114 



PLANT PHYSIOLOGY 



depletion of the reserve organs of the seed is usually accompanied 

 by their shriveling and desiccation and finally their detachment 

 from the plantlet after the embryo has attained complete inde- 

 pendence. With the endosperm, this is always the case. Fre- 

 quently it is entirely dissolved and absorbed by the seed, so that 

 only the empty seed coat remains. When the reserve organs are 

 cotyledons and are carried above ground, they usually become 

 green and begin to function like real leaves, simultaneously 

 increasing in size. 



Fig. 22. — Grain of wheat in longitudinal section: A — previous to germination; 



B — in the beginning of germination. 



The further growth of the embryo consists in an increase in 

 size of all the initial organs of the embryo, the rootlet first, then 

 the leaves and the stem. With the gradual enlargement of the 

 seedling, it continually develops new organs that were absent 

 in the seed, viz., new leaves, roots, branches, stems, etc. Gradu- 

 ally the parts that have been formed earlier complete their 

 growth. At least, they cease to increase in size, and in later 

 stages of development in the plant, completely mature tissues 

 and organs may be distinguished from the embryonic tissues 

 that then appear at the ends of the roots and stems and that 

 consist of continuously dividing and growing cells. 



In the course of their growth, many plants, especially the 

 perennials, differ essentially from animals. When the animal is 



