EVAPORATION OF EVEHGREEN AND DECIDUOUS TREES. 241 



expected to yield satisfactory results. Now, in the preceding experi- 

 ments, the plants were unquestionably injured by the treatment they 

 received, and what is still worse, they were injured to an unequal 

 extent, some of them being rendered far more sickly than the 

 others. If means cannot be devised to prevent the evaporation 

 of moisture from the surface of the soil without tying bandages 

 round the stem, it is far better to calculate the amount of water 

 thus lost, as Hales did, and subtract it from the total loss at each 

 weighing. If all tlie other circumstances are perfectly similar, 

 this loss would not vary mvich ; it would be pretty nearly constant 

 with each of the ditferent plants. It is also to be remarked, that 

 in thus absolutely preventing all evaporation from the enclosed 

 soil, excepting that which took place through the leaves of the 

 plant, the soil was altogether cut off from contact with the ex- 

 ternal air, and thus another, and by no means unimportant con- 

 dition was introduced. 



In experiments of this sort it is very desirable not to commence 

 them the very day the plants have been transplanted ; some little 

 time should be allowed to elapse before the experiment is begun, 

 so that the plants may become accustomed to the new conditions 

 under which they are placed ; and in every case where possible 

 two or three similar plants should be taken for each experiment, 

 instead of single individuals. 



For the most part the exi)erimental results are pretty nearly 

 what might have been expected, though of course there are ex- 

 ceptions and irregularities, which must be attributed to special 

 interfering causes. There would seem to be a remarkable dif- 

 ference between the several plants, as regards the relation existing 

 between the temperature and the rateof evaporation, independent 

 of the dryness or moisture of the air. On comparing the tables 

 together, it will be found that in the case of the Portugal Laurel, 

 Holly, Larch, and Sycamore, the maximum evaporation occurs at 

 the same time as the maximum of temperature, namely, between 

 the 2ord of July and the 22nd of August. This, however, is 

 not the case with the other plants. In the case of the Oak and 

 the Deciduous Berberis, the maximum evaporation occurs after the 

 greatest heat, the greatest quantity of water being evaporated 

 between the 22nd of August and the 21st of September, though 

 the average temperature then was more than 8 degrees lower 

 than it had been during the previous four weeks. Exactly the 

 reverse is the case with the remaining plants, for the maximum 

 evaporation with the Spruce-Fir, Evergreen Berberis,Yew, Laurel, 

 and Ash, in each instance preceded the maximum of heat. On re- 

 ferring to the table showing the evaporation from the Evergreen 

 Berberis, it will be found that from the 23rd of June to the 23rd 

 of July, when the average temperature was below 62°, the daily 



VOL. VI. s 



