^ INTERNAL TEMPERATURE OF LEAVES. 247 



Table XIX.— January 23, 1906. 



Dead Leaf (brown and brittle) of Magnolia sphenocarpa. 



„, , .. Calculated 



Time. Conditions. m ' ^ ^ Temperatui-e Difference. 



Temperature. o^Leaf. 



A.M. 



10 .42 . . Leaf in sun (little 



breeze) .. 26 -I 



11. 2 .. Do. .. 26-3 



11. 7 .. Leaf in Sim (air still) 26-5 

 11 . 25 . . Leaf in sun (little 



breeze) .. 26-6 



11.29 ..Do. .. 26-6 



11.34 .. Junction only shaded 26-8 



11.40 ..Do. . . 27- 1 



1 1 . 43 . . Shade removed . . 27 



45-5 .. 19-4 



48-7 .. 22-4 



SOO .. 23-5 



49-2 ... 22-6 



49-7 .. 231 



35-7 .. 8-9 



43-1 .. 16-0 



48-8 .. 21-8 



It is thus seen that such a leaf attains in the sun a tempera- 

 ture of 23° C. above that of the surrounding air, and reaches 

 50° C. This is a temperature which would be injurious to the 

 living leaf, and the fact that the living leaf does not attain to 

 it may be put down to two causes : (i.) the living leaf, on 

 account of its colour being green, not dark brown, absorbs 

 less of the sun's radiation, i.e., it is more transparent ; (ii.) 

 the living leaf is cooled by the loss of water in transpiration. 

 An attempt was made to estimate the effect of transpiration 

 in another way. This consisted of placing the thermo- 

 j unction between two leaves placed one on top of the other 

 with their edges bound together in a cardboard frame fastened 

 with clips. The small space between the leaves in which the 

 thermo-junction lay was thus a closed space. 



When the leaves were placed with their stomatal surfaces 

 facing each other, transpiration was largely diminished. It 

 was difficult to fit up the leaves, so that their edges should be 

 closely pressed together all round forming a quite .closed inner 

 space. In actual experiments the leaves were only approxi- 

 mately so placed, so that a small amount of communication 

 between the inner air space and the outer air was possible. 

 The leaves so placed were contrasted with a pair of leaves, 

 whose stomatal surfaces faced outwards, thus allowing of free 

 transpiration from these surfaces. On the whole the contrast 

 is between pairs of leaves, alike in other respects, in one of 

 which there is free stomatal transpiration, and in the other this 

 transpiration is very largely diminished. 



