176 



LECTURES ON 



Fig. 14. 



the soil. In the light of the facts we have set forth, this view is not 

 for a moment admissible. 



In seeking the means by which the dissolved matters of the soil 

 find entrance into the plant, we must have recourse to the same 

 agency which accounts for the imbibition of its gaseous food. Differ- 

 ent liquids or solutions of different solids in the same liquid, if capable 

 of mixture at all, exhibit the osmotic or diffusive tendency, which 

 has been considered in case of gases. 



If a tall vessel be partly filled with salt and then completely with 

 water, the salt as it dissolves forms a solution much heavier than 

 pure water, which therefore tends to remain unmixed at the bottom 

 of the vessel. In fact it is easy to add the water so carefully that at 

 first no salt shall be perceptible by taste or otherwise near the surface. 

 In time, however, although every possible means of mechanical 

 admixture be perfectly avoided, the salt will diffuse into the pure 

 water until every portion of the liquid be uniform in composition. 



Diffusion will take place equally well through porous membranes, 

 provided they are capable of being wetted by (have surface attraction 

 for) at least one of the liquids. 



The apparatus shown in figure 14 is one commonly employed to 

 illustrate the fact of liquid diffusion. The tube a 

 is fastened to the neck of a bladder filled with brine, 

 solution of sugar, or other dense liquid, and the 

 latter is immersed in the water of the large vessel. 

 Immediately water passes inwardly to the brine 

 {endosmose) and salt passes outwardly to the water 

 (exosmose.) The endosmose being more rapid than 

 the exosmose, the brine shortly rises in the tube to 

 a considerable height. 



The rapidity and even the direction of the osmose 

 is greatly dependant on the nature of the membrane. 

 Alcohol and water diffuse into each other without 

 difficulty when brought into direct contact; if we 

 separate them by a bladder we find that water will 

 rapidly pass into the alcohol, but the reverse flow 

 will take place with great slowness, for the reason 

 that alcohol cannot wet the surface of this membrane. 

 On the other hand india-rubber is readily moistened 

 by alcohol but not by water; and if a thin sheet of 

 this substance be interposed between these liquids, 

 it will be seen that alcohol passes the membrane 

 into the water much more rapidly than water tra- 

 verses in the opposite direction. 



Schacbt has made observations on the cell-mem- 

 brane of the Caulerpa prolifera, a plant presenting 

 single cells of sufficient size for such purposes, and 

 found that it admitted of all the phenomena of diffusion exactly as 

 manifested by other membranes. 



The rootlets of a plant being immersed in the water (or moisture) 

 of the soil, act towards it as the bladder filled with brine in our 



