376 GAMETOPHYTE OF ANGIOSPERMS 



and that the antipodal cells are a remnant of the numerous vege- 

 tative cells of the gametophyte. The peculiar formation of the 

 endosperm nucleus through the fusion of the two polar cells is 

 looked upon as a nourishing device to give the endosperm nucleus 

 the power to grow and form a tissue that supplements the anti- 

 podal cells and so takes the place of the nourishing gametophyte 

 of the gymnosperms. According to this view, the endosperm is 

 a delayed prothallial growth which does not take place until 

 fertilization is effected. There is considerable evidence to justify 

 this conclusion. In the Pteridophyta the archegonia are formed 

 at the close of the prothallial development. Among the species 

 of the Gymnospermae there are several examples indicating that 

 the archegonia are formed at earlier and earlier stages in the 

 development of the gametophyte. Possibly we have in the 

 angiospermae a final condition where the homologues of the 

 archegonia are developed at the very start of the gametophytic 

 growth. The nicety of such a sequence of development is alto- 

 gether admirable. In the ferns, if fertilization is not effected, 

 the prothallial growth is wasted, but in the angiosperms there is 

 no prothallial growth without fertilization. 



The male gametophyte presents several features suggestive of 

 the gymnosperms. The germination of the microspores usually 

 begins within the sporophylls, and by the time that they are shed 

 and carried to the stigma, their nuclei have already divided once 

 and each spore consists of a large tube cell and an antheridial cell 

 (Fig. 272, B). There is very rarely a trace of the vegetative 

 cells of the gametophyte, as in the cycads and pines, since the 

 necessity for these cells no longer exists. The stigma is generally 

 provided with minute outgrowths or papillae derived from the 

 epidermal cells which serve to hold the microspores (Fig. 269, s). 

 These cells of the stigma usually secrete a sugary solution which 

 nourishes the microspores and causes a continuation of their ger- 

 mination. It is noteworthy that these microspores may be made 

 to germinate by placing them in sugar solutions, but the approxi- 

 mate strength of the solution on the stigma must be determined 

 in order to prepare a solution suitable for their development. 

 The tube cell, stimulated by the secretion of the stigma, ruptures 



