CAUSES OF THE ITOTIOK OF JUICES. 401 



mm. When we consider the vast root-surface exposed 

 to the soil, in case of a vine, and that myriads of root- 

 lets and root-hairs unite their action in the compara- 

 tively narrow stem,' we must admit that the apparatus 

 above figured gives us a very satisfactory glance into the 

 causes of bleeding. 



Motion of Nutritive or Dissolved Matters; Se- 

 lective Power of the Plant. — The motion of the sub- 

 stances that enter the plant from the soil in a state of 

 solution, and of those organized within the plant is, to a 

 great degree, separate from and independent of that 

 which the water itself takes. At the same time that 

 water is passing upwards through the plant to make 

 good the waste by evaporation from the foliage, sugar or 

 other carbhydrate generated in the leaves is diffusing 

 against the water, and finding its way down to the very 

 root-tips. This diffusion takes place mostly in the cell- 

 .tisBue, and is undoubtedly greatly aided by osmose, i. e.. 

 By the action of the membranes themselves. The very 

 thickening of the cell-walls by the deposition of cellulose 

 would indicate an attraction for the material from which 

 cellulose is organized. The same transfer goes on sim- 

 ultaneously in all directions, not only into roots and 

 stem, but into the new buds, into flowers and fruit. 

 We have considered the tendency to equalization between 

 two masses of liquid separated from each other by pen- 

 etrable membranes. This tendency makes valid for the 

 organism of the plant the law that demand creates sup- 

 ply. In two contiguous cells, one of which contains 

 solution of sugar, and the other solution of potassium 

 nitrate, these substances must diffuse until they are 

 iningled equally, unless, indeed, the membranes or some 

 other substance present exerts an opposing and prepon- 

 derating attraction. 



In the simplest phases of diffusion each substance is, 

 to a certain degree, independent of every other. Any 

 36 



