the Intrinsic Transpiring Power of the Plant . 353 
and being protected by the epidermis, is therefore not subject to the direct 
influence of movements of the outside air. In the case of the atmometer, 
however, the whole of the path of the diffusion stream is exposed to air 
movements, and it is theoretically possible, by means of a sufficiently rapid 
current of air, to reduce the moisture in the air close to the evaporating 
surface to the same concentration as that in the general atmosphere. In the 
case of the leaf, it is only at the surface of the epidermis, and not of 
the evaporating mesophyll, that this minimal concentration can be obtained, 
so that it is to be expected that changes in the speed of air movement will 
have less influence on the transpiration rate of the plant than on the rate of 
water loss from the atmometer. This difference is easily demonstrated by 
experiment. 
Experiment L 
A shoot of Eupatorium adenophorum was set up in a potometer 
and removed to a dark room where conditions of temperature and humidity 
could be kept constant. In addition, the conditions of air movement were 
under control, the plant being placed in an air-flue (described elsewhere, 2) 
fitted with a fan by means of which any desired rate of air movement could 
be produced. The plant could be kept in comparatively still air by 
stopping the fan and closing the ends of the flue with pieces of wood. 
Transpiration was determined at 30-minute intervals by weighing, and 
the quantity of water absorbed by the plant was measured by readings 
of the graduated tube of the potometer. The evaporating power of the air 
was determined by weighings of a Livingston standard atmometer placed 
close to the shoot, which was subjected to still air and moving air 
alternately for varying periods. The results are shown in Table I. 
Table I. 
„ . , 
Half-hour 
Absorption 
Transpiration 
Atmometer 
T 
JrCVlOCL 
period 
by shoot 
in mg. 
loss in 
Conditions. 
TIO, 
ending : 
in c.c. 
{T). 
mg. (E). 
E 
I 
12 noon 
0-21 
213 
108 
i *97 
Still air. 
2 
12.30 p.m. 
0-31 
318 
190 
1.67 
Wind. 
3 
1.0 „ 
0-32 
312 
188 
1 -66 
Speed of 
4 
1.30 „ 
o -35 
320 
187 
1-71 
7 metres 
5 
2.0 „ 
0-31 
322 
188 
I ‘7 I J 
per min. 
6 
2.30 „ 
0-26 
223 
no 
2*03 
Still air. 
7 
3 -o „ 
0*25 
230 
115 
2*00 
n 
8 
3-30 >> 
o *33 
335 
196 
1*71 
Wind. 
9 
4.0 „ 
o *33 
334 
*93 
i “73 
>> 
10 
4-30 „ 
o -34 
34 ° 
! 94 
i -75 
The numbers in the column — 3 although not reduced to unit area and 
unit time, are proportional to ‘ relative transpiration 
If ‘ relative transpiration ’ is to be taken as a measure of the ‘ transpir- 
