SALTS IN THE ASCENDING WATER. 2^^ 



menon in question is far from being explained by that. However, here again we 

 recognise the significance of transpiration for the nutrition of the plant ; since it 

 is clear that water strongly charged with nutritive salts requires to be supplied to 

 the green organs of assimilation in less quantity than if it contained but little of 

 these substances. 



In concluding the preceding remarks, and as an introduction to what follows, 

 I may now briefly advert to the question whether the water ascending in the wood 

 cell-walls, which evaporates in the leaves, does really carry with it the nutritive 

 matters — the sulphates and phosphates of the alkalis and alkaline earths — dissolved 

 in it, and transport them into the assimilating cells of the leaves. This question 

 is justified, because for a long time the transport of these materials was supposed to 

 take place in another manner. It was assumed that the living cells of the root- 

 cortex, abounding in sap and containing protoplasm, take up these substances from the 

 water of the soil surrounding the roots, according to the laws of endosmosis ; and 

 that likewise, proceeding from cell to cell, from the roots right up into the leaves, 

 endosmotic processes transport the salts into the organs of assimilation, without the 

 co-operation of a continuous current of water. What is here considered is a 

 movement of the molecules of salt dissolved in the water, independent of a current 

 of the water itself. If we suppose an isolated cell, lying in water which contains 

 saltpetre, for example, while none is present in the cell, then a certain quantity 

 of the molecules of this salt will, in spite of the cell-membrane and protoplasm, 

 pass into the cell; and if this stands in connection with a series of other cells, 

 these also will gradually take up the molecules of salt. This would take place to an 

 extent so much the greater if the salt-molecules were decomposed in the interior 

 of the distant cells, and by this means an endosmotic equilibrium prevented from 

 re-establishing itself. This form of the movement of material certainly occurs in 

 submerged water-plants, in root-parasites growing underground, and here and there 

 also in the tissues of the land-plants ; but considerations of the most various kinds 

 led me years ago to the view that in transpiring land-plants, and especially in shrubs 

 and trees, the vital conditions of the plants could not be correlated in this manner. 

 The endosmotic movements of the salt-molecules referred to, are so slow that they 

 could not possibly supply the foliage of a tree with the large quantities of nutritive 

 salts which there co-operate in assimilation, and are actually shown to be present 

 in the leaves by analysis. When I had fully substantiated the ideas, already pointed 

 out incidentally by Unger, that the transpiration current of land-plants moves not 

 in the cavities but in the substance of the cell-walls of the wood, the question 

 arose whether the water ascending in the wood cell-walls is not perhaps pure 

 water, or whether it carries with it the soluble salts contained in the soil, and 

 conveys them into the leaves. The probability of the latter assumption was in- 

 creased by the consideration that we have to deal with only extremely dilute 

 solutions — water in which the dissolved molecules of the salts concerned are 

 contained only in extremely small quantity; perhaps i : 2000 or even less. 

 Further, it was to be observed that even in the ordinary endosmotic movement 

 salt molecules go through the substance of cell-walls and protoplasm. I was thus 

 able to conclude, further, that also the few molecules contained in the ascending 

 current are carried forward in the substance of the wood walls, simultaneously with 



