246 SMITH : 



In this, the thinnest leaf used, the fall during the first minute 

 was more rapid still. In thi-ee minutes the leaf had practically 

 attained the temperature of the new conditions. Thus, in 

 the contrast between the ten minutes taken by the succulent 

 leaf and the three minutes taken by the thinner leaves, we 

 seem to have a purely physical effect due to the greater value 

 in the former of the proportion ^^1^— - Where there is 

 a greater mass to be heated up in proportion to the surface 

 receptive of radiation, a longer time is taken to reach the 

 higher temperature and the same holds for the reverse process 

 of cooling. If the mass is small in proportion to the surface, 

 both higher and lower temj)eratures are reached more quickly. 

 These facts, however, do not seem to produce any permanent 

 difference between the two classes of leaves with respect to the 

 temperature to which they attain even under varying condi- 

 tions of sunshine and shade, as in the case of the thinner leaves 

 the more rapid cooling is balanced by the more rapid heating. 

 It will be showTi later what a powerful agent in reducing leaf 

 temperature movement of air is, and it is important to notice 

 that in this respect succulent leaves are much less affected by 

 the breeze than thinner leaves. (See Table XVI. , 11 . 21 a.m.) 

 This difference must be of considerable importance under 

 natural conditions, since a slower cooling during air movements 

 must tend in the direction of keeping the succulent leaf at a 

 higher temperature than that of the thin leaf. 



Estimation of Effect of Transpiration on 

 Temperature. 

 Theory indicates (for a full discussion of the point, see Brown 

 and Escombe (4) ) that the loss of water by transpiration is a 

 most important factor tending to lower the temperature of 

 transpiring leaves. In order to get some idea of this effect 

 quantitatively, the internal temperature of a dead and brittle 

 leaf of Magnolia was ascertained. This leaf does not, it will 

 easily be seen, present the same case as a living leaf, minus the 

 transpiration effect. The colour is quite changed from green 

 to a dark brown, and no doubt the dead leaf is less transparent 

 to radiation on that account. It may nevertheless be of some 

 interest to give the figures obtained. 



