256 VEGETABLE PHYSIOLOGY 



CHAPTER XVII 



DIGESTION 



WE have noticed in studying the deposition of reserve food- 

 stuffs that the forms in which they exist in the reservoirs 

 differ in many respects from those which they assume for 

 purposes of transport or translocation. They are generally 

 insoluble in water or cell- sap, and almost always indiffu- 

 sible, whereas they travel in the form of soluble, diffusible 

 bodies. The removal of them from the seats of storage 

 takes place at times which are dependent on the resump- 

 tion of activity of growth or development ; and as such a 

 removal involves the resumption of the travelling forms, 

 they must undergo a process which, from analogy with 

 similar processes in the animal body, may be described as 

 digestion. Each must, after such treatment, be presented 

 to the protoplasm of the growing cells in much the same 

 form or condition as that in which it was first constructed 

 from the simple bodies which the plant absorbed from its 

 environment. This is necessary in all cases, because, 

 as we have already noticed, the storage forms are not 

 directly assimilable by the protoplasm, but have under- 

 gone a certain modification in the process of their deposi- 

 tion. 



The process of digestion in plants is chiefly intra- 

 cellular, and takes place in all cells in which reserve 

 materials occur. It is only occasionally that we find it 

 taking place on the exterior of the plant that is, not in 

 the interior of a cell. In a few cases we find it carried 

 on in connection with the absorption of nitrogenous or 

 proteid food, as has been already shown in a preceding 



