362 HOW CROPS GROW. 
Rapid Motion of Sap in the Stem.—In the stem of the 
plant we have commonly a resistance to root-action, so far 
as a flow of liquid is concerned. The ducts and sieve- 
cells,—in conifers, the wood-cells—though offering visibly 
continuous channels for the transmission of juices, are 
nevertheless in most cases extremely small, and while they 
raise liquids with enormous capillary force, they retain 
them with the same force, and continuous motion can only 
be the result of a correspondingly energetic disturbance. 
The root-action which can sustain a column of mercury 
many inches, or one of water many fect high, in a wide 
tube, is greatly neutralized by capillarity as we ascend the 
stem from the root, or the root from its young extremities. 
Root-action is, however, unsteady in its operation, and 
when it declines from any cause, it is capillarity which acts 
rapidly within the ducts and visible channels to supply 
waste by evaporation. 
Motion of Nutritive or Dissolved Matters; Selective 
Power of the Plaut.—The motion of the substances that 
enter the plant from the soil in a state of solution and of 
those organized within the plant is to a great degree sep- 
arate 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 carbohydrate 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-tissue, and is undoubtedly greatly aided 
by osmose, i.e, by the action of the membranes them- 
selves. The very thickening of the cell-walls by the dep- 
osition of cellulose would indicate an attraction for the 
materiat from which cellulose is organized. The same 
transfer goes on simultaneously in all directions, not only 
into roots and stem, but into the new buds, into flowerg 
and fruit. We have cansidered the tendency to equaliza- 
tion between two masses of liquid separated from each 
