1904] DARWIN: MOVEMENTS OF STOMATA 85 
any way indicate the cooling effect of transpiration, because the 
leaf used is one cut from the parent plant, in which the stomata 
had closed; the leaf simply acts as a good conductor. 
In most experiments a leaf with stomata shut was placed on 
the control bulb, the experimental leaf being on the other. 
This has not merely the advantages pointed out, but it has the 
more important effect of protecting the control bulb from 
draughts of air, and therefore from changes in temperature. 
The effect is evident when both bulbs are covered with cotton 
wool. The line drawn is then quite free from the zigzags so evi- 
dent in the figures published. The following experiment shows 
that it is important to choose carefully the body placed on the 
control bulb. 
EXPERIMENT 46, fig. 37, Dec. 18, 7899. DRY AND WET GYPSUM. 
A small block of dry gypsum wrapped in tin-foil on bulb R; a similar 
block soaked in water on bulb B; the tin-foil was pierced with pin holes so as 
to allow only a moderate degree of evaporation, 
A.M, 
11:7 Battery on to balance, and then turned off so that the blocks of gyp- 
sum might be placed on the bulbs. 
11:25 Battery on: the B-cool fall is owing to the evaporation and conduct 
P.M. ing power of the damp gypsum (B). 
12:14 Doors and windows of the greenhouse opened, producing the follow- 
ing changes: 
Dry bulb, °C. Wet bulb, °C. Humidity of air, 
per cent. 
12:13 : “ 15.0 12,2 70 
12:18 - - 4493 7.8 58 
12:27 . - 8.5 6.0 66 
12:43 - - ee Bh 6.0 75 
The air was thus cooled, and was at first rendered drier; the 
increased evaporation should therefore have cooled B still more, 
but the curve shows that it became relatively warmer; this must 
be due to the fact that the block of damp plaster (B) took longer 
to cool. It was clearly a temporary effect, since at 12: 43 the 
curve returned to nearly the position occupied before the tem- 
perature was lowered. 
At 12:43 (about) the windows and doors were shut, and a 
reverse movement of the same temporary character followed. 
