44 PHYSIOLOGY OF NUTRITION. 



All analytical work must be performed away from direct sun- 

 light. Bat when the Carbon dioxide has been led into the eudio- 

 meter we may expose the apparatus for some hours to direct 

 sunlight, in order to ensure very vigorous assimilation. Imme- 

 diately after exposure, the leaf is removed from the eudiometer, 

 being rotated a little as it passes through the mercury, in order 

 to liberate any bubbles of air which may be clinging to it. After 

 about two hours, when the apparatus has quite cooled down, we 

 read off the gas volume, and introduce some potash solution from 

 a small pipette, by warming the pipette with the hand while 

 keeping it closed at the top. After absorption of the Carbon 

 dioxide which has escaped decomposition, the volume of the gas 

 is once more determined. 



For experiments concerning gas exchange in assimilation, leaves 

 of Prunus laurocerasus or Nerium are suitable. Before intro- 

 duction into the eudiometer, the leaves must have been exposed 

 to the light so that they may contain no absorbed Carbon dioxide 

 in their tissues. The leaves are left in the eudiometer exposed 

 to the light for three to six hours, which is long enough, 

 especially in direct sunlight, to ensure decomposition of a large 

 quantity of the Carbon dioxide introduced. As regards the 

 amount of Carbon dioxide to be led into the eudiometer, it may 

 be remarked that 6-8 c.c. is sufficient. In many cases, when viz. 

 the intensity of the sunlight to which the leaves are exposed is 

 very considerable, it is desirable to hang over the eudiometer a 

 double-walled bell-glass filled with water (see Fig. 9), to prevent 

 undue heating of the leaf and of the gas in the apparatus. Or, if 

 the light is too intense, we may shade the apparatus to some extent, 

 for which purpose paper screens serve very well. If we desire 

 to study the influence of coloured light on the rate of decom- 

 position of Carbon dioxide, we suspend over the eudiometer 

 double-walled bell-glasses filled with coloured fluids, and prevent 

 access of light from below, say, by means of black oil-cloth. (See 

 Pfeft'er's cited treatise.) 



We are at present, however, particularly interested only in the 

 behaviour of leaves in mixed white light. Under the influence of 

 such light notable quantities of Carbon dioxide are decomposed 

 in a short time, and if we carefully carry out our investigations in 

 the manner described, we shall be convinced that the volume of 

 gas remaining in the eudiometer after exposure of the leaves to 

 sunlight is as large as before exposure. Very small differences, 



