24 4 PRINCIPLES OF BOTANY, ETC. 
feet does the surface of all the great and leafy plants 
present to the atmosphere, and how great must their 
number be, for instance, in a full grown leafy oak- 
tree? According to. Hales’s experiments, the mois- 
ttre which ascends from the leayes of plants by 
transpiration, is very great. A sunflower, three 
feet high, transmitted in 12 hours about one pound 
and four eunces avoirdupois. When dew fell, this 
transpiration ceased entirely, and the leaves ab- 
sorbed two or three ounces of it. When there was 
no dew, then the transpiration during night amount- 
ed to only three ounces. He made many other s!- 
milar experiments, and the transpiration was always 
considerable in the day time. Mr Watson put a 
glass vessel of 20 cubic inches capacity inverted on, 
grass, which had been cut during a very intense 
heat of the sun, and after many. weeks had passed 
without rain; in two minutes tinie it was full of 
drops which run down its sides. He collected these 
on a piece of muslin, carefully weighed, and repeat- 
ed, the experiments for several days between twelve 
and three o’clock. And, from this he was led to 
calculate, that an acre of land transpired, in 24 
hours, 6400 quarts of water. | 
§ 240, 
As the hfe of animals greatly depends on external. 
war mth, plants likewise need. a certain degree of it. 
Plants of warm countries want, more of it dan those 
which belong to cold regions. These are facts 
which need, no further demonstration. But whether 
plants, like animals, have a fixed and peculiar. de- 
gree 
