2O2 



GROWTH AND WORK OF PLANTS. 



empties the two sperm nuclei. One of these sperm nuclei unites 

 with the egg nucleus. This is fertilization. 



319. Development of the embryo. The fertilized egg now 

 grows and divides into two cells, and these into more cells, form- 

 ing the young embryo which lies in the sac within the ovule. This 

 is why the term embryo sac is employed for this structure. At the 

 time of fertilization the endosperm begins to form, and develops 

 the tissue, some of which is used in the growth of the embryo, and 



Fig. 154- 



Seed of violet, external view, and 

 section. The section shows the em- 

 bryo lying in the endosperm. 



PC 



Fig. 155- 



Section of fruit of pepper (Piper 

 nigrum), showing small embryo lying 

 in a small quantity of whitish endo- 

 sperm at one end, the perisperm occu- 

 pying the larger part of the interior, 

 surrounded by pericarp. 



the rest is stored as food, either by the side of or around the 

 embryo, or in the cotyledons, according to the kind of seed, for 

 use by the young seedling. 



320. Formation of the seed. We are now ready to learn 

 how the seed is formed. In the bean, pea, squash, and many 

 other seeds, the embryo uses up all of the endosperm which is 

 formed, storing up in the cotyledons what is not used in making 

 the tissues of the different parts of the embryo. During this 

 process nearly or quite all of the inside portion of the ovule has 

 been used as food for the developing endosperm. There remain 

 then the walls of the ovule (the integuments), which make the 

 walls of the seed. Inside lies the embryo, with sometimes a 

 papery remnant of the interior of the ovule, as in the squash, 

 pumpkin, etc. In the forming grain of corn, wheat, castor bean, 

 etc., the embryo uses up only a portion of the endosperm, so 

 that in the seed there are embryo and endosperm, surrounded 



