176 



STORAGE OF FOOD AND WATER 



The Kinds of Stored Food. — The best evidence of what 

 constitutes the food of plants is found in seeds. What occurs 

 stored up there must be plant food; and if we find, as we do, 

 that the same stufE is stored in the branches, trunk and roots of 

 mature plants, then we know that the food required for the 



embryo plant in the seed is the same as 

 that needed by the adult plant. 



Both in seeds, bulbs, tubers, etc., and 

 in the general plant body are found 

 nitrogenous and non-nitrogenous foods. 

 Of the latter class starch, fats, and oils, 

 are far the most common forms used in 

 storage; glycose, laevulose, and sac- 

 charose are much employed; and cellu- 

 lose, inulin, glucosides, and mucilage 

 less frequently. Different kinds of pro- 

 teids and amides are the chief repre- 

 sentatives of the nitrogenous class. 



Starch. — Starch occurs in the form of 

 definite grains, either suspended in the 

 cytoplasm or lying loose in the vacuoles. 

 It is insoluble in the cell-sap and is one 

 of the most stable and permanent forms 

 in which food is stored. The sizes, 

 shapes and markings of the grains vary a great deal in different 

 plants, and even in different parts of the same plant. The starch 

 grains are, as a rule, much larger in special storage parts, as in 

 fleshy roots and stems, and in the endosperm of seeds than in 

 the ordinary stems and roots, and they range in diameter in 

 different species from .002 mm. or less to nearly . 2 mm. 



Starch grains in the special storage organs usually have char- 

 acteristic shapes and markings for the different kinds of plants. 

 The forms are spheroidal, ovoidal, ellipsoidal, or polyhedral 

 where the grains crowd one another. Rarely rod and dumb- 

 bell shapes are found. The markings are in the form of con- 

 centric and excentric striations and more or less irregular cracks. 



Fig. 96. — Camera-lucida 

 drawing of tangential section 

 through the wood of grape- 

 vine. MS, cells of medullary 

 ray, and n, of xylem paren- 

 chyma, packed full of starch. 



