THEORY OF OSMOSE. 179 



however, entirely fails to account for the familiar fact that the 

 sap is often pressing into trees, like the birch, with tremen- 

 dous force, several weeks before there is the slightest activity 

 in the buds. 



Dutrochet discovered the principle called osmose, which 

 causes unlike fluids separated by a thin septum to flow to- 

 gether with difierent degrees of rapidity. Thus, if a solu- 

 tion of sugar be separated, by a thin membrane, from pure 

 water, the water will pass through into the sugar freely while 

 a minute portion of the sugar will ent^r the water, the result 

 being a large increase in the volume of the sugar solution. 

 This force, under favorable circumstances, will overcome the 

 force of gravitation so as to cause the rise of water in a tube 

 to a considerable height. 



Professor Graham has more recently investigated this sub- 

 ject and learned that dissimilar fluids and gases have a ten- 

 dency to mingle their molecules, and to do so with some 

 freedom through permeable septa in accordance with the law 

 of osmose. He has divided all soluble substances into two 

 classes, namely, crystalloids and colloids. The former, like 

 common sugar and salt, difi'use themselves readily through 

 solvent fluids and membranous partitions, while the latter, 

 like glue and starch, are, comparatively, non-ditfusive. 



The general principle of osmose has been almost univer- 

 sally adopted, without any considerable attempt at demonstra- 

 tion by physiological experiments, as the chief cause of all 

 the motions which occur in the contents of vegetable cells, 

 such as the absorption of water by the rootlets, the ascent of 

 the crude sap to the leaves, and the general transference of 

 all nutrient matters to the parts where they are deposited and 

 assimilated. 



There are many difficulties in the way of accepting this 

 charmingly simple hypothesis. Among these may l)e named 

 the fact that there are found in the different adjoining cells of 

 plants entirely distinct substances which do not mingle, as in 

 the brilliant petals of floAvers, where superimposed layers of 

 cellular tissue contain fluids of unlike colors. The cambium, 

 also, which evidently does not penetrate the sap-wood, 

 readily finds its way through hundreds of feet of its proper 

 conducting medium. Again, "the organic contents of plant- 



