90 ESCAPE OF WATERY VAPOUR FROM THE LEAVES. 



matter of familiar observation that the water gradually diminishes in 

 bulk, and will at length entirely disappear, even when evaporation into 

 the air is entirely prevented. The water which ihns disap{)ears is taken 

 up by the roots of tlie plant, is carried up to the leaves, is there spread 

 out over a large surface exposed to the sun and to the air, and in the 

 form of vapour escapes in considerable proportion through the pores of 

 the leaves and diffuses itself through the atmosphere. 



The quantity of water which thus escapes from the surface of the 

 leaves varies with the moisture of the soil, with the species of plant, 

 with the temperature and moisture of the air, and with the season of the 

 year. According to the experiments of Hales, it is also dependent on 

 the presence of the sun, and is scarcely perceptible during the night. 

 He found that a sun-flower, 3i feet high, lost from its leaves during 12 

 hours of one day 30, and of another day 20 ounces of water, while during 

 a warm night, without dew, it lost only three ounces, and in a dewy 

 night underwent no diminution in weight.* 



This loss of warery vapour by the leaf is ascribed to two different 

 kinds of action. First, to a natural perspiration from the pores of the 

 leaf, similar to the insensible perspiration which is contiifually proceed- 

 ing from the skins of healthy animals ; and second, to a mechanical 

 evaporation like that which gradually takes place from the surface of 

 moist bodies when exposed to hot or dry air. The relative amount of 

 loss due to each of these two modes of action respectively, must differ 

 very much in different species of plants, being dependent in a great 

 measure on the special structure of the leaf. In all cases, however, the 

 natural perspiration is believed very greatly to exceed the mere mechan- 

 ical evaporation — though the results of Hales, and of other experimen- 

 ters, show that both processes proceed with the greatest rapidity under the 

 influence of a warm dry atmosphere, aided by the direct rays of the sun. 



Among the several purposes served by this escape of watery vapour 

 from the surface of the leaf, it is of importance for us to notice the direct 



* When Ihe escape of vapour from the leaves is more rapid than the supply of water from 

 the roots, tlie leaves droop, dry, and wither. Such is sometimes the case with growing 

 crops in very hot weather, and it always happens when a twig or flower is plucked and sep- 

 arated from the stem or root. When thus separated the leaves still continue to give off wa- 

 tery vapour into the air, and consequently the sap ascends from the twig or stalk to supply 

 the place of the water thus exhaled. 



But as the sap ascends it must leave the vessels empty of fluid, and air must rush in to 

 fill the empty space. This will continue till nearly all the fluid has risen from the stem into 

 the leaf, and the vessels of the wood are full of air. But if the stem of the twig or flower be 

 placed in water this liquid will rise into it, air will be excluded, and the freshness and bloom 

 of the leaves and flowers will be longer prestjrved. If the water into which they are intro- 

 duced contain any substances in solution, these will rise along with the water, and will grad- 

 ually make their way through all the vessels of the wood, till they can be detected in the 

 leaves. By this means even large trees may in a short time be saturated with saline solu- 

 tions, capable of preserving them from decay. It is only jiecessary to cut down or saw 

 through Ihe tree and insert its lower extremity info the prepared solution, when the action 

 of the sun and air upon the leave.s will cause it spontaneously to ascend. Tims corrosive 

 sublimate (the subject of Kyan's Patent) may be injected with ease, or pyroligni/e o/iron, 

 (iron dissolved in wood vinegar,) which Boucherie recommends as equally efiicient and 

 much more economical, [Ann. de Chim. et de Phys. Ixxiv. p. 113.] The process is finislied 

 wlien the liquid is found to have risen to the leaf. Coloured solutions may in the same way 

 be injected and the wood tinged to any required shade. One of the chief benefits attendant 

 upon" the cutting of wood in the winter, appears to be that the absence of leaves prevents the 

 exhaustion of the sap and the ascent of air into the vessels of the wood — the oxygen of this 

 air tending to induce decay. But the sap may be retained, and the air excluded almost as 

 effectually, at any other season of the year, by stripping the tvze of its leaves and branches a 

 few days before it is cut down. 



