Vegetable Physiology. 245 



transpiring under the influence of sun-light, be carried into a 

 Jark room, the transpiration is at once, and almost entirely 

 checked, and if the stomata are examined, they are found to be 

 closed. This transpiration, or exhalation, is regarded as a 

 kind of evaporation from the interior of the plant, and appears 

 to be controlled by the stomata, which by admitting or ex- 

 cluding the air, permit or check it, according to the influence 

 of light upon themselves. This is one way in which light 

 influences the growing plant. If light be excluded, exhalation 

 is prevented, and the fluid which is transpired must pass off 

 by the slower process of evaporation, and absorption soon 

 ceases, since the tissues can contain no more moisture. The 

 share which the leaves have in promoting and maintaining the 

 absorption of fluid by the roots, can thus be understood. The 

 exhalation which takes place in the leaves occasions a new 

 demand for fluid from below, as the combustion of oil in the 

 wick of a lamp gradually exhausts the supply. If the flame 

 be extinguished, the absorption ceases, and if the leaf no 

 longer exhales, the root ceases to absorb. This connexion is 

 shown by an experiment, which also goes far to explain the 

 cause of the rising of the sap in spring, after it has remained 

 stationary during the winter. If a vine be growing on the out- 

 side of a hot-house, and a single shoot be trained within, in 

 mid-winter, the warmth to which the latter is exposed, will 

 cause its buds to swell and unfold ; while those on the outside 

 are inactive. A demand for fluid will thus be occasioned 

 along this particular branch, and this will be supplied by that 

 existing in the vessels below. When these are emptied they 

 will be again supplied from the parts below them ; and thus 

 the motion will be propagated to that division of the roots, 

 whose fibres are connected with those of the vegetating branch, 

 which will absorb fluid for its support, while all the others are 

 completely at rest. In the spring of the year, when the cheer- 

 ful rays of the sun call the whole of the buds into activity, the 

 whole of the roots are similarly affected ; and that the sap 

 begins to move in the upper branches, before it commences 

 ascending in the trunk, h^s been shown by experiment, 

 notches having been cut at intervals, by which the period of its 

 flow could be ascertained in each part. 



