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THE INDIVIDUAL ORGANISM 



"seed," and phyton, "plant"). Seeds are not eggs; they are embryonic 

 plants enclosed in tough seed coats. By the time the seed is ripe, the 

 embryo has developed far enough so that the principal parts of the body 

 of the plant can be distinguished. In the dicotyledons, of which the bean 

 may be taken as an example, the greater part of the seed generally is 

 made up of two greatly swollen leaves (cotyledons) , filled with a store of 



embryo 



hypocolyl 



roots 



radicle 



Fig. 9.3. Germination of a seed plant, the bean. 



starchy food that carries the developing plant through the period of 

 germination and establishment. The cotyledons enclose and partly 

 conceal the rudiments of the rest of the plant; these rudiments are the 

 radicle or embryonic root, the hypocotyl or embryonic stem, and the 

 plumule or embryonic leaf shoot. The entire embryo is in a resting state, 

 with all metabolic activities reduced to a very low point. 



Germination. Under proper conditions of moisture and temperature 

 the embryonic plant becomes active and begins to grow. The seed coat 

 splits, and the radicle and plumule emerge. The elongating radicle is 

 positively geotropic and negatively phototropic — i.e., it turns toward the 

 center of the earth under the influence of gravity and away from light. 



