Sept. 8, 1887] 



NATURE 



445 



instance. We are told by vegetable physiologists that the Coniferae 

 when raised in total darkness 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 ven- 

 ture 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 statements 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 

 par 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 constitution, one being a basic nitrogenous colour- 

 ing 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 very peculiar properties ; it 

 is the phyllocyanin of Fremy ; 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 acid, giving it up to the protoplasm 

 or whatever the agent may be that effects its actual decomposition 

 under the influence of light. The advantage of this arrangement 

 would consist in this, that the carbonic acid would be presented 

 in a more condensed state to the agent which effects its decom- 

 position than if it were merely contained in a watery solution, 

 but more loosely combined, and therefore more easily accessible 

 than if it were united to a strong base such as potash or lime. 

 The carbonic acid having been disposed of, the other two con- 

 stituents would be in a state to take up fresh quantities of car- 

 bonic acid, and so on. Chlorophyll would therefore act as a 

 carrier of carbonic acid in the plant, just as hemoglobin serves 

 to convey oxygen in the animal economy. Numerous objections 

 may of course be raised to the theory of which I give an outline ; 

 I only throw it out as a tentative explanation, showing that the 

 function of chlorophyll maybe, in' part at least, chemical, and 

 that we need not suppose it to be endowed with the marvellous 

 and exceptional powers usually ascribed to it. Other and more 

 probable explanations will doubtless suggest themselves when 

 this difficult subject has been more thoroughly worked out. 

 Eventually, too, it will be found, I imagine, that physical forces 

 as well as chemical affinities play a part in this as in every other 

 process of the vegetable economy. In the case of cholorophyll 

 there can be no doubt that the green colour and the peculiar be- 

 haviour towards light have something to do with its action, but 

 on this point it is not necessary for the chemist to pronounce 

 any opinion. I may take this oportunity of mentioning the 

 important experiments of Sachs and Pringsheim on the optical 

 properties of chlorophyll in their relation to assimilation in 

 plants, as they are probably not so well known to chemists as to 

 botanists. 



What I have said may serve to show that the very first steps 

 of the process whereby organic or organized matter is formed in 

 plants are hardly understood. We understand still less the 

 further steps leading to the production of the more complex 

 vegetable bodies — acids, alkaloids, fatty matters. Granted that 

 we were able to trace the formation in the plant of a compound 

 of simple constitution, such as oxalic or formic acid, how far 

 should we still be from understanding the building up of such 

 compounds as starch, albumen, or morphia ? The syntheses so 

 successfully and ingeniously carried out in our laboratories do not 

 here assist us in the least. We know the steps by which alizarin 

 is artificially produced from anthracene ; but can anyone for an 

 instant suppose that the plant commences in the same way with 

 anthracene, converting this into anthraquinone, and having acted 

 on the latter first with acid, then with alkali, arrives at last at 

 alizarin ? Indeed the plant never contains ready-formed alizarin 



at all. What we observe from the commencement is a glucoside 

 a compound of alizarin and glucose, which, so far as we see, is 

 not gradually built up, but springs into existence at once. When - 

 we think of the complicated process by which indigo is produced 

 in the laboratory with the various substances and appliances re- 

 quired, and then see how in the minutest seed-leaves of a plant 

 like woad a still more complex substance, indican, is found ready- 

 formed, we stand confounded at the simplicity of the apparatus 

 employed by the plant, and are obliged to confess that we have 

 no conception of the means whereby the end is attained. The 

 same difficulties occur in other cases, and it will therefore pro- 

 bably be conceded that the synthetic process carried on in 

 plants, from the first step to the last, are not in the least 

 understood. 



It might be supposed that after all the labour and attention be- 

 stowed on the inorganic constituents of plants we should know 

 something of the part played by these constituents in the processes 

 of assimilation and nutrition, but here the obscurity is as great 

 as elsewhere. We know by experiment that certain inorganic 

 matters — potash, lime, magnesia, iron oxide, phosphoric acid — 

 are essential to the growth of plants ; but of their mode of action, 

 or of the reason why certain plants require potash salts, others 

 lime, and so on, we know nothing. Phosphoric acid is no 

 doubt an essential constituent of the protoplasm of the 

 plant ; but why cellulose, of which the various organs chiefly 

 consist, should require mineral matters, which do not enter 

 into its composition, for its formation and building up is still a 

 mystery. 



The department of chemistry which relates to the decomposi- 

 tion of organic and organized matters presents problems almost 

 as difficult of solution as those relating to their formation and 

 building up ; that is to say, the phenomena observed do not 

 apparently obey the same laws as those prevailing in the in- 

 organic world. When I began my chemical studies the difference 

 in this respect between mineral and organic compounds was less 

 clearly seen than at present. The conversion of alcohol into 

 acetic acid, the putrefaction of animal and vegetable matter were 

 thought to be simply due to oxidation ; they were phenomena, it 

 was supposed, exactly similar to the rusting of iron, the tarnish- 

 ing of metals, the fading of colours. That a third body was 

 required to initiate and continue the process of decomposition, 

 that organic matter in contact with purified air would remain un- 

 changed for any length of time — was not known nor suspected. 

 I am not quite sure whether spantaneous decomposition — i.e. the 

 splitting up of a complex body without the intervention of an 

 external agent — might not at that time have been considered 

 possible. In order to explain the phenomena of fermentation, 

 the decomposition of sugar into alcohol and carbonic acid, for 

 instance, we had only the theory of contact— devised by Ber- 

 zelius and Mitscherlich, the latter of whom used to expatiate on 

 the subject at great length in his lectures. When this ghost of 

 a theory was laid by Liebig, who suggested an intelligible ex- 

 planation of the phenomena in accordance with the facts then 

 known, it was felt to be quite a relief, as affording a resting- 

 place — if only a temporary one — for the mind. The brilliant 

 researches of Pasteur, which have thrown so much light on the 

 action of the insoluble organized ferments, I need only refer to, 

 as they are so widely known, even outside scientific circles ; and 

 since also investigations such as his cannot be discussed without 

 some reference to biological questions, which cannot be entered 

 on now. I will confine myself therefore to a few remarks on the 

 unorganized or soluble ferments, one of which I had occasion to 

 examine when engaged in investigating madder and its colouring 

 matters. These ferments, the type of which is diastase — a sub- 

 stance found accompanying starch in the seeds of plants — are 

 soluble in water, perfectly neutral, devoid of all definite form, 

 and though apparently inert, able when acting within the sphere 

 in which Nature has placed them to cause changes and decompo- 

 sition of the most profound character. Their action excludes 

 everything in the shape of vitality, and yet it is as mysterious and 

 unaccountable as anything that the vitality of the organized 

 ferments is capable of effecting. Indeed, in vegetable, and 

 especially animal, organisms they seem expressly intended for the 

 attainment of certain ends necessary for the well-being, or even 

 the existence, of the organism, insomuch that it has been sup- 

 posed, with some show of reason, that it is to bodies of this class 

 existing within the cells of organized ferments, but not separable 

 by any means at our disposal, that the changes produced by the 

 latter are really due. 



A great deal of attention has been paid to the product and 



