388 PHYSIOLOGY 



contains an enzyme which digests proteins. That the products enter 

 the plant and are advantageous has been shown by comparing fed and 

 unfed plants in the same pot. Those on whose leaves tiny bits of meat 

 and egg were placed were larger and thriftier, and had more flowers, 

 as well as more and larger seed, than the ones which grew under identi- 

 cal conditions without feeding. 



The capture of insects probably supplements a scanty supply of nitro- 

 gen obtained from the soil nitrates; but too little is known of the ecology 

 of such plants to establish this explanation as at all conclusive. 



A fuller discussion of most of the topics of this chapter will be found 

 in Part III. 



5. THE STORAGE AND TRANSLOCATION OF FOOD 



Surplus food. — A part of the food made by a plant is promptly util- 

 ized in the making of new tissues (growth) and in the repair of the pro- 

 toplasm which has undergone changes in the course of its activity. It 

 is often said, also, that a part of it is oxidized directly to furnish energy 

 for growth and other work; but it is at least doubtful whether this is 

 true. 1 However that may be, most plants, at least at some period of 

 their existence, make more food than they actually use at the time. The 

 surplus is then stored for a longer or shorter time, until it is required. 

 But it may never be used. 



Storage places. — Accumulation may take place in the very part 

 where the food is made; but usually, if there is any room there, it is 

 insufficient; and to judge from the infrequent storage in food-forming 

 organs these two functions are not fully compatible. So when there is 

 any considerable surplus of food, it migrates to some more or less spe- 

 cialized storage organ. In the lower plants these are relatively simple, 

 for ordinarily such plants make little excess food. In Marchantia, 

 for instance, the colorless parenchyma of the lower part of the thallus 

 is accounted the storage region. In the pteridophytes and sperma- 

 tophytes, any one of the larger organs, root, stem, or leaf, may become 

 the seat of food accumulation. In many cases there is marked change 

 in structure and form. 



Parenchyma increased. — The characteristic change in structure con- 

 sists of an exaggerated development of parenchyma, in which chiefly the 



1 The matter wjll be discussed further in the section on Respiration, p. 403. 



