70 ANATOMY AND PHYSlOLOGr OF 



witlioutj in the same proportion as it is evaporated from the 

 leaves. A proof that the evaporation of the leaf actually in- 

 creases the absorption, is again furnished by the experiments of 

 Hales, according to which the quantity of water that a shoot ab- 

 sorbs is in direct proportion to the number of its leaves, and the 

 quantity of water absorbed sinks to one half when half the leaves 

 are cut off the shoot ; the experience also speaks in favour of it, 

 that during winter the root of a plant standing in the open air, 

 for instance, a vine or a hazel bush, begins to absorb if one of its 

 shoots is introduced into a hot-house, and the unfolding of its 

 leaves caused by the action of heat. Liebig (" Researches on the 

 Movement of the Juices in theAnimctl Organism/' Untersiick 

 hher Safibewegung^ cfcc, 68) has also shewn the influence wliieh 

 is exerted by evaporation at one point even in apparatus artifi- 

 cially contrived, and which, when it is assisted by the pressure of 

 the atmosphere, is capable of causing the fluid to flow through 

 the membrane against the laws of endosmose. In plants, this in- 

 fluence not only suffices to increase the absorption, and cause it 

 to commence under circumstances in which it did not otherwise 

 occur, but is even powerful enough in plants which have been 

 poisoned, to carry the poisoned fluid in great abundance upward 

 through the already dead lower part of the plant. 



Ohserv, I hei^e refer to experiments "wMch I have made both on Firs 

 and Dicotyledonous trees in regard to the absorption of pyrohgnite of 

 iron, the diffusion of which through the plant is readily perceived by the 

 dax'k colour. Young trees sawn off and placed with the cut surface in 

 the fluid, became filled with it, when they had white wood like the Bnch, 

 in all their parts from below upwards, and continued to convey the fluid 

 upwards in this way through the lower part of the stem, after all their 

 ceils were saturated with it and their cell-membranes were infiltrated 

 with it through their entire thickness : under which circumstances we can 

 certainly not imagine them to have retained a remnant of vitahty. 



a. Diffusion of the Sap in the Plant 



The mode in which the fluid taken up by the cells situated at 

 the surface of the rind of the root becomes diffused in the plant, 

 is a subject which lies in far deeper obscurity than the absorption 

 of the cells in contact with watery nutriment In the lower 

 'plants which are composed of single cells, as Protococcus, there 

 can be no movement of the sap, and even in such as are com- 

 posed of simple rows of cells, like the Confervas, each cell seems 

 to elaborate independently the nutriment it takes up. In the 

 Lichens we have already an indication in the different structure, 

 and especially in the green colour of the internal layer, that here, 

 where indeed no distinct organs exist, the different layers of the 

 thalius are endowed with unlike physiological functions ; we can 

 scarcely imagine this without an exchange of the juices of the 



