272 Plant Physiology 



dew and Nepenthes, secrete enzymes which act from with- 

 out the absorbing organs, and likewise the fungi produce 

 enzymes at least slowly diffusible. The exact method 

 of action of the protein enzymes is not yet clear, but it is 

 generally assumed to be hydrolytic. It is necessary that 

 protein enzymes be produced in plants in some quantity 

 at the time of germination to effect the movement and use 

 of stored products, for, as has been shown already, indif- 

 fusible protein compounds are common in such organs. 

 Moreover, at the maturity of the plant or of any vegeta- 

 tive organs of the plant much of the solid protein and pro- 

 toplasmic material is converted and accumulated in the 

 seed, and in the recovery of this from organs which have 

 ceased to grow there is, of course, much economy. 



158. Conduction of digested foods. From what has 

 been said regarding the action of digestive enzymes, it is 

 apparent that there is required an effective means of trans- 

 location, as may be demanded, for digested materials to 

 and from the leaves, shoots, storage organs, and seeds. 

 Diffusible organic substances in complex plants seem to 

 require, then, for rapid diffusion to and from active organs 

 specialized paths or tissues. This demand is met through 

 the phloem of the vascular bundles, in which the sieve 

 tubes occur (Fig. 6, w, o). The large protoplasmic connec- 

 tions between the rows, of sieve cells evidently permit a 

 movement more rapid than simple diffusion. 



The sieve tubes are important in the movement of such 

 products as shown by direct and indirect evidence : thus, 

 by the fact that sieve tubes in particular contain a quan- 

 tity of simple organic substances ; by the absence of such 

 products in so great a quantity in xylem, pith, or cortex; 



