ol 
there is no avoiding interference on the part of the cobalt paper itself ; 
but nearly all of the light to which the plant has access comes from above, 
and the disturbing effect of cutting off that from below is correspondingly 
moderate. 
The wind affects the transpiration in two ways—by constantly changing 
the air immediately outside the stomata and by agitating the leaves and 
thus causing a circulation within the intercellular spaces and an egress 
and ingress through the stomata. The disturbance of the first of these 
effects is inoperative in this case because of the drying action of the cobalt 
paper ; and as the pinna as a whole remains fairly movable, only the small 
part between the slides being rigid, and the intercellular spaces are con- 
tinuous, the interference with the circulation is at most but partial. The 
disturbance of the transpiration by cutting off the wind is therefore not a 
serious matter. 
That the cobalt-chloride test of transpiration places the leaf in an 
abnormally dry atmosphere is a great and unavoidable objection. Even 
if the plant did not react to this condition other than as a surface of water 
would, namely, by more rapid evaporation, this error would be very diffi- 
cult to control; for while the blue paper must constantly surround itself 
with very dry air, this medium becomes damper as the paper turns red.*? 
In practice, the matter is far from being as simple as if we were studying 
evaporation from a water surface only. When the cobalt paper is applied 
to a surface with open stomata it suddenly makes an increased demand on 
the water vapor in the intercellular spaces which are in immediate contact 
with the open pores, and most particularly on the water in the guard 
cells themselves. An abnormally active escape results, this in turn causes 
the stomata to close, checking the loss, and this process presently brings 
the transpiration below the normal. Thus, the decreased illumination 
and abnormal dryness work together in reducing the transpiration, and 
their combined effect is to cause the paper to change color, at first more 
rapidly than the normal transpiration would make it do so, but afterwards 
much more slowly. 
That the rapidity of reddening of the cobalt paper comes far from 
indicating the actual rapidity with which the plant loses water is clearly 
shown in the last preceding table, in which the time intervals are those 
required for the reddening to take place. As the first column shows, the 
initial reddening took place in one-fiftieth of the time consumed in the 
last interval. The second column shows the relation of the reddening to 
the actual, though abnormal, transpiration. The area of this pinna was 
75.4 square centimeters. Of this, 19 square centimeters was under the 
slide, leaving 56.4 square centimeters free; 56.4 centimeters of cobalt 
paper would absorb 0.26 gram of water in changing color. Evidently 
“Very soon after paper in contact with actively transpiring leaves is really red, 
water begins to precipitate on the glass. 
