ANGIOSPERMS 



383 



tube in bringing the sperm cell to the egg. The third nucleus of 

 the egg apparatus is that of the egg cell. 



547. Fertilization. When the pollen tube has reached the 

 embryo sac it opens at the end, and the sperm cells with some of 

 the protoplasm are emptied. One of the sperm nuclei unites 

 with the egg nucleus bringing about fertilization. The second 



Fig. 379- 



Two- and four-celled stage of embryo sac of Lilium. The middle one 

 shows division of nuclei to form the four-celled stage. (Easter lily.) 



sperm, in a number of plants, has been found to join the two 

 polar nuclei in the center of the embryo sac, the three nuclei 

 fusing to form the primary endosperm nucleus. This is called 

 double fertilization, 



548. The endosperm of the Angiosperms. The primary 

 endosperm nucleus, formed by the fusion of the two polar nuclei 

 (and sometimes by triple fusion with the second sperm nucleus), 

 develops the endosperm. The nucleus divides successively into 

 a large number of free nuclei arranged in the protoplasm, which 



