144 



PRACTICAL BOTANY 



leaf -like in its appearance, as in buckwheat and the castor bean, 

 its activity differs from that of the permanent leaves. The rate 

 of transpiration for equal areas of such cotyledons, when com- 

 pared with that of the later leaves, 

 has been found to be from one 

 and a half to two times as great. 

 It is easy to see that in a 

 general way the readiness with 

 which some cotyledons assume 

 the character of temporary foliage 

 leaves depends on their com- 

 parative freedom from deposits 

 of plant food. For this reason 

 some of the most leaf-like coty- 

 ledons, like those of the buck- 

 wheat and the morning-glory, are 

 found in seeds with abundant 

 endosperm. 



134. Action of enzymes on re- 

 serve material of seeds. 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 saw- 

 dust or on moist blotting paper. 

 Evidently the plant food at the 

 start must all come from the 

 seed, and the removal of most 

 of the reserve food of the seed 

 greatly retards the growth of the 

 seedling (Fig. 134). 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 are so quickly withdrawn from them and transferred to the 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 soluble with difficulty or quite insoluble in water 



FIG. 134. Pea seedlings growing 

 in water 



A, deprived of both cotyledons; /?, 

 with cotyledons uninjured 



