The two ends of the microscopic aqueous canals, in plants of the ordinary sort, are the active imbibing root-hairs, above mentioned, and the exposed surfaces of the aerial parts, chief of which are the leaves. The lower terminal is buried beneath the soil from which it receives its supply, and consists of a sac with a very thin and elastic wall, lined with a delicate film of living protoplasmic substance. We may imagine one of these cells many hundred times enlarged, its contents consisting of a thin syrup, but slightly more dense than the liquid in which it is immersed. As time passes, there is a flow inward of the less dense liquid and an increase of the wall tension of the cell. This tension might be observed by pricking the wall, when from the pinhole the liquid would spurt for some distance. The same pressure might aid in the passage of the liquid from the sac to the one next adjoining, and in that way a flow would be set up from the less to the more dense cell contents.
A homely and common illustration of this osmosis or membrane diffusion is seen in the action of sugar upon ripe strawberries, the sugar taking the thin juice from the cells and making a syrup that finally surrounds the berries. Place dried prunes in a dish of warm water, and a similar exchange is demonstrated, but in this case the flow is into the dry cell contents, and the prunes finally become plump. There has been a transportation of liquid in both these instances, and it has been from one cell to another through the whole necklace, so to say, of many beads, from the surface to the innermost cell or vice versa, as the case may be.
Let us ascend a tall tree, figuratively, and study microscopically the upper terminals of the lines of water-carriers. Here we find the leaves in great numbers, presenting, possibly, acres of actual visible surface to tile drying influence of the almost constantly changing air. But if we note the exceedingly porous structure of a leaf, how one cell touches its fellows at but few points and the bulk of the space is intercellular, the actual surface exposed to the atmosphere is a hundred times more than the naked eye reveals. As with the soil terminal, so here the end of the transportation line divide up into a million parts. In the former, each is for the reception of liquid; in the latter, they are all places of unloading. The drying air sweeps over them, and something of their contents is vaporized and is gone from the plant. But this evaporation increases the density of the cell contents and were there no reserve the tissues would wither, dry up and become dead, as is the case when a branch is cut off or grass is mown in the meadow.
If we apply the law illustrated in the dried prunes, it will be seen that each surface cell in the loose pulp of the leaf is dependent upon the next below it, and that, in honoring the draft upon it, is making a physical call upon another, and so the line is established, like men
