346 GENERAL BOTANY 



while in the gymnosperms (cycad, spruce, and pine) the gameto- 

 phyte is a definite cellular structure which bears true archegonia. 

 The male gametopliyte is also greatly reduced in Capsella, as 

 in the mandrake (Fig. 85, 6) and the iris (Fig. 204, a). Pollina- 

 tion and fertilization take place in the manner already described 

 for the mandrake and the bean. Double fertilization probably 

 occurs in Capsella, although this has not been definitely investi- 

 gated. In this process, as shown in Fig. 204, 6, the nucleus of 

 one of the male cells unites with the egg, while that of the other 

 combines with the polar nuclei to form the endosperm nucleus. 

 The fertilized egg cell develops into the embryo, while the endo- 

 sperm nucleus initiates the formation of the endosperm. 



THE EMBRYO, ENDOSPERM, AND SEED 



The embryo. After fertilization the egg secretes a cellulose 

 wall and then divides by repeated mitoses, forming an elon- 

 gated, rodlike proembryo (Fig. 203, D). The upper cell of this 

 proembryo forms the embryo proper by cell division and differ- 

 entiation, while the remaining cells form the suspensor (E, F) 

 which supplies the embryo with nutriment and anchors it within 

 the megaspore, or embryo sac. The two cotyledons bud out 

 from the upper half of the embryo, while the root grows from 

 the lower portion (6r). In If the proembryo is shown in posi- 

 tion in the embryo sac, with the lower cell of the suspensor 

 occupying an enlarged portion of the sac next to the micropyle. 



The endosperm. In Capsella, as in other angiosperms, the 

 endosperm begins to form by repeated divisions of the endosperm 

 nucleus, formed, as indicated above, by the union of one male 

 nucleus and the two polar nuclei. 



The numerous endosperm nuclei thus formed accumulate in 

 the peripheral layer of cytoplasm which surrounds the large 

 central vacuole of the embryo sac, where they form a layer of 

 free nuclei (Fig. 203, H, e). These free nuclei never form a per- 

 manent cellular endosperm in Capsella, since they are gradually 

 absorbed by the growing embryo, which in a later stage of its 

 development fills the embryo sac. 



