Chlorophyll

Chlorophyll, the green colouring matter of the leaves and young stems of plants, a substance of great physiological importance. In the lowest algae, such as the Protococcaceae and the "gonidia" of lichens, it exists diffused throughout the protoplasm. In Spirogyra it colours spiral bands of the protoplasm only; in Zygnema, stellate masses; and in other Conjugatae, portions of other forms; but in higher plants (exclusive of all fungi and of a few parasites and saprophytes among flowering-plants in which it is absent) it colours certain differentiated granules of the protoplasm known as chloroplastids. These are formed independently of the action of light; and in the leaves of ferns and the cotyledons of gymnosperms, if the temperature be high enough, they become green in the absence of light. Normally, however, i.e. among angiosperms, they are either colourless (in the absence of iron) or coloured yellow by etiolin (in its presence) until exposed to light. For the development of their green colour they require (1) light of moderate intensity, especially the yellow end of the spectrum, (2) heat, approximating to an optimum temperature between 25° and 30° C, and (3) iron. The chemical composition of chlorophyll is not certainly ascertained, but it contains carbon, hydrogen, nitrogen, oxygen, and perhaps phosphorus and iron. It is insoluble in water, but in alcohol, ether, or benzol yields a bright green solution having a brilliant red fluorescence (q.v.). It gives an absorption spectrum with seven bands, four narrow between the red and the green and three wide ones at the blue end. Many observers consider this spectrum to indicate two distinct colouring-matters, one yellow and the other blue-green. In the absence of iron the green colour is not produced, but it is quickly assumed on the plant being watered with a solution of iron. Chlorophyll-granules multiply by division, and seem interchangeable with the colourless leucoplastids formed in the dark, and with the variously-coloured chromoplastids of ripening fruits, which contain various crystalline colouring substances. Chloroplastids are few in number in the epidermis, but occur especially in the "palisade-tissue," or hypoderm of the leaf, and in the phelloderm, a layer of the primary cortex of the stem. The function of chlorophyll is to bring about the decomposition of the carbon-dioxide of the air absorbed by the plant, liberating an equal bulk of oxygen to that in the absorbed carbon-dioxide. This process, known to vegetable physiologists as "assimilation," is also termed the chlorophyllian process, and, as the source of practically all the carbon in the organic world, is a nutritive process (not respiratory) of primary importance. This process only goes on in the presence of light, preferably the less refrangible rays. It is believed to consist in the formation of a glucose from the carbon-dioxide and the water taken in by the roots, the first visible result of it being the appearance in the chloroplastids of starch-grains which may originate from the glucose or grape-sugar by dehydration. It is by the oxygen liberated in this process that green plants in daylight purify the air. Some protozoa, the freshwater sponge, Hydra viridis, and some planarian worms form chlorophyll-corpuscles in their protoplasm, and are thus capable of feeding upon atmospheric carbon-dioxide, or, as it has been called, of aerobiosis, holophytic or autophytic nutrition, as if they were plants.