240 EVAPORATION OF EVERGREEN AND DECIDUOUS TREES. 



ware pots, made without any holes at the bottom, so that there 

 was no need to make any correction for the water lost by eva- 

 poration ; tlie plants were weighed three times a day for some 

 weeks, and the thermometer was noted at each weighing. 

 The chief facts observed were, that the plants perspired more 

 of a morning than of an afternoon ; that they very often absorbed 

 moisture by the leaves during the night ; and that the proportion 

 perspired was generally in the direct proportion of the tempera- 

 ture of the day. 



These simple experiments are, all of them, perfectly satis- 

 factory, and as far as they go, are no doubt quite trustworthy. 

 It is remarkable that during the last 130 years hardly a single 

 new fact of much importance lias been added to tliis department 

 of vegetable physiology, and that our knowledge of this im- 

 portant branch of the economy of vegetation is very little ex- 

 tended beyond wiiat it was at the time of Hales. One reason of 

 this certainly is, tliat the observers wlio followed him began to 

 refine upon the simple mode of experimenting which he em- 

 ployed, and introduced complicated and unnatural forms of 

 experiment, the results of which are, for the most part, of but 

 little value. Thus the numerous and laborious experiments of 

 Bonnet, undertaken chiefly at the suggestion of Calandrini, to 

 ascertain the relative power of absorbing moisture by the superior 

 and inferior surfaces of the leaf, were far from satisfactory, 

 because, though his object was to measure the power of absorbing 

 aqueous vapour, his experiments in fact all tended to a different 

 effect, namely, the power of the leaves to absorb water, when 

 placed in contact with it, by the upper or lower surface. The 

 experiments of Bonnet, even on the direct absorption of water, 

 do not really give a true indication of the evaporating power of 

 the leaves, because they were made on single leaves and not on 

 entire plants ; they consequently did not fairly represent the 

 sound and perfect leaves of a growing plant. 



The very numerous series of experiments, on the evaporation 

 from leaves, detailed in the preceding pages, are highly valuable, 

 because they extend over a considerable period of time, and tliere- 

 fore are less under the influence of the various interfering causes 

 which generally introduce errors into such investigations. At 

 the same time, however, they are by no means unexceptionable, 

 for there are several matters connected with them whicli are open 

 to doubt and uncertainty. The first great condition of all such ex- 

 periments in every case is, that the ])lants must be brought into a 

 healthy condition at the commencement of the experiment, and kept 

 in a healthy state all through it ; if the plant is rendered sickly and 

 unhealthy by the conditions of the experiment, it is plain that, 

 tlie circumstances being forced and unnatural, they cannot be 



