Temperature. 71 



picking were kept for 24 liours before experimentation at fairly 

 constant temperature, witli 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 deteimining 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 values so obtained by calculation are 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 



