228 VEGETABLE PHYSIOLOGY 



they are only intended to compensate for regular or acci- 

 dental intermittence in the translocatory stream to the 

 parts in question. The food is temporarily stored in the 

 ordinary parenchymatous cells or in the sheaths of the con- 

 ducting tissue, and no special arrangements are made to 

 receive it. It is often of accidental occurrence deposited 

 suddenly and gradually or rapidly removed. Such deposi- 

 'tion and re-absorption form, indeed, one of the features of 

 the transporting mechanisms. 



We may now pass to the consideration of the forms in 

 which the different foods present themselves in these 

 reservoirs of storage. It is not surprising that we find 

 here a great deal of variety, even in any particular class 

 of food. The more prolonged the stay in the reservoir, 

 the more complex usually is the structure which the 

 nutritive substance assumes. 



We may deal in the first instance with the stores of 

 carbohydrates. We have already noticed that in the 

 great majority of cases these take the form of starch. In 

 the chloroplasts in the leaf-cells the starch grains are 



laid down as minute bodies, showing hardly 

 ,* any trace of structure and crowded together 



in the substance of the plastid till they are 



almost in contact with each other (fig. 107). 



The deposition is due to the protoplasm or 

 FIG. IDT. STABCH gtroma of the plastid, and does not depend 



GBAINS IN THE 



BODIES OF CHLO- in any way upon the colouring matter, the 



BOPLASTS. X250. .. , . . n . , .. 



presence of the latter influencing only the 

 other function of the chloroplast, the synthesis of sugar, 

 as we have already seen in a previous chapter. The 

 process is thus one of true secretion, and the deposition 

 of the starch originating at several centres in the plastid, 

 several granules are coincidently formed. The number, 

 however, is not constant. 



In the more permanent reservoirs of starch it usually 

 happens that the cells are so charged with the grains that 

 they appear to contain nothing else. Fig. 108 shows a 



