Transpiration of Aqueous Vapor by the Leaves of Plants.
Professor McNab, of Cirencester College, England, has recently published an important series of experiments on this subject. The plant experimented on was in all cases the common cherry-laurel, (Prunus laurocerasus,) and the fluid to determine the rapidity of ascent, lithium citrate, a very small quantity of which can be detected by means of the spectroscope. Dr. McNab divided the results under the following heads:—1. Quantity of water in the leaves. The mean of several experiments gave 63.4 per cent. 2. Quantity of water which can be removed by calcium chloride, or sulphuric acid, in vacuo. This was found to be from 5.08 to 6.09 per cent. 3. Amount of transpirable fluid in the stem and leaves, 7.58 per cent. The remainder, from 56 to 57 per cent., was therefore determined to be fluid in relation to the cell-sap of the plants. 4. Rapidity of transpiration in sunlight, diffused light, and darkness. The results given are:—
In sunlight, 3.03 per cent in an hour; in diffused daylight, 0.59 per cent.; in darkness, 0.45 per cent. 5. Amount of fluid transpired in a saturated, and in a dry atmosphere in the sun, and in diffused daylight. In sunshine, the experiments gave 25.96 per cent. in an hour, in a saturated atmosphere; 20.52 per cent. in a dry atmosphere; in the shade, the results were reversed, nothing whatever in a saturated, 1.69 per cent. in a dry atmosphere. These results strikingly confirm the earlier experiments of Dehérain, that evaporation from leaves is due to light, and not to heat, and that it proceeds equally in a perfectly saturated atmosphere. 6. Quantity of water taken up by leaves when immersed in it. The mean of several experiments gave 4.37 per cent. in one and one-half hours. 7. Quantity of aqueous vapor absorbed by leaves in a secluded atmosphere. This was found to be nil, again confirming the statement of M. M. Prillieux and Duchartre that plants absorb no moisture whatever in the state of vapor through their leaves. 8. Differences in the amount of fluid transpired by the upper and under side of leaves in the sun and in diffused daylight. From the upper surface in sun, 1.74 per cent. in an hour, from the under surface, 12.83 per cent.; from the upper surface, in diffused light, 2.82 per cent. in forty-eight hours, from the under surface, 16.08 per cent.; from both sides, when coated with collodion, 0.86 per cent. in sun, 2.56 per cent. in diffused light. 9. Relation of fluid taken up, to that transpired and that retained by the plant. Increase of weight of branch, in saturated atmosphere, diffused daylight, in forty-eight hours, 7.34 per cent., in ordinary atmosphere, 7.14 per cent., in darkness, 8.01 per cent. 10. Rapidity of ascent of fluids. From 4 7-12 inches in ten minutes to 8 7-12 inches in ten minutes. 11. Influences of gases on transpiration. Transpiration of fluid in oxygen in one hour in sun, 12.77 per cent., in atmospheric air, 7.5 per cent., in carbonic acid, 4.01 per cent., in nitrogen, 1.97 per cent. The bad weather and the lateness of the season terminated the experiments before several points of interest could be fully determined. A. W. B.—From the American Naturalist, March, 1871.