198 



APPLIED BIOLOGY 



egg-nucleus, and the combined nucleus is the nucleus of the 

 fertilized egg-cell. This process of fusion of sperm-cell with 

 egg-cell is fertilization. Distinguish be- 

 tween pollination and fertilization as 

 defined above. 



Cell-division. The result of fertiliza- 

 tion is to stimulate the egg-cell to begin 

 a series of cell-divisions to form a group 

 of many cells (Fig. 62, A, C). These 

 cells become the embryo, which is the 

 part of the seed able to develop into 

 a new plant. While the egg-cell is de- 

 veloping into the embryo, the surround- 

 ing cells of the ovule are forming other 

 parts of the seed. In all seeds there is 

 the outer covering or seed-coat of one 

 or more layers ; and in many seeds there 

 is more or less tissue lying between the 

 seed-coat and the embryo. This is com- 

 posed of cells stored with food to be 

 used later when the seed is sprouting or 

 germinating. The bean seed is an ex- 

 ample of one with an embryo and seed- 

 coat only. 



A castor-oil bean ( 138) is one with 

 an embryo, a seed-coat, and a large mass 

 of food-storing tissue between embryo 

 and seed-coat. Such food tissue outside of the embryo is 

 called endosperm (meaning within the seed). 



In flowers like four-o'clock and buckwheat, which have one 

 ovule and form one embryo, the development of the embryo 

 and its surrounding parts (endosperm and seed-coats) is 

 associated with enlargement and change in the wall of ovary. 

 This forms an outside case close to the seed so that the fully- 

 developed seed is inclosed in a sort of double seed-coat. 



FIG. 62. Stages in de- 

 velopment of a seed- 

 plant embryo. The 

 four upper cells in A 

 divide many times 

 (B, C) and form the 

 embryo shown in D 

 with two cotyledons 

 (c), epicotyl (p),and 

 hypocotyl (K). The 

 row of cells shown be- 

 low the embryo in 

 each figure holds it in 

 position in the ovary. 

 (From Strasburger.) 



