CYTOLOGY OF REPRODUCTION IN ANOIOSPERMS 143 



indirect genetical evidence which indicates that it does enter in some 

 species (page 232). As the two gametic nuclei unite, their aspect may 

 vary as it does in animals: both may have a thready structure much like 

 that of the metabolic stage ; the male may be smaller and more compact ; 

 one or both of them ma}^ be in the prophase of mitosis, the maternal 

 and paternal genomes being distinguishable in the next ensuing division. 

 In any case the chromosome complement of the fertilized egg consists of 

 two genomes: diplosis has occurred in syngamy. After a cross in angio- 

 sperms, therefore, the nuclear material of the zygote and the individual 

 which it eventually becomes is derived from both parents, whereas its 

 cytoplasm may come from the mother alone. 



While the above events are taking place, the other male gamete 

 nucleus takes up a position near the two polar nuclei somewhere in the 

 sac. All three then undergo a triple fusion, formmg a primary 

 endosperm nucleus wth three genomes. Sometimes the two polars 

 have fused by the time the male nucleus arrives, the male then being 

 added. All orders of fusion of the three have been observed and, as 

 in the egg, they may be in the metabolic state or in some stage of the 

 prophase. In sacs with more than the usual number of nuclei, more 

 than three may unite to form the primary endosperm nucleus. The 

 union of one male nucleus with the egg nucleus while another unites 

 with the polar nuclei is called double fertilization and is a process peculiar 

 to angiosperms. 



The time elapsing between the arrival of the pollen on the stigma and 

 the syngamic union varies greatly, as the following examples show: 

 rye, 7 hours; maize, 18 to 24 hours, in spite of the length of the style 

 (the silk); Jimson weed, 25 hours; box elder, 40 to 72 hours; Indian pipe 

 (Monotropa), 5 days; pecan, 5 to 7 weeks; red oak, 13 to 14 months. 

 These periods vary, of course, with temperature. 



Embryogeny and Seed Development. — Pollination and syngamy set in 

 motion a number of reactions in the ovary and ovules which result in 

 the development of a fruit with seeds. This involves alterations in every 

 part, from the fertilized egg to the tissue of the pistil and sometimes the 

 other floral organs. 



The fertilized egg develops into the embryo, which is to become the 

 sporophytic plant of the next generation. It divides several times 

 transversely, forming a few cells in a row (Fig. 105), and then, beginning 

 in the terminal cell (toward the center of the sac), divisions in the longi- 

 tudinal and other planes accompany its lateral growth. The stage of 

 embryonic development reached by the time the ovule becomes a mature 

 seed varies widely: in some plants the embryo is a small undifferentiated 

 mass of cells, most of the seed being occupied by endosperm to b(; 

 described below; in others the embryo is larger, has differentiated its 



