238 



SMITH : 



It appears from the blank tests that the apparatus is quite 

 capable of registering the internal temperature of a leaf to an 

 accuracy of -5° C. Experiments were then devised to see if 

 different temperatures were obtained when the junction was 

 threaded through different parts of the leaf. For this purpose 

 a leaf of Magnolia sphenscarpa with fairly prominent midrib 

 and side veins was used. The junction was placed first of 

 all in the midrib, then through one of the side veins, then in 

 the lamina where there were no prominent veins. 



In considering tables V. and VI. it must be borne in 

 mind what difficulties present themselves in getting accurate 

 comparison. In the first place, as will be shown more fully 

 later, the breeze was a most powerful factor in reducing the 

 temperature of the leaves. No attempt was made to measure 

 it accurately, but it has been roughly indicated in the tables as 

 slight, moderate, or strong. It was not often possible to get 

 readings in still air. On January 22, for example, the interval 

 between 10.9 a.m. and 10.33 a.m. was filled up in taking 

 observations when some amount of breeze was blowing, and the 

 two readings given were the highest obtained in still air. On 

 January 23 no reading in still air was obtained at all, and the 

 reading given was the highest temperature obtained during the 

 observations, though even then some breeze was felt. Again, 

 as the morning advanced, the sun got more powerful, and under 

 similar cloudless conditions in still air the highest readings were 

 always obtained near to noon. The difl^erences in the above 

 table between the temperatures registered in the three positions 

 of the junction can all be ascribed to these changes of conditions , 

 and it seems that no change due to the change of position can 

 be discerned. The readings from 10.^ a.m. to 11 . 19 a.m. are 

 progressively higher as time goes on, and are just what we 

 should expect from the more powerful sunlight, so that the 

 change from midrib to side vein seems to have had no efifect. 

 The lower temperature of 38-3 obtained at 10.8 a.m. on the 

 following morning with the junction in the lamina itself is 

 very near to the 38-4 obtained at 10.9 a.m. on the previous 

 morning, and there seems no reason to doubt that, if the air 

 had been quite still and the experiment continued later, 

 temperatures as high as those of the previous day would have 



