Chlorophyll, as its name implies, is leaf-green, the ordinary colouring matter of vegetation. Microscopic examination of plant tissues shows it some- times to tinge the whole protoplasm of the cell (though never the cell sap). Much more generally, however, it is collected into definite bodies, the so-called chlorophyll granules, or less frequently, as in some of the lower algae, arranged in starlike masses (Zygnema) or spiral bands (Spirogyra). A brief immersion of the specimen in alcohol suffices to dissolve out the chlorophyll, and leave the form of the chlorophyll bodies unaltered; and the solution may then be conveniently studied in a fresh state, as it gradually fades on keeping. It is of a deep rich green by transmitted, but of characteristic deep red colour by reflected light; in a word, is highly fluorescent. The absorption-spectrum is quite characteristic; yet the complex nature of the substance is strongly argued for on the ground of the following experiment. Shake up the alcoholic solution with benzol, and when the two fluids have had time to separate, we find the green to have disappeared. The lighter alcohol above is now of a strongly yellow hue, while the subjacent benzol is of a distinctly bluish green. To these colouring matters the names Xanthophyll and Cyanophyll are commonly given. While their separate spectra are quite characteristic, their combined spectrum seems to restore that of chlorophyll. The latter substance resembles the blue-green colouring matter of some of the lowest algae (Oscillatoria, &c.), while the yellow substance recalls that of many buds and flowers. Others, however, maintain that these substances are really the result of a destructive decomposition of true chlorophyll; while Gautier has even prepared green needle-like crystals which he regards as chlorophyll in a pure state. The chemical composition of chlorophyll is also a matter of considerable difficulty, both on account of its own easy decomposability, and its being necessarily accompanied by other extractive substances; thus the question of the presence or absence of iron has been much debated, a matter which seems to indicate variability of composition. This is further borne out by the labours of spectroscopists. Thus in addition to the familiar well-defined analogous colouring matters which characterise respectively the blue-green algae, the brown and the red (see SEAWEEDES), researches such as those of Sorby and others seem to indicate a number of varieties too numerous even for enumeration. This varietal range, however, may best be approached after considering the conditions in which chlorophyll is formed. Of the chemical factors little can be said save that the presence of iron in the soil seems to be quite indispensable; the physical conditions are, however, simpler.
A sufficient temperature is necessary to the formation of chlorophyll; for seedlings from 4° to 8° C. being a minimum, while the process is said to take place most rapidly, on the average, at about 35° C. The most important condition is, however, the presence of light; with rare exceptions, plants cannot form any chlorophyll in darkness. The blanching of celery or leeks is a familiar instance of this, or conversely, the green coating acquired by a potato-tuber where it happens to have been denuded of its covering of earth. Shoots formed in darkness form colouring matter indeed, but only of a pale-yellow hue, the so-called etiolin; after a very brief exposure to light, especially if direct sunshine, chlorophyll appears, apparently by direct transformation of the former. The other colouring matters of plants are sometimes in protoplasm granules (chromoplasts), but more frequently also in solution in the sap; and these frequently are present in such abundance, especially in the epidermis, &c., as to mask the chlorophyll altogether, as happens especially in so many of our hothouse plants with decorative leaves. The general tendency of research is to indicate that all the phenomena of plant colour seem intimately connected with what we may term the general life-history of chlorophyll. Thus the yellows and reds of young shoots and leaves in spring may be interpreted as having some relation to the development of chlorophyll, if not absolutely stages of the process; while the same suggestion arises with regard to the colouring matters of flowers, which are similarly to be regarded as imperfectly vegetative shoots, although in this case through the onset of the reproductive function, instead of merely by reason of immaturity. Finally also with regard to the autumnal hues, in which the chlorophyll pigment seems to be disintegrating through colour-stages analogous to those of its evolution in spring, or of its arrestment in the flower. In favour of such a view evidence is forthcoming from either end of the vegetable kingdom. Thus Cienkowski has shown that in the case of certain unicellular algae, which like the common Protococcus of rain-water, possess red pigment as well as green when passing into the resting stage, the quantity of red may be increased in proportion as autumnal conditions are reproduced by artificially lowering the temperature, and vice versa. The same experiment may be made with the common Sempervivums, Sedums, &c., whose leaves become more or less red in autumn or winter; those of Thuja (Arbor Vita) similarly become brownish, but recover themselves in spring. Among almost all cultivated plants, variegated varieties tend to arise, that is to say, we have certain cell-areas of the leaf-parenchyma destitute of chlorophyll. The conditions of this are obscure (see VARIEGATED PLANTS); but it is noteworthy that this loss of chlorophyll may also take place in lower plants, so that there is considerable ground for regarding at least many apparent fungi as simply algae which have degenerated in this respect through parasitism. Phanerogamous parasites like Toothwort (q.v.) or Dodder (q.v.) similarly become almost completely blanched.
Chlorophyll apparently identical with that of green parts of plants, can be experimentally demonstrated in the tissues of certain animals; and although this has in many cases been shown to be due to the presence of symbiotic algae (see SYMBIOSIS), there remain cases—e.g. Hydra viridis, &c., and at the very least Paramacium viride, in which we have undeniably intrinsic chlorophyll, and this of truly vegetable function.
The development of our knowledge with respect to the functions of chlorophyll is still far from complete, and may in any case be more conveniently treated under LEAF and VEGETABLE PHYSIOLOGY (q.v.); suffice it therefore to state here the elementary and essential fact that its presence is in every case constantly associated with the process of assimilation, or elaboration of new products by help of the energy of sunlight, upon which the continued nutrition of the green plant depends; and of which the formation of starch with decomposition of carbonic anhydride and disengagement of oxygen are the most obvious chemical results. A fuller account of the state of knowledge and opinion upon this portion of the subject will be found in the articles referred to.