NUTRITION 



389 



food accumulates. This may be the parenchyma of the cortex, or of 

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

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

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

 also that the reservoirs of food are usually located in parts that 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 surplus. 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 

 seems to be the reverse of the process by 

 which the storage forms of food are pro- 

 duced. 



Storage forms. — The storage forms of food 

 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. 660-662), 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 starch. 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; that is, it is composed of a multitude of microscopi- 

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



Fios. 660-662. — Starch 

 grain and leucoplasts of Pdli- 

 onia : 660, simple starch grain 

 with leucoplast in position; 

 661, leucoplast alone of a simi- 

 lar grain; 662, leucoplast of 

 a twin grain, x 900. — After 

 Meyer. 



