Temperature on the Rate of Growth in Pisum sativum. 29 
was reduced to the minimum possible. About ninety were required, since 
about 20 per cent, are injured or deformed or of extremely large or small 
size. It was thus hoped to eliminate unconscious selection of the soaked 
peas. From then onwards, too, the germinating apparatus and all dishes 
used were periodically sterilized as a precautionary measure. Under these 
conditions a third series was carried out, and it agreed exactly with 
the first. 
Detailed figures are given in Table I, and the results are plotted 
in Text-fig. 3. Table [I shows the average initial length, average growth 
in 22| hours, together with the standard deviation and probable error of the 
mean for each experiment. The experiments are arranged in chronological 
order. 
Second Series. 
In the second part, the peas were grown in the following way. They 
were soaked and germinated as before in the electric thermostat, but 
remained in the plaster-of-Paris apparatus for only one day, at the end 
of which the roots were about 5 to 10 mm. long. Then they were placed in 
tubes measuring i*8 cm. in diameter and 10 cm. in height, with corks bored 
eccentrically to admit the thermometer during the experiment, and having 
a triangular piece cut out at one side so that the air in the tube was in con- 
tact with the outer air at all times. In the cork a capillary glass tube was 
fixed so that it lay along the side of the tube, and the pea was fixed to the 
cork by a pin so that the root was in contact with the side of the tube, 
between it andja slip of filter-paper and just beside the capillary tube. The 
filter-paper dipped in water. The root was thus supplied with sufficient 
moisture at all temperatures and air had free access to it. It was found that 
without the capillary tube the filter-paper adhered to the glass, and the 
resistance thus offered to the growth of the root was sufficient to cause 
curvatures. Plate I, Fig. 2, is a photograph of peas so grown. By this 
method a large proportion of the peas grew diagrammatically straight, and 
only a small proportion were incapable of accurate measurement. A slight 
curvature, such as that shown in the tube to the right in the photograph, is 
of no significance, since the experimentation-time is so short that the 
inclination does not appreciably change. In these cases, readings are taken 
with the eye-piece slightly rotated so that the micrometer scale is perpendi- 
cular to the axis of the root. 
With the magnification used, a reading 1 on the micrometer scale 
represented 0*056 mm. O'l \ 
A beaker served as a water-bath, and the temperature was regulated 
by a micro-burner and Roux’s regulator. Except at low temperatures, the 
temperature of the water remained constant to within 0-5°. For tempera- 
tures below 1 5° a slow current of water flowed through the beaker ; a conical 
