EVAPOKATION OF EVEEGKEEN AND DECIDUOUS TREES. 237 



porating power of different leaves, contrasting togetlier in parti- 

 cular those of evergreens and those of deciduous plants. Before 

 making one or two remarks which these experiments suggest, it 

 will perhaps not be out of place to say a few words respecting 

 some former investigations on the same subject. 



It is more than a liundred and fifty years since Dr. Woodward 

 published in the 20th volume of the Philosophical Transactions 

 of the Royal Society, an account of some experiments on vege- 

 tation, having for their especial object the evaporating power of 

 the leaves; these experiments were curious, and excited a good 

 deal of interest at the time they were published. Some of the 

 conclusions drawn from them were tolerably accurate, but from 

 the vague and uncertain views which were then generally enter- 

 tained respecting the growth and nourishment of plants, the very 

 facts themselves became to a very great extent mystified and 

 confused, so that their practical value was greatly diminished. 

 Dr. Woodward's experiments were made with weighed bottles of 

 water, having a piece of parchment tied over their mouths, in 

 which a small aperture was made just sufficient to admit tlie stem 

 of a plant, but not so small as to confine or impede its growth. 

 As the water evaporated, fresh was added from time to time, a 

 register being kept of the quantity added, as well as of that 

 which was lost by evaporation. The plants were placed side by 

 side in a window, where they were equally exposed to sunshine ; 

 and the experiment was continued from the 20th of .July, 1691, 

 to the 5th of October in the same year. The following was the 

 result of one of these comparative experiments : — 



Spearmint, in spring water 

 Ditto, in rain water. 

 Ditto, in Thames water 

 Nightshade, in spring water 

 Lathyris, in spring water . 



Orif^inal 

 Weight. 



Grains. 



27 

 28 

 28 

 49 

 98 



Final 

 Weight. 



Grains. 



42 



45 



54 

 106 

 101 



Water 

 Evaporated. 



Grains. 

 2558 

 3004 

 2493 

 3708 

 2501 



Proceeding to compare together the increase in weight of the 

 plant with the quantity of water it had given off, Dr. Woodward 

 showed that in the case of the three plants of Spearmint, it was 

 respectively as 1 to 170, to 171, and to 95 ; whilst in the instance 

 of the Nightshade it was as 1 to 65, and in that of the Lathyris 

 as 1 to 714. It is evident, however, that from such experiments 

 no very satisfactory or accurate conclusion could well be drawn ; 

 they were repeated and varied in different ways, and similar re- 

 sults were obtained. One of the most curious of these experi- 



