92 CIRCULATION AND USES OF PLANT FOOD 



In a similar manner, protein seems to be changed and trans- 

 ferred to various parts of the plant. Some forms of protein are 

 soluble and others insoluble in Avater. White of egg, for example, is 

 slightly soluble, but can be rendered insoluble by heating it so that 

 it coagulates. Insoluble proteins are digested within the plant; 

 how and where is but slightly understood. Soluble proteins pass 

 down the sieve tubes in the bast and then may be stored in the 

 bast or medullary rays of the wood in an insoluble form, or they 

 may pass into the root, fruit, or seeds of a plant, and be stored 

 there. This stored food becomes of immense value to mankind, 

 for it forms not only our cereal, potato, and other crops, but also 

 our fruits of all kinds. 



Plants with Special Digestive Organs. — Some plants have 

 special organs of digestion. One of these, the sundew, has leaves 



which are covered on one 

 side with tiny glandular hairs. 

 These attract insects and later 

 serve to catch and digest the 

 nitrogenous matter of these in- 

 sects by means of enzymes 

 /^/ ^'# ' '.>. I poured out by the same hairs. 



§ i Another plant, the Venus's fly- 



Three modified leaves, which get nitro- trap, catches insects in a seusi- 



gen from captured insects: 1 pitcher ^^^^ j^^f ^^j^-^j^ £^1^3 ^^^ 



pJant; 2, sundew; 3, Venus s flytrap. i i i i • o '^ 



holds the msect fast until en- 

 Z3anes poured out by the leaf slowly digest it. Still others, called 

 pitcher plants, use as food the decayed bodies of insects which fall 

 into their cuplike leaves and die there. These few plants are 

 somewhat like those animals which have certain organs in the 

 body set apart for the digestion of food. 



Summary. — The raw^ materials which have been absorbed by 

 the roots pass through conducting tubes up the stem to the leaves, 

 where they are manufactured into food. This food passes down 

 the stem, as a liquid, in the sieve tubes, until it reaches a place 

 where it is used to build tissue or is changed to a solid form and 

 stored. 



Summary of the Functions of Plants and Animals. — The processes 

 which have just been described; with the exception of food making, 



