ESTTERNAL TEMPERATURE OF LEAVES. 249 



The readings of the black-bulb-in- vacuum thermometer are 

 given and show a slight progressive increase in the intensity of 

 the sun's radiation. Readings of the humidity are given, as 

 change in this factor is likely to affect transpiration consider- 

 ably. These readings show fairly uniform conditions through- 

 out, though on the whole the humidity decreases a little. It 

 will be seen that the conditions with respect to movement of air 

 vary considerably, so that it is only on two or three occasions 

 that still air conditions were obtained. This renders compari- 

 son of the results difficult. To get a reliable comparison it 

 would be best to take the average of a series of readings in 

 each case. Since, however, no attempt was made to measure 

 the conditions as to rate of air movement, it might be mis- 

 leading to do so, because the amount of breeze in one series 

 might not be comparable to the amount in another. It seems 

 best to compare the readings obtained in still air. If we do 

 so, we get stomata inwards 168 C above surrounding air, 

 stomata outwards 15*7 C. above surrounding air, stomata 

 inwards 18*8 C. above surrounding air. The increase from 

 16 '8 to 18*8 may be put dox^Ti to the increase in intensity of 

 radiation (see figures for black bulb thermometer) as the 

 morning advanced. Taking the average of these two 17*8, 

 it is 2*1 above the highest reading for stomata outwards. 

 This difference may be fairly attributed to the diminishing of 

 the transpiration in the former case. It will be seen that a 

 difficulty occurred, with the apparatus used, in comparing 

 two leaves or sets of leaves, in that the conditions of air 

 movement and intensity of sun's radiation often changed very 

 rapidly, and in a way which could not easily be measured. It 

 might occur, therefore, that during the time occupied in 

 setting up a leaf for comparison with one previously used the 

 conditions had changed considerably. For instance, a breeze 

 may have sprung up or the sun have become more or less 

 obscured. Under such conditions comparative experiments 

 were useless. In order to get over this difficulty, two sets of 

 apparatus were henceforth used. The leaves to be compared 

 could be set up side by side, and thus conditions were equalized. 

 Since only one galvanometer was available, the readings could 

 not be absolutely simultaneous, and the method was to take 



