Temperature on the Rate of Growth in Pisum sativum. 27 
constructed by Professor Johannsen and not as yet described. A photo- 
graph of it appears in Plate I, Fig. 1, and the apparatus is made in the 
following manner : A dish of suitable size is chosen and fitted in the foot 
with a plate of cork. The whole is lined with paraffin, and pieces of wire, 
or needles, are stuck in the cork in pairs, at such distances that two rubber 
tubes, which are placed round them, press closely together. The dish is 
filled with plaster of Paris, and when it has set the rubber tubes are pulled 
out, the form removed, and the apparatus is ready. A lid as shown in the 
photograph is quite simply made with a similar lined dish and a smaller 
plate of cork, covered with paraffin, to press into the soft plaster of Paris. 
Convenient sizes for the germination of peas are : whole apparatus, 
10 x 15 cm., and rubber tubes 0-7 and 0-5 cm. in diameter, o-6 cm. apart. 
For use the apparatus is thoroughly moistened and is set in a dish con- 
taining a little water. In the experiments proper, as distinct from the first 
a 
/ 
/ 
Text-fig. 2. 
two days’ growth, instead of the porous plaster-of-Paris lid, a bell-jar lined 
with filter-paper was used to cover the peas, and a petri dish with NaOH 
was placed above the peas to prevent the accumulation of C 0 2 . 
The peas were soaked for 22 \ hours, and then placed seventy alto- 
gether, each day, on this apparatus and left for 4 6\ hours. At the be- 
ginning of the third day after soaking, therefore, the thirty-five most 
uniform in length were selected and measured. The method of measure- 
ment was as follows : the triangular piece of the seed-coat burst up 
in germination was removed, and a piece of millimetre-paper placed 
behind the root so that a centimetre line came exactly behind the point 
marked a in Text-fig. 1. The length of the root was read off in millimetres, 
so that Professor Johannsen’s method of measuring ‘ up to ’ was used ; that 
is, the next higher millimetre was always taken as the reading. In Text- 
fig. 2, which shows the paper in place, the length is 15 mm., but, if the 
