166 On the Transpiration of Plants. [March, 
house than when exposed to the open air; this, no doubt, accounts 
for the fact that more was lost at night while indoors than when 
exposed, as may be seen by subtracting the amounts evaporated 
by day, in the table, from the whole amounts given off in twenty- 
four hours, the average at night having been 540 grains per night 
while indoors, and only 422 grains per night outside. Tempera- 
ture and the relative humidity of the atmosphere would therefore 
seem to influence transpiration at night, the weather, apart from 
the conditions mentioned, having been about the same at night in 
the two cases. These results also show that the process is at 
least twice as active when the plant is exposed during the day as 
when kept in the house; and yet, as before intimated, the average 
temperature and the complement of the dew point were higher 
during the experiments made indoors than when the plant was © 
out of doors. This would go to show that sunlight and currents — 
of air are, one-or both of them, great modifiers of this process. 
It is interesting to notice that this plant evaporated 100 grains 
more than its own weight (4 ounces) in twelve hours. 
Plant No. 4, Hydrangea arborescens, a shrubby plant in æsti- 
vating stage, 2 feet high; weighed, in a green state with roots 
cleaned, 4 ounces 250 grains; leaves alone, 2 ounces 250 grains; 
evaporating surface of leaves, 744 square inches; complete weight — 
of plant fitted for experimentation, 7 pounds 11 ounces 240 
grains. 
It was found on taking up this plant, after the experiments had 
been made on it, that the quantity of earth its roots had to draw 
moisture from was rather too small, notwithstanding this circum- 
stance, however, the results obtained are full of interest. They 
are the following : 
Place of Dùration of |. Los Loss by |Avera’e) Average 
Ex.| experiment,| experiment. weight by day, temper- io 
evapora’n| t2 hours. | ature, | point. 
Weather. 
a. 
a 
I\In opén air Day and night) 3010 gr.'2450 gr.) 71.° | 54.6° Clear, windy. 
I " s 2305 "1610 e i 7T. 55-8 |Clear, calm 
C 
AII “ “, Taaa “ |1940 * | 75.5 | $9.2 |Clear. 
IV . " 2515 “* |2045 “| 75. | 57-5 |Clear. 
È Indoors. u 1460 = | 975 “ | 79; 1 58.7 Clear, 
VI " 1370 “ | 900 * | 8, | 59.3 iClear. gu 
| os proportion to the extent of evaporating surface, this plant 
| did not exhale as much as the Fuchsia; whereas the relation be 
= tween the weights of the two plants, and the quantity evaporate? 
-by eask; oe is about the same. In the case of t : 
