144 OUTLINES OF PLANT LIFE. 



another it must be done by altering them into soluble sub- 

 stances, that is, by digestion. (See *\ 182.) 



This is accomplished by means of enzymes of different kinds, adapted 

 to effect the alteration of various foods. The most abundant enzyme is 

 diastase, which has the power of altering starch into a sugar. Enzymes 

 fitted to transform proteids are also found in considerable amounts. 



When the foods have been brought into a soluble condi- 

 tion, they dissolve in the water present. If one cell then 

 contains more of that particular substance, say sugar, than its 

 neighbor, the sugar particles will pass into the neighboring 

 cells until the amount is equal. If this sugar is being used 

 up in growth or repair, or is altered into another substance 

 at any point, a constant stream of particles of sugar moves 

 toward the point at which it is disappearing. Thus from 

 the food sources it is transferred to the reservoirs and stored 

 in suitable form. Thence, when needed, it is redissolved 

 after digestion and carried to the active parts which utilize it. 

 This movement may be hastened if elongated cells are pro- 

 vided along the more important lines of travel. This is 

 done in the bast strands. The movement is made still easier 

 also in these by the perforation of the ends of some of the 

 elongated cells, so that there is less resistance to movement. 

 Foods, therefore, travel chiefly in the bast bundles and in 

 either direction as may be necessary. 



EXERCISE XXXIII. 



To show in what tissues food most readily travels. 



Girdle as in Exercise XXV A a shoot of willow. Cut it off 5 cm. 

 below ring. Place shoot in water. After some weeks note where new 

 roots are formed. Why ? 



To show the digestion of starch by diastase. 



Powder a handful of malt in a mortar or obtain ground malt. To 25 

 grams of the powder add 100 cc. of water ; stir well together ; allow 

 mixture to stand (with occasional stirring) one to two hours ; filter ; pre- 

 serve the filtrate. Take i gm. of starch and rub it up in a dish with 



