CAUSES OF THE MOTION OF JUICES. 405 



and colored the epidermis, the vascular and cambial tis- 

 sue, and the parenchyma of the leaf-veins, keeping 

 strictly to the cell-walls, but in no instance communi- 

 cated any color to the cells containing chlorophyl. 

 (Phytopathologie, Leipzig, 1868, p. 67.) We must infer 

 that the coloring matters either cannot penetrate the 

 cells that are occupied with chlorophyl, or else are chem- 

 ically transformed into colorless substances on entering 

 them. 



Sachs has shown in numerous instances that the juices 

 of the sieve-cells and cambial tissue are alkaline, while 

 those of the adjoining cell-tissue are acid when examined 

 by test-paper. (Exp. Phys. der Pflanzen, p. 394.) 



When young and active cells are moistened with solu- 

 tion of iodine, this substance penetrates the cellulose 

 without producing visible change, but when it acts upon 

 the protoplasm, the latter separates from the outer cell- 

 wall and collapses towards the center of the cavity, as if 

 its contents passed out, without a corresponding endos- 

 mose being possible (p. 224). 



We may conclude from these facts that the membranes 

 of the cells are capable of effecting and maintaining the 

 separation of substances which have considerable attrac- 

 tions for each other, and obviously accomplish this result 

 by exerting their superior attractive or repulsive force. 



The influence of the membrane must vary in character 

 with those alterations in its chemical and structural con- 

 stitution which result from growth or any other cause. 

 It is thus, in part, that the assimilation of external food 

 by the plant is directed, now more to one class of 

 proximate ingredients, as the carbhydrates, and now to 

 another, as the albuminoids, although the supplies of 

 food presented are uniform both in total and relative 

 quantity. 



If a slice of red-beet be washed and put into water, 

 the pigment which gives it color does not readily dissolve 



