METABOLISM 271 
plastid is present is brought about in the same way, though 
it is not so long continued and the formation of successive 
lamine does not take place. 
The method by which aleurone grains arise in the meshes 
of the cytoplasm is of a precisely similar character. 
The formation of fat is due to the same kind of behaviour 
on the part of the protoplasm. It can be observed most 
easily in the case of certain fungi when they are livmg under 
such conditions as prevent their bemg properly nourished. 
The protoplasm in the hyphe diminishes in quantity, the 
vacuolation becomes considerable, and their cavities are 
found to contain large drops of oil. In the cells of seeds 
such as those of the castor-oil plant, in which large quantities 
of oil are stored as reserve materials, the deposition of fat 
can be studied. Sections of the cells should be stained 
with osmic acid, which colours fatty substances brown or 
black according to the quantity of them which is present. 
When fat is beginning to be formed the substance of the 
protoplasm becomes faintly granular, but the granules are 
more or less transparent and cannot be seen without 
staiing. On the application of osmic acid they display 
their fatty character by becoming brown. In cells which 
are a little older the granularity is more marked, as the 
separate granules have increased in size and many have 
run together, forming small droplets. The staining is 
darker in these cells, the larger granules becoming quite 
black. In still older cells the whole substance becomes 
intensely black and the protoplasm can be seen to be 
saturated with the oil. If the latter is removed by treat- 
ment with ether, the living substance is found to have 
diminished in amount. There has been a formation of 
fat by the protoplasm, and the latter has evidently pro- 
duced it by a decomposition of its own substance, for it has 
become reduced in bulk. 
The elaioplasts, to which reference has been made, 
behave similarly, their substance diminishing at the same 
time that the fat or oil makes its appearance. 
