SEEDS AND SEEDLINGS 



165 



the cotyledons remain underground, inclosed in the seed 

 coats, and gradually empty their stores of reserve food into 

 the growing seedling. In the bean the cotyledons come out 

 of the ground but never become 

 leaf-like, while in the squash, 

 castor bean, buckwheat, and 

 morning-glory (fig. 144) they 

 emerge from the ground and 

 become short-lived leaves. 



157. Reserve food of seeds 

 digested by enzymes. One of 

 the most surprising things about 

 the early growth of seedlings is 

 the rapid way in which many 

 kinds begin to grow even in 

 sawdust or on moist blotting 

 paper. Evidently the plant food 

 must all come from the seed in 

 the beginning, and the removal 

 of most of the reserve food of 

 the seed greatly retards the 

 growth of the seedling (fig. 146). 

 It is not at once clear how the 

 proteins and the starch of some 

 seeds and the oil or cellulose of 

 others are so quickly withdrawn 

 from them and transferred to 

 the growing plantlet. Most of 

 the reserve substances found in 

 seeds are difficultly soluble or 

 quite insoluble in water or the 

 watery sap of plants, but the 



insoluble substances, before being transferred into the seed- 

 ling, are transformed into soluble ones. This is due to the 

 action of certain substances known as enzymes or soluble fer- 

 ments. An enzyme as found in seeds is a substance secreted 



FIG. 146. Pea seedlings growing 

 in water 



A, deprived of both cotyledons; B, 

 with cotyledons uninjured 



