86 ON THE tL ANT-CELL. 



These changes require no further explanation than the decomposition 

 of water, the setting free of oxygen, and the separation of a smaller or 

 larger number of equivalents of water ; processes which we know con- 

 stantly present themselves in the decomposition of organic substances. 



One of the most important of the proximate principles is undoubtedly 

 dextrin. In all formative fluids, according to Mitscherlich and Mulder, 

 dextrin presents itself as the primary substance out of which all the 

 other assimilated matters are formed. In the various changes which 

 these matters undergo, the nitrogenous bodies seem to be the means of 

 effecting changes in the other bodies, whilst they themselves remain 

 unchanged. This phenomenon has got various names without any ex- 

 planation of it being given. Berzelius calls the process catalysis; 

 Mitscherlich, the contact of substances ; and Liebig the activity of ap- 

 prehending bodies. A number of such chemical facts are known ; thus 

 sulphuric acid, with heat, converts starch into dextrin and sugar and 

 alcohol into ether ; diastase changes starch into dextrin and sugar ; albu- 

 men, protein, &c. convert sugar into alcohol. Liebig's explanation of 

 the phenomenon as a communication of motion is founded on the notion 

 of the existence of ultimate atoms, and is otherwise untenable. Could 

 we explain better this phenomenon of one of the assimilated substances 

 facilitating the changes which go on in the others, we should have yet 

 to explain the changes which produced the nitrogenous substances. 



The most important of these changes appears to be the decomposition 

 of water, but we are at a loss to know whose calculations to adopt. 

 Almost all plants need for their growth the influence of light. Here 

 also we have a need of experiments to determine the action of the par- 

 ticular rays of the sun-light, as of the coloured, the calorific, and the 

 chemical. Only thus much is known from De Saussure's experiments : 

 that under the influence of light the carbonic acid of the air is fixed in 

 the cells, and combines also with hydrogen ; a process which will not go 

 on when light is excluded. That in this case light can be supplied 

 through hydrogen, appears to be proved by an interesting experiment of 

 Humboldt's.* 



34. In the formation of the assimilated matters, many sub- 

 stances become free, which, either through their natural affinities, 

 or the effect of contact, or predisposing affinity, form new com- 

 binations either amongst themselves, or with the non-assimilable 

 substances which may have been absorbed at the same time. All 

 substances formed in this way I call secretions (materia secreta) of 

 the cells. Some of these are universally present, as free oxygen, 

 or at least when they have vegetated under definite circumstances, 

 as the green colouring matter (chlorophyll). There are others 

 whose formation depends on especial circumstances, as conia, so- 

 lania, and the like. The chemical changes by which such sub- 

 stances are produced are for the most part concealed. Two points 

 remain to be noticed here: 1. That these secretions would be 

 frequently injurious to the cells were they not neutralised by in- 

 organic substances taken up- from without or by newly formed 

 organic matters: thus, oxalic acid combines with lime, and the 



* Floras Fribergensis Specimen, p. 180. [See also, on this subject, Hunt's Reports 

 in the Transactions of the British Association, 1847 ; and Draper on the Chemistry of 

 Plants TRANS.] 



