The Angiosperms 157 



tube, as it pushes its way through this conducting 

 tissue until it finally reaches the opening of the 

 ovule, which it enters and fertilizes the egg in much 

 the same way as we have already described for the 

 gymnosperms. The highly modified structures of 

 the carpels, so different from the open leaf-like ones 

 of most gymnosperms, is one of the most striking 

 characters of the angiospermous flower. 



The effect of fertilization is the development of 

 the embryo from the egg and the hardening of 

 the outer tissues of the ovule, which now be- 

 comes a seed, resembling very closely the seed of 

 the gymnosperms. A marked difference in the 

 seed may be noted, however. The " endosperm," 

 or gametophyte tissue which surrounds the em- 

 bryo, and which in the gymnosperms is developed 

 before fertilization, in the angiosperm arises 

 subsequent to fertilization, and is the result of 

 the division of the endosperm-nucleus, which is 

 formed by the fusion of the two polar nuclei (Fig. 

 1 6, C, />). The latter may also sometimes fuse 

 with one of the male nuclei from the pollen-tube. 

 The homologies existing between the endosperm of 

 the gymnosperms and the angiosperms are not, 

 therefore, entirely clear. It has even been claimed 

 that the endosperm of the angiosperms is rather 

 in the nature of an embryo than part of the gameto- 

 phyte, but we are inclined to the older view that it 

 really belongs to the gametophyte as it does in the 

 gymnosperms, and that the nuclear fusion which 



