228 ' LECTURE XIV, 



much more readily if Maize, Tobacco, Beans, Cabbage, or other plants are allowed to 

 develop their roots, not in earth, but in aqueous solutions of nutritive matters. If the 

 glass vessel containing the nutritive solution and the roots of the plant is closed by 

 a halved cork, which allows the stem to pass through and fit tight, the- level of the 

 nutritive solution is seen to sink from day to day ; and in the course of several days 

 the whole of the fluid within reach of the roots disappears from the vessel. The 

 whole of this water has ascended through the stem into the green leaves, there to 

 be evaporated. If the student wishes to convince himself of this, which of course is 

 scarcely necessary on a little reflection, it suffices to cover a plant arranged as 

 described with a bell-glass previously cooled : the aqueous vapour escaping from the 

 leaves soon becomes condensed on the glass wall, and runs down in the form of drops. 

 Of course no measure of the evaporation in the open air would be obtained by this 

 means, because the space under the bell-glass becomes nearly saturated with aqueous 

 vapour, and then almost entirely prevents further transpiration from the leaves. 

 A substance which absorbs water (e. g. sulphuric acid or calcium chloride) could be 

 brought under the bell glass, however, and the vapour exhaled by the plant would 

 then be absorbed by this substance, and the saturation with vapour of the air sur- 

 rounding the leaves prevented : the increase in weight of the substance named would 

 then indicate the quantity of water exhaled in a given time. If plants with large 

 leaves stand in spring-time or autumn behind a closed window, it is noticed in the 

 morning that at those parts of the glass near the upper surfaces of the leaves, there 

 is a deposit of water, which has evaporated frorii the leaves and been condensed on 

 the cold window panes. 



It would be quite in vain, however, to attempt to give from researches of 

 the kind above described, an exact account of how much water a plant exhales in 

 the form of aqueous vapour, and takes up through the roots, in a day or in a 

 week. Transpiration depends upon the environment, quite as much as upon the 

 organisation of the plant; experience shows that the formation of vapour from 

 the leaves is increased as the warmth and dryness of the air, and especially the 

 intensity of the light, increase. In moist air, or when fog, dew or rain cover 

 the surfaces of the leaves with water, it is obvious that but little aqueous 

 vapour, or even none at all, can escape from these organs : the transpiration, and 

 accordingly the amount of the water moving in the plant, thus depend upon the 

 alteration of external circumstances ; and so far as experience teaches, the well-being 

 of the plant is not interfered with within a wide range of play of these circumstances. 

 However, the extremes must be avoided, since just as the long continued prevention 

 of transpiration is injurious to a land-plant, so a too energetic formation of vapour 

 in the leaves during hot sunshine may lead to the result that the roots do not at the 

 same time absorb as much water as is given oflf at the leaves; hence the latter 

 droop or wither — a phenomenon observed often enough during dry weather on 

 very hot July days, but which passes away without injury when the temperature sinks 

 at night, and the relative moisture of the atmosphere rises, and the transpiration 

 diminishes so far that the supply of water from the roots suffices to make the leaves 

 again turgid and fresh— i. e., to fill them with water. 



Although it is not possible to give an exact idea of the quantity of water 

 sent upwards in a given plant during a period of vegetation, it is nevertheless of some 



