24 



GENEEAL BOTANY 



Cotyledon 



that the growth of branches and leaves at regular nodal inter- 

 vals, and their cyclic and spiral arrangements on the main stem 

 and its branches, are not all that is necessary for the proper 

 adjustment of these organs to the environment. This fact is 

 most strikingly illustrated in the growth of young plants from 

 the seed, where the parts of the embryo, originally folded in 



the seed, must 

 gradually expand 

 and adjust them- 

 selves to the air, 

 light, and soil for 

 the absorption and 

 use of raw food ma- 

 terials. The growth 

 and development 

 of the embryo and 

 seedling of the 

 common pea fur- 

 nishes a good il- 

 lustration of the 

 movements which 

 take place in the 

 adjustment of a 

 young plant to 

 the new condi- 

 tions presented to it as it emerges from the seed and the soil. 

 The seed of the pea is composed of a dense outer covering, or 

 seed coat, inclosing the embryo proper, which entirely fills the 

 seed coats. This embryo consists of two fleshy cotyledons, or 

 food-storing leaves, which comprise the bulk of the embryo. 

 Between the two cotyledons are found two or three minute 

 leaves, constituting the first bud of the young plantlet, and a 

 stemlike body, the hypocotyl, which bears the cotyledons and 

 the plumule. These parts are all shown in their proper relation 

 in the early stages of germination of the pea (Fig. 13, a), just 

 after the plumule and hypocotyl have broken through the 

 seed coat. 



FIG. 13. Stages in the development of a seedling of the 

 garden pea 



a-c, emergence of the embryo from the seed and of the 



plumule from the soil; c-e, erection of the plumule and 



growth of the lateral roots ; /, mature seedling with leaves 



and roots adjusted to light and soil 



