630 EEPOKT— 1887. 



organised body, which appears at once with all its characteristic properties, lilte 

 Minerva springing fully armed from the head of Jove. If we are to adhere to the 

 facts so far observed, we must conclude that the plant does not proceed as we 

 should do in the laboratory, beginning with the more simply constituted compounds 

 and advancing to the more complicated, but that the reverse process is the one 

 actually adopted, the supposed intermediate products being in fact the result 

 of retrogressive metamorphosis. This conclusion is, however, so much opposed to 

 ordinary chemical views that one cannot feel surprised at the constantly repeated 

 attempts to clear up the question. There can be no doubt indeed that much here 

 remains to be done and to be discovered. 



Intimately connected with this subject is that of chlorophyll, the green colouring 

 matter of leaves, which is always found wherever the process of assimilation in plants 

 is going on, and nowhere else, and is therefore doubtless an essential factor in the 

 process. What part it plays in this process is, in my opinion, still unknown. Its 

 action is probably in part chemical, in part physical, and this adds, it may be, to 

 the difficulty of understanding it. It is generally supposed that it is chlorophyll 

 which by its direct action on the carbonic acid and water with which it comes 

 into contact leads to the formation of organic matter with elimination of oxygen. 

 But this is, I think, a mere assumption — an error due, like many others, to a 

 mistaken use of terms. The chlorophyll of chemists is simply an organic coloiu'ing 

 matter, like alizarin or indigo, but being in the vegetable cell intimatety associated 

 with other matters, vegetable physiologists have attributed to the action of one, 

 and that the most obvious, constituent what is really due to the complex, perhaps 

 even to some quite other constituent of the complex. It is difficult to understand 

 how the chlorophyll of chemists can be endowed with the remarkable and excep- 

 tional properties attributed to it by physiologists ; it is a chemical entity, nothing 

 more. It may indeed be said that chlorophyll only acts as it is stated to do 

 when enclosed within the vegetable cell, but this merely amounts to saying that 

 its action is not purely chemical, but is controlled by the vitality of the cell, which, 

 I suppose, means the action of the protoplasm. If chlorophyll is the agent 

 whereby the decomposition of carbonic acid and water is efi'ected, how, it may be 

 asked, is the agent itself produced ? It does not come from without ; the plant 

 must be able to form it in the first instance. We are told by vegetable physiolo- 

 gists that the coniferre when raised in total darliness from seeds produce 

 chlorophyll. In light or in darkness I am convinced it is the same ; the plant 

 forms chlorophyll as a means to an end. What the end is we know ; it is the 

 assimilation of carbon and hydrogen to form organic matter. How does the 

 chlorophyll assist in attaining this end ? 



In propounding a new theory in reply to this question I venture to claim your 

 indulgence, such as has been accorded to some of my predecessors and others who 

 at these meetings of the British Association have been permitted to make state- 

 ments and use arguments of a novel or paradoxical character, which, if they effect 

 nothing else, at least afford a relief to the usual routine of scientific reasoning. 

 My experiments on chlorophyll have led me to infer that the constitution of 

 that body is much less simple than it is generally supposed to be. I do not 

 mean by this that chlorophyll is a mixture in the usual sense ; everyone who has 

 paid any attention to the subject knows that ordinary chlorophyll consists of 

 several colouring matters, some of which are yellow, not to mention fatty matters 

 which are unessential. What I mean to say is this, that the pure green substance, 

 the chlorophyll ;;ar excellence, does not belong to the same class of bodies as alizarin 

 or indigo, but contains three elements, each of which is essential to its constitu- 

 tion, one being a basic nitrogenous colouring matter, the second a metal or a 

 metallic oxide, the third an acid, the three together constituting green chlorophyll. 

 The basic colouring matter is a body of A'ery peculiar properties : it is the phyl- 

 locyanin of Freray : the metal may be iron or zinc, the acid I will suppose to be 

 carbonic acid. Now the plant having formed its colouring matter, the metallic 

 oxide being present in some form or other, and the carbonic acid being supplied 

 by the atmosphere, all the necessary conditions co-exist for the formation of 

 chlorophyll. The compound is an unstable one ; it easily parts with its carbonic 



