254 LECTURE XV. 



the water. These considerations receive full experimental confirmation by the 

 fact that even a salt (lithium nitrate) not belonging to the ordinary nutritive 

 materials answers the condition required by me, and ascends in the wood walls with 

 the transpiration current. If, as already mentioned, a weak solution of lithium 

 is poured on the roots of a land-plant, the latter continues to transpire, and after 

 one or two hours the lithium is already found in the leaves situated fifty to two hun- 

 dred centimetres above the roots. If the leaves be cut off and immersed, with the 

 exclusion of the stalk, in pure water, lithium may be recognised in the water after a 

 few hours, having come out of the epidermis by diffusion. It is by no means to be 

 supposed that this rapid movement of the lithium in the plant has taken place 

 by endosmose from cell to cell : it has evidently ascended with the transpiration 

 current in the wood cell-walls, and in the same manner the nutritive salts men- 

 tioned are likewise sent into the leaves. What has been said, however, does 

 not exclude the fact that endosmotic movements of salt-molecules take place where 

 necessary even in vroody plants : when the cells of the living cortex, or the 

 assimilating cells of the transpiring leaves decompose phosphate of lime or sulphate 

 of magnesia and the like, in their interior, molecules of these salts, ascending with 

 the water in the wood walls, will be able to enter by endosmose into the interior 

 of the cells named. We shall obtain a correct idea on the whole if we suppose 

 that the water ascending in the wood extends in the first place into all the cell-walls, 

 even of the parenchymatous tissues of the cortex and leaves, and takes with it the salt 

 particles dissolved in it; thus the cell-contents become surrounded on all sides by 

 a layer of water, which is between the molecules (micellae) of the cell-membrane. 

 The cell-wall framework of the entire plant contains at the same time the store of 

 water and salts for all the cells. If one of the latter requires water, it finds it in the 

 first place in its wall ; if, on the other hand, it requires a number of salt molecules, 

 these also are contained in the water of the cell-wall, and can enter at once into the 

 interior. Everything is of course far more simple in submerged water-plants, which 

 can take up water and the salts dissolved in it at every point of their surface ; with 

 the slender structure and the numerous large intercellular spaces of the water-plants, 

 the path which the materials in the cell-walls have to describe is rarely greater than 

 I mm., and even the slowest movement suffices to satisfy the requirement. 



The question has for a long time been ventilated whether the leaves are not able, 

 at least under certain circumstances — e. g. during continuous rain, or when they 

 are covered with dew — to absorb water and substances dissolved in it. Numerous 

 researches directed to this end have yielded no satisfactory result w^hatever; 

 but it is by no means difficult to apprehend the essential points without that. If 

 the whole plant is filled and turgid with water, and particularly the leaves, it is 

 not clear how the latter are to take up more water from without; if, on the other 

 hand, they are drooping, and not quite filled with water, it depends upon the consti- 

 tution of the cuticle whether and how rapidly they are able to absorb water. 

 The fact is, that drooping shoots, especially of wood-plants, when inverted and 

 plunged in water, so that the cut end of the stem projects, often remain drooping 

 for days in spite of the surrounding water, and thus absorb none ; and it is 

 obvious that this is the case in general so long as a layer of air clings to the 

 surface and prevents the wetting of the leaves. If the leaves become actually wetted. 



