272 VEGETABLE PHYSIOLOGY 
The decomposition of the protoplasm in the formation 
of fat is not accompanied by much reconstruction, so that it 
is soon very greatly diminished in amount, while the fat, 
the product of the katabolic processes, increases. 
The appearance of fat in the two cases described seems 
to demand two different explanations. In the cells of the 
seeds and in the elaioplasts it is to be regarded as a 
storage of reserve materials. In the starved hyphe of 
the Fungus it appears to be due to the decomposition 
of protoplasm under conditions of grave disturbance of 
nutrition, if not of approaching death. In both cases, 
however, it is derived from the breaking down of the living 
substance, though the decomposition of the latter is due 
to such different causes in the two cases. 
One of the most important of the secretions of plants is 
the green colouring matter, chlorophyll, which we have 
already seen ig present in the form of a solution in the 
meshes of the chloroplasts. The formation of chlorophyll 
is a more specialised process than any of those which we 
have just been considering, and is dependent upon a 
variety of conditions. It probably involves not only the 
self-decomposition of the protoplasm, but also other 
processes which take place within the substance of the 
chloroplast. 
The gpecial conditions necessary for the formation of 
chlorophyll are—Ist, access of light; 2nd, a particular 
range of temperature; 8rd, the presence of a minute 
quantity of iron in the plant ; 4th, access of oxygen. There 
are a few exceptions to the rule that chlorophyll can be 
formed only in light ; the embryo in the seed of Huonymus 
europeus is green at the time the seed is ripe, though it is 
surrounded by a thick red protecting coat which is opaque, 
Seedlings of Pinus also are green when they are raised 
from seeds in light which is insufficiently strong to enable 
chlorophyll to be formed in seedlings of Dicotyledons grown 
side by side with them. A few other cases also are known. 
Tf an ordinary plant is cultivated from the seed in darkness, 
