From this we infer that the change of colour in the leaves during autumn is simply dependent on a chemical alteration of the green colouring-matter by light.
Mulder, after showing from a large number of facts that wax along with a green colouring-matter exists in leaves and unripe fruits,— wax, with a red colouring-matter, in the red leaves which appear in autumn, and in the red fruits,—and wax with a yellow colouring matter, in the yellow leaves of autumn, and in the yellow fruits—gives a lengthened chemical description of Chlorophyle, for an account of which we must refer to the original work.
From Mulder's experiments, and those previously instituted by Berzelius, it appears that the green colouring-matter of the leaves is readily decomposed into three different substances, one yellow, another blue, and a third black ; and that according to the proportion of these three mixed with the green, a different kind of green must be produced. Hence the difference in the green colour of different leaves depends not only on the presence of more or less Chlorophyle, but also on the different mutual proportions of these three colouring-matters.
The quantity of pure C. Chlorophyle contained in the leaves is exceedingly small ; according to Berzelius it is not more than the amount of pigment in dyed cotton.
If a tincture of pure Chlorophyle be exposed to the action of the sun the green colour becomes in a few hours converted into a yellow. When a solution of pure Chlorophyle in ether and hydrochloric acid was kept for five months in a bottle half full, the green was entirely changed into a yellow. From these experiments we learn, first, that the green colouring-matter is decomposed and a yellow one left, both with and independently of the influence of light ; and secondly, that in all probability a similar decomposition (accompanied by a repro duction) of green colouring-matter and green leaves is constantly going on under the influence of light. Mulder conceives that the continual decomposition of the green colouring-matter may be in part the origin of the wax, since the quantity of the latter is found to have increased when the same leaves are analysed later in summer. In consequence of the continuance of this reproduction, the leaves remain green ; when it stops, the leaves become yellow as in autumn.
It is worthy of notice that decomposed Chlorophyle yields a blue colouring-matter ; it is this which is no doubt present in the skins of many fruits, as for instance those of the grape ; the exact nature of the chemical change is not clearly understood.
It is very obvious that the influence of light will convert starch into Chlorophyle. Every part of an amylaceoua root becomes green on exposure to light. The parts of plants which become green (all without exception) contain starch ; and in autumn as this green colour decreases the starch also decreases, and finally cannot be detected by the iodine-test. Hence starch ceases to form B. Chloro phyle under the influence of light, the B. Chlorophyle being a complex substance consisting chiefly of wax. The change of starch into B. Chlorophyle may be explained in much the same manner as its conversion into fat. •The wax contained in the leaves and other parts ofplants may be chemically represented by the formula C„ H„ 0. Now if no other products be simultaneously produced we may suppose the wax obtained from the starch in the following manner :— 5 equiv. of starch . . . . 0„ With 10 of water . . . . nio Make . . . . . . And 4 of wax . . . . . O.
Leaving to be given off . . . 0„ That is to say, 5 equivalents of starch yield 4 equivalents of wax, and give off 56 equivalents of oxygen. This fully explains the phenomenon why plants, while becoming green, evolve oxygen, and further indicates the use of starch in the leaves.
Mulder has, as far as we are aware, made only one ultimate analysis of pure C. Chlorophyle—that from poplar leaves; from this analysis he calculated the formula C„ N 0„.
"Properly speaking," Mulder observes, "the green colouring-matter in the leaves has nothing to do with the evolution of oxygen ; on the contrary, the colourless C. Chlorophyle, which seems to be every where present, becomes green by the absorption of oxygen. Ilence a small portion of the oxygen produced from the conversion of starch into wax is employed for this purpose, and is not mixed with the atmosphere. But this is just the reason why C. Chlorophyle is not formed by the exhalation of oxygen ; it only becomes green instead of white, as it previously was. This can only happen when there is an abundance of oxygen, and this we have seen to be the case when starch is converted into wax. We may therefore assume as proved that white Chlorophyle diffused throughout the whole plant, will become green in proportion as starch is converted into wax ; because it is enabled, in such proportion, to take up oxygen—to become oxidised, just like white indigo.