250 
Thomas and Ferguson . — On the 
equivalent area of the transpiring surface of the plant two sets of calibrating 
vessels, three in each set, were employed. In one set (vessels C, E, and G) 
the dishes were filled to 5 millimetres from the rim, in the second set (D, F, 
and I) to 3 centimetres. The following figures were obtained : 
Table III. 
Radius. 
Loss in 
weight. 
Duration o J 
experiment . 
h. 
m. 
Plant 
.... 
6*45 grammes. 
68 
12 
C. 
8-26 cm. 
29*55 
68 
23 
E. 
7"°3 
23-38 
68 
26 
G. 
5*84 
16-55 
68 
2 
D. 
7-48 
16-80 
68 
10 
F. 
6*45 
12-15 
68 
10 
I. 
5 *°° 
7 * 5 i 
68 
6 
The mean rate of evaporation of the plant is therefore 0-0946 gramme per 
hour. If we assume, as we have shown to be justifiable, the area law for D, 
F, and I, we obtain, in determining the equivalent water surface (A) of the 
plant by means of the equation 
7 T a j 2 E. 
E 
(a) from D, A — 67-45 sq. cm., 
(/ 3 ) from F, A = 69-34 sq. cm., 
(y) from I, A — 67-48 sq. cm., 
giving a mean value of 68-09 sq. cm - f° r A. 
In reducing the data obtained from C, E, and G, log E was plotted 
against log a in the usual way, and the resulting straight line showed that 
the appropriate value of n was 1-58. Hence using the equation 
losa _ «log <h- log^ + log^ 
s 2 11 
we obtain, 
(6) from C, a 2 — 3-168 cm. or A — 31*53 sq. cm., 
(e) from E, a 2 = 3-126 cm. or A = 30-70 sq. cm., 
(C) from G, a 2 — 3-216 cm. or A = 32-49 sq. cm. 
The mean value of A is therefore 31-57 sq. cm. 
Hence we see that the plant loses water at the same rate as a circular 
water surface 31-6 square centimetres in area filled to a distance of 5 milli- 
metres from the rim of the containing vessel ; or at the same rate as a circular 
water surface 68* 1 square centimetres in area filled to a distance of 3 centi- 
metres from the rim. The vital importance of specifying this distance is 
here shown to be very obvious. 
