280 VEGETABLE PHYSIOLOGY 



storage of reserve materials. In the starved hyphse 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, how- 

 ever, 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 is 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 pro- 

 cesses which take place within the substance of the chloro- 

 plast. 



The special conditions necessary for the formation of 

 chlorophyll are 1st, access of light; 2nd, a particular 

 range of temperature ; 3rd, 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 Euonymus 

 europceus 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. 

 If an ordinary plant is cultivated from the seed in darkness, 

 the resulting seedling will not be green, but will have a 

 yellowish-white colour. When its tissues are examined 

 with a microscope, the plastids will be found in the cells, 

 but they will be tinged with a pale yellow pigment known 

 as etiolin. When the latter is exposed to light it will 

 rapidly become green, being in fact converted into chloro- 

 phyll. The etiolin is in the first instance secreted by the 



