Page:Popular Science Monthly Volume 14.djvu/380

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366
THE POPULAR SCIENCE MONTHLY.

found to transpire hourly per metre of foliage sixteen grammes. It was then inserted into a tube of water, and subjected to the pressure of a column of water two and a half metres high. Under these conditions the evaporation mounted to fifty-five grammes per hour, and the branch at the end of five hours weighed more than at the commencement.

The general result of these experiments shows the mutual working of the various parts of the plant with reference to the phenomena of transpiration. The roots, absorbing water from the soil by endosmose, direct it toward the stem. Whether the motive force here is injection by the roots or absorption resulting from the transpiration in the green parts of the plant, or a union of both, is a question still unsettled. The

PSM V14 D380 Importance of stem leaves.jpg
Fig. 1. Fig. 2.

stem serves not only as a passage for the water to reach the leaves, but also as a reservoir to be drawn on during rapid evaporation. In the leaves the sap is concentrated by the transpiration, and the matters in solution enter into the cell formation, or, changed by the action of light, are distributed throughout the plant by the descending sap. The circulation would be quite similar to that in an animal, were it not for the irregularity. While the supply of water from the roots varies but slightly, the loss by evaporation from the leaves is subject to the greatest fluctuations, according to the temperature and hygroscopic condition of the surrounding air. During these periods the leaves draw on their stock of constitution water and the supply in the stem; and when both fail, the phenomenon of wilting ensues.