366 HOW CROPS GROW. 
those of the stomata excepted. Sachs found that solution 
of indigo quickly entered the roots of a seedling bean, 
but required a considerable time to penetrate the stem, (p. 
289.) THallier,in his experiments on the absorption of 
colored liquids by plants, noticed in all cases, when leaves 
or green stems were immersed in solution of indigo, or 
black-cherry juice, that these dyes readily passed into and 
colored the epidermis, the vascular and cambial tissue, 
and the parenchyma of the leaf-veins, keeping strictly to 
the cell-walls, but in no instance communicated any color 
to the cells containing chlorophyll. (Phytopathologie, 
Leipzig, 1868, p. 67.) We must infer that the coloring 
matters either cannot penetrate the cells that are occupied 
with chlorophyll, or élse are chemically 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. (Aap. 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 cach other, and obviously accomplish this result 
by exerting themselves superior attractive or, repulsive 
force. 
The influence of the membrane must vary in character 
with those alterations in its chemical and structural consti- 
tution which result from growth or any other cause. It is 
thus, in part, that the assimilation of external food by the 
