222 VEGETABLE PHYSIOLOGY 



storage reservoirs must be materially different from those 

 in which they travel. We have already seen that one of 

 the conditions of the continuous formation of any one of 

 them is the removal of it from the seat of its construction 

 as soon as its amount exceeds a certain limit. If this is 

 not secured, the sap of the constructing cells soon contains 

 as much of the body in question as it will hold, and then no 

 more is made. The removal is dependent upon the depo- 

 sition of the substance from the sap in some way which 

 lessens the concentration of its solution in the latter. We 

 find accordingly that the bulkier reserve materials are very 

 frequently deposited in solid forms, sometimes amorphous, 

 sometimes granular, and sometimes crystalline. Other 

 cases are known as well, in which they remain in solution 

 in the sap of particular cells, but in these cases they are 

 retained in such cells through the difficulty or impossibility 

 of diffusing through the cytoplasm. They are generally 

 formed inside these cells from some particular constituent 

 of the travelling stream, much as are those which become 

 insoluble, and once formed, they are unable to pass out of 

 the vacuole. 



In considering the forms which the various reserve 

 food materials assume in the reservoirs they occupy, we must 

 then remember that they are not a simple accumulation of 

 food pabulum in the form in which it is of immediate use. 

 Granted that the plant in the first instance forms certain 

 materials on which its living substance draws at the place 

 where it is originally constructed, then, so long as the 

 immediate needs are in excess of the amount prepared, 

 there is no alteration in such materials ; they are at once 

 utilised by the living substance in the processes of nutrition 

 and growth. But as soon as the supply exceeds the imme- 

 diate demand, the surplus is not simply retained unchanged 

 in the cell, nor does it overflow unchanged to contiguous cells 

 where demand exceeds supply, or where provision is made 

 for storage. The storage forms, whether retained in the 

 cells of construction or transferred to others, are different 



