1 86 STORAGE OF FOOD AND WATER 



dextrose (glucose, grape sugar) is the most common form in 

 which sugar is transported, and when it is to be stored in this 

 form there is little left for the storage cells to do beyond taking 

 it in and aiding in its accumulation after its concentration has 

 become greater than in the surrounding tissues. Undoubtedly 

 the plasma membranes in particular are active in this work, not 

 only allowing but helping the sugar to accumulate when, governed 

 alone by the recognized laws of diffusion, after its concentration 

 in the storage cells equalled that in surrounding cells, its con- 

 tinued entrance would be impossible. 



When the dextrose coming to the storage cells is transformed 

 to saccharose, as in the sugar-beet root, the change is appar- 

 ently advantageous to storage since saccharose is less diffusible 

 and has less osmotic power than dextrose. 



It may be assumed that when sugar in solution is stored in 

 cells the osmotic pressure within the cells becomes very great, and 

 saccharose would be more advantageous in high concentrations 

 since its osmotic pressure is only about half that of glucose. 



The reserve cellulose, amyloid, and similar thickenings of 

 endosperm cell-walls intended for food on the germination of 

 the seed, as in the case of the date, Tropceolum ma jus, Impa- 

 tiens balsamina, etc., appear to be formed by the decomposi- 

 tion of exterior portions of the plasma membrane, or by the 

 more direct transformation of dextrose, which comes to the 

 cells abundantly while storage is going on. No special proto- 

 plasmic organ for this work has been discovered, and it may 

 be assumed that the exterior plasma membrane does the same 

 work that it is supposed to do in the building of the ordinary 

 cell-wall (see page 5). 



It is noteworthy that in the construction of the cellulose, 

 amyloid, etc., food reserves, the protoplast has accomplished 

 without a special organ the same kind of chemical work as 

 that done in the storage of starch where special organs, namely 

 the leucoplasts, are necessary. 



The formation of the proteid reserve foods can be followed 

 to a certain extent where they appear as definite granules (aleu- 



