sac as nourishing substance 

 for the developing embryo. The seed of the angiosperms conse- 

 quently typically contains, in addition to the gametophyte and 

 sporophyte of lower seed plants, a third generation. This is 

 known as the trophophyte or endosperm, and it is distinguished 

 from gametophyte and sporophyte, not only by its peculiar mode 

 of origin, but also by the fact that in its nuclear divisions three 

 times as many chromosomes are present as in the gametophyte 

 and one-half more than in the sporophyte. This cytological 

 condition is doubtless due to the three nuclei which are fused to 



