TRANSACTIONS OF SECTION B. 631 



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 arrange- 

 ment would consist in this, that the carbonic acid would be presented in a more 

 condensed state to the agent which effects its decomposition 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 constituents would be in 

 a state to take up fresh quantities of carbonic acid and so on. Chlorophyll would 

 therefore act as a carrier of carbonic acid in the plant, just as haemoglobin serves 

 to convey oxygen in the animal economy. Numerous objections may of course be 

 raised to the theory of which I here give an outline ; I only throw it out as a tenta- 

 tive explanation, showing that the function of chlorophyll may be, 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 explana- 

 tions 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 chlorophyll there can be no 

 doubt that the green colour and the peculiar behaviour towards light have some- 

 thing to do with its action, but on this point it is not necessary for the chemist to 

 pronounce any opinion. I may take this opportunity of mentioning the impo^'taut 

 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 organised 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. AVe 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 com- 

 mencement 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 required, and then see how in the minutest seed-leaves of 

 a plant like woad a still more complex substauce, iudican, 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 probably 

 be conceded that the synthetic processes 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 bestowed 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 tlie 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 wbj- cellulose, 

 of which the various organs chiefly consist, sliould 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 decomposition of organic and 



