318 



ELEMENTARY STUDIES IN BOTANY 



to water and oxygen offered by the testa, and in the case of 

 many seeds this will make a great difference in the prompt- 

 ness of germination. 



Embryo. The young plant enclosed in the seed, often 

 called " the germ," is the structure that is to work and grow 

 and escape from the testa. The living substance (proto- 

 plasm) of the embryo is in a dormant stage, which means that 

 it is inactive ; and it needs the water to put it in a proper 

 condition for activity. As a general rule, the longer dor- 

 mancy is continued, the less active the embryo is when 

 aroused. The germinating power of a seed is called its 

 viability, and viability diminishes as dormancy is prolonged. 

 This is not true for all seeds, for in certain cases changes 

 are necessary in the embryo itself during dormancy before 

 germination can begin. In these cases, therefore, viability 

 increases as long as these changes in the embryo are occurring, 

 but after the embryo has " ripened," 

 the viability diminishes as germination 

 is postponed. 



It is evident that the testa and the 

 embryo of the plants we cultivate must 

 be better understood before we can 

 secure in every case prompt germina- 

 tion at the time of greatest viability. 



Food supply. In many seeds, as in 

 the cereals, the food supply is packed 

 around the embryo (Fig. 1), or at one 

 side of it, as in corn (Fig. 2), forming 

 a distinct region of the seed, called the 

 endosperm. In other seeds, as peas and 

 beans, the endosperm has been used up 

 by the growing embryo, and the food 

 substances have been redeposited in the seed-leaves (cotyle- 

 dons) of the embryo, which become large and fleshy. In this 

 latter case the embryo occupies all the space within the testa 



FIG. 2. Section of a grain 

 of corn, showing the endo- 

 sperm at one side of the 

 embryo : fuller explana- 

 tion in text on p. 349. 

 After FRANK. 



