146 Ingvar Jorgensen and Walter Stiles. 
picking were kept for 24 hours before experimentation at fairly 
constant temperature, with their stalks in water in covered beakers 
exposed to diffuse light. It was found that the previous history of 
the leaf, especially as regards nutrition and temperature changes, 
is very important in determining the amount of assimilation. The 
significance of this we shall refer to later. 
For each experiment a fresh leaf was employed and this was kept 
with the cut end of its stalk in water in order to keep down loss of 
water from the leaf through transpiration. The leaf was’contained in 
a chamber through which air containing a known quantity of carbon 
dioxide was passed at a known rate. Analysis of the outflowing gas 
gave the necessary data for determining the intake of carbon dioxide. 
As a source of light, incandescent gas or Keith high pressure gas 
was used. For the work described in the second paper, sunlight 
alone was used. 
It is assumed that assimilation and respiration take place 
simultaneously in the leaf, and for this purpose the respiration at 
each temperature was obtained by measuring the output of carbon 
dioxide in the dark. On adding this to the value of the ‘apparent 
assimilation,’ the ‘true assimilation’ is obtained. 
At higher temperatures the respiration is more difficult to 
estimate, for oscillations occur much too big to be accounted for 
by experimental errors. Also, during an assimilation experiment, 
the respiration is constantly changing on account of the assimilation. 
An approximation to its value was therefore obtained by measuring 
it in the dark before and after an assimilation experiment and taking 
the mean value. 
Another complication has to be taken into account when high 
intensities of light are used. It was recognised by Brown and 
Escombe (1905) that light falling on a leaf would bring about a 
rise in temperature of the leaf, and they endeavoured to calculate 
this from a knowledge of other conditions of the leaf. Blackman 
and Matthaei (1905) show that the values obtained by Brown and 
Escombe depend on the values of six other quantities which are 
not all known, and hence the value obtained by calculation is not 
very likely to be correct. 
Blackman and Matthaei therefore made direct measurements 
of the internal temperature of the leaf by means of small thermo¬ 
couples of copper and constantan. One junction was embedded 
in the midrib of the leaf, and the other kept in a water bath. The 
internal temperature of the leaf was measured by bringing this water 
