16 EVAPORATION FROM LEAVES. 



the result of the elasticity of the stem released of the pressure of the 

 portion cut off; and I claim only to have shown, as yet, how water is 

 taken up by trees from the soil, and that endosmose may supply an 

 explanation of the rising of the sap to the highest twigs, and to 

 the extreme ramifications of the most distended boughs. 



Exosmose and endosmose are found to take place freely through 

 two tissues, the surfaces of which are contiguous to each other, the 

 double tissue acting much as a homogeneous tissue, thicker than 

 either of them would do ; and in the knowledge of this — ^the know- 

 ledge of the fact that sap does rise from the rootlet to the stem, the 

 knowledge that capillary attraction cannot explain all the phenomena 

 of the case, the knowledge of the fact that no other vis a tergo has 

 been discovered, and the fact that by endosmose and exosmose aU 

 the phenomena of the case are explicable, — it seems not unreasonable' 

 to conclude that thus it is that the rise of the sap may be effected. 



At this stage of our consideration of these phenomena the determi- 

 nation of this is of importance, mainly, in satisfying us that we have 

 not lost sight of the moisture in tracing, or endeavouring to trace, its 

 course from the rootlets to the leaves. It wiU subsequently again 

 demand our attention. It is the absorption of moisture from the soil 

 and the emission of a large portion of this into the atmosphere with 

 which we have chiefly to do in studying the meteorological effects of 

 forests on the humidity of a climate. It is by the spongioles that it 

 is absorbed, and by the leaves that it is emitted. The transmission 

 of that moisture from the spongioles to the root is, in so far as our 

 present study is concerned, not unimportant, but its importance is 

 only secondary in view of the absorption and emission ; and to the 

 means whereby this passage of moisture from plants by their leaves 

 is effected would I next direct attention. 



The passage of moisture from plants by their leaves is effected 

 by means of what may be described, in mechanical phrase, as a 

 beautifully simple self- regulating ventilating-evaporating apparatus. 

 The operation of this I would illustrate thus : In the oxalis as in the 

 clover we have a leaf composed of three leaflets. In an oxalis 

 brought from Guatemala the whole leaf is almost transparent, and it 

 is said that six sets of vessels can be distinctly traced — three pairs, 

 one pair proceeding from each leaflet ; and it is conjectured that in 

 other plants a similar arrangement exists ; but in most plants it is 

 difficult to separate and distinguish the different sets of vessels. 

 These vessels, with less or more of cellular matter, constitute the 



