88 ON THE PLANT-CELL. 



III. Of the Excretion of Substances from the Plant-Cell 



35. The endosmose whereby fluids are introduced into cells 

 necessitates an exosmose, consequently a small quantity of the 

 contents of the cell pass out. In this case there is no elective 

 power of the cell to be assumed, but all that is dissolved in the 

 cell, with the secreted matters, are exposed to a modification which, 

 as in endosmose, is regulated by the relation of the substances in 

 the inside to those on the outside of the cell. 



In this place we must speak of "the theory of excretion by the roots. 

 But, first, we must regard this process as it takes place in the individual 

 cell, for of such is the external part of the root composed. In this case 

 we find that where endosmose takes place there also exosmose must 

 exist ; and the denial of excretion by the one process whilst absorption is 

 admitted by the other, as is done by Meyen*, is highly unphilosophical. 

 This, however, is a different question from that as to whether the plant 

 has the power of rejecting those substances which are injurious to its 

 life. We cannot conceive of an endosmose without an exosmose ; but 

 there is no sense in which we can say that the plant has the power of 

 getting rid exclusively of that which is injurious to it, because the 

 assumption of injurious and non-injurious substances is altogether gra- 

 tuitous. 



The substances which are thrown out from the cell during exosmose 

 may become changed at the moment of their exit by contact with the 

 substances passing inwards, so that in many cases it is not improbable 

 that it is impossible to learn what is truly the product of exosmose. 

 With this case we have one remarkable analogy. During the process of 

 germination, starch, by virtue of the gluten (diastase), is converted into 

 dextrin, and this again into sugar, and the sugar is ultimately converted 

 into other substances, during which changes carbonic acid is fixed, and 

 acetic acid is set free (according to Becquerel) ; at the same time acetic 

 acid is never found free in germination. In fermentation the gluten 

 changes the starch into gum and sugar, and separates this into carbonic 

 acid and alcohol, which is easily (as, for instance, with soft platinum) con- 

 verted, in contact with oxygen, into acetic acid. The analogy is so strik- 

 ing in this case, that we cannot avoid supplying by hypothesis the failing 

 link, and supposing that alcohol also is formed during germination, and is 

 immediately converted by union with oxygen into acetic acid, which is 

 then separated. 



Two points demand attention here, which modify the process of exos- 

 mose considerably. The one is the decided affinity between substances 

 without the cell, and which are free to follow this attraction ; and, secondly, 

 the attraction which similar substances have for each other. In a fluid 

 in which two salts are dissolved, we may produce a crystallisation of either 

 one or the other, according as we throw into the solution a crystal of one 

 or the other. In this way a cell appears to give out especially the mat- 

 ters which are found surrounding it in greatest quantity. At least, in 

 this way we may explain the fact that the cells surrounding the gum- 

 passages secrete the largest quantity of gum. These points will be recon- 

 sidered when we speak of the root. 



* Physiologie, vol. ii. pp. 27. 524. 



