NUTRITION 



3»9 



food accumulates. This may l>c the parenchyma of the cortex, or of 

 the vascular bundles, or of the pith; <>r all may be involved. One note- 

 worthy point is that the storage tissues arc composed of live cells, even 

 though, as in -nine ferns, they arc very thick walled. It is to be observed 

 also that the reservoirs of food are usually Located in part- thai persist 

 through a dry or cold season unfavorable to growth, and that have rudi- 

 mentary growing points capable of quick and vigorous development 

 by using the adjacent suqilus. So the seeds, 

 bulbs, tubers, rhizomes, etc., are organs of 

 propagation, and by way of attaining that end 

 become also organs of storage. (See Part III 

 on seeds, bulbs, and tubers.) 



Storage cells active. — The storage of food 

 is not merely a stuffing of passive cells with 

 surplus food; it involves the activity of the 

 storage cells themselves, at least for the ac- 

 cumulation of the food, and usually also for 

 the mobilization when this food is about to 

 travel to growing regions where it is subse- 

 quently used. The process of mobilization is 

 commonly called digestion (see p. 397), and J^ JSSirfKS 

 seems to be the reverse of the process by onia: 660, simple starch grain 



which the storage forms of food are pro- £* '^^'f in ***».! 

 & » 661, leucoplast alone of asimi- 



duced. lar grain; 662, leucoplast of 



■After 



The storage forms of food a Uv,n s ram - x 9°°-- 

 Meykr. 



Storage forms. 

 are chiefly starches, sugars, hemi-celluloses, 

 inulin, fats, and proteins. From this list it will be apparent that carbo- 

 hydrates predominate, and quantitatively they form much the greater 

 part of stored food. 



Starches. — Starches are stored in the form of grains, many having a 

 form characteristic of the plant in which they are found. The grains are 

 organized by the activity of cell organs called leucoplasts or amyloplasts 

 (figs. 600-062), which seem to take the material as it comes to the cells, 

 perhaps as glucose, and combine it into larger and more complex mole- 

 cules, that finally become Stan h. This is disposed in the interior of the 

 leucoplast as one or more grains, which at length stretch it enormously, 

 or even rupture it. The actual structure of the grain is believed to be 

 that of a spherite; thai is, il is composed of a multitude of mil roscopi- 

 cally minute, threadlike crystals, radiating from its organic center. If 



