Page:An introduction to physiological and systematical botany (1st edition).djvu/216

 186 damp cellar, or immersed in water, the leaves revive, by which their power of absorption is also proved. Hence the use of a tin box to travelling botanists, for the purpose of restraining the evaporation of plants, and so preserving them fresh for some days till they can be examined, as well as of reviving faded plants, if the inside of the box be moistened before they are shut up in it.

Dr. Hales found that a plant of the Great Annual Sunflower, Helianthus annuus, lost 1lb. 14oz. weight in the course of twelve hours in a hot dry day. In a dry night it lost about 3 oz.; in a moist night scarcely any alteration was observable, but in a rainy night it gained 2 or 3 oz. The surface of the plant compared with that of its roots was, as nearly as could be calculated, in the proportion of five to two; therefore the roots must have imbibed moisture from the earth of the pot in which the plant grew, and which was all previously weighed, in the same proportion of five to two, otherwise the leaves would have faded. The same experiment was made on the vine, the Cabbage, &c., with various results as to the exact degree of