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others are so quickly withdrawn 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 Sa consid 
seeds are difficultly soluble or Pe ee 
quite insoluble in water or the with cotyledons uninjured 
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 
Fic. 146. Pea seedlings growing 
