Vegetable Physiology. 247 



we j(rht only four and a half grains; so that twenty and a half 

 trains had disappeared by exhalation. 



Experiments of this last kind maybe very easily performed by 

 an y one who has command of a pair of scales adapted to weigh 

 small substances ; and it is well that the student should avail 

 himself of such opportunities of learning how to " put Nature 

 to the question" in matters of this simple character, in order to 

 cultivate habits of accuracy and caution, which are useful in 

 every condition of life. Let him take several leaves of different 

 plants,— such, for example, as the Vine, Oak, Elm, Beech, 

 Lime, Apple, Pear, — weigh them separately, and estimate as 

 nearly as he can the comparative surface presented by each. 

 He should then place their foot-stalks in glasses or bottles of 

 equal size, into which has been poured a certain weight of 

 water, carefully ascertained to be the same in each. All 

 these should be placed in similar circumstances for a certain 

 time, and a corresponding glass should be kept, without a leaf, 

 in order to estimate the amount lost by evaporation from the 

 surface of the water. By ascertaining how much had been 

 absorbed by each leaf, and the weight each had gained, he 

 would thus be easily enabled to calculate the quantity it must 

 have exhaled ; and then by comparing this with the extent of 

 the surfaces of the different leaves, he would estimate the pro- 

 portional rapidity of the process in the various species he had 

 chosen. Care should be taken to select, in the first instance, 

 trees in equal stages of growth, and leaves of a similar degree 

 of freshness and development.^ 



Whenever the vapor given off from the skins of animals, for 

 any cause exceeds the amount which the atmosphere can take 

 up, it collects into drops, and forms sensible perspiration. 

 In like manner, when the sun first rises, some plants exhale so 

 rapidly, that the lower temperature of the air causes the fluid 

 to accumulate in drops at the points of the leaves. This is 

 often mistaken for dew, but that it is really condensed perspira- 

 tion may be readily understood, by observing that it occurs not 

 only on plants which are in the open air, but also on those under 

 shelter, and to which the dew could not possibly have had access. 



* Cyclopedia of Natural Science. 



