METABOLISM 271 



plastid is present is brought about in the same way, though 

 it is not so long continued and the formation of successive 

 laminae 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 living under 

 such conditions as prevent their being properly nourished. 

 The protoplasm in the hyphaa 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 

 staining. 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 elai'oplasts, to which reference has been made, 

 behave similarly, their substance diminishing at the same 

 time that the fat or oil makes its appearance. 



