447 



CHAPTER VIII. 



RESERVE MATERIALS, AND THEIR DEPOSITION. 



We have seen that the amount of food constructed in the 

 mesophyU ceUs from the crude materials absorbed is often con- 

 siderably in excess of their immediate need. A certain amount 

 is, no doubt, used up at once by the living substance of each 

 cell, the assimilation of the food being the culmination of the 

 anabolic processes, as protoplasm is the most complex material 

 existing in the plant. The remainder is otherwise disposed of; 

 no doubt a certain quantity is always retained in the cell to 

 supply the needs of the protoplasm when actual construction is 

 intermitted. Evidence of the temporary accumulation of a local 

 reserve store is afforded by the occurrence of the starch grains 

 which the chloroplastid forms inside its own substance. A 

 great deal of the manufactured organic matter is, however, con- 

 stantly leaving the cells in which it is formed, and passing to 

 other parts of the plant. Even the temporary accumulation of 

 starch soon disappears ; if a plant, which has been vigorously 

 forming it in its chloroplastids during a summer's day, so that 

 at evening there is a great amount deposited there, is examined 

 early next morning, the leaves are found to be devoid of 

 starch. This is not brought about by its having been used 

 during the night, for if the path of removal is obliterated, as it 

 may be by severing the petiole, the leaf is found as full as on 

 the previous evening. 



Evidently, then, the surplus food manufactured in the 

 mesophyll is transferred from the seat of its elaboration to other 

 parts of the plant. To bring about this removal of the starch and 

 inferentially that of the proteids, a certain alteration of them 

 must be effected, the mechanism of which will be discussed 

 later. The starch is converted again into sugar, and possibly 

 some of the sugar may be further changed, though, no doubt, 

 sugar is the general form of the travelling carbohydrate. The 

 sugar which has not been transformed into starch may travel 

 as it is, or some modification of it, still a sugar, though of 

 another description, may be formed. That sugar is readil}^ re- 

 moved from the leaf may be inferred from several considerations. 

 We can find but little of it in the mesophyll of the leaf, though 

 we know it is constantly bemg produced. We do find it, however, 

 fairly easily in the bast of the fibro-vascular bundles of the leaf, 



