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 
ag 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 
