INTERNAL TEMrERATTTRE OF LEAVES. 295 



anthooyan would aid in promoting this transpiration. Much 

 of what has been suggested in the latter part of the paper is 

 tentative and provisional. In two directions especially the 

 evidence is in great need of further extension; namely, (1) in 

 the extension of observation of the facts of new rapid growth 

 in evergreen trees in the tropics, and (2) in further chemical 

 investigation of the reserve stores in such trees. The former 

 investigation is now impossible for the writer of tliis paper, 

 but with regard to the latter, at any rate, so far as relates to 

 the relative distribution of the chief mineral foods in new 

 twigs and older branches, he hopes to bring forward some 

 evidence in a subsequent paper. 



In conclusion, I wish to express my thanks to Dr. F. F. 

 Blackman for suggestions and help in this investigation, es- 

 pecially with regard to Part I,, to Dr. J. C. Willis, at whose 

 suggestion the investigation forming Part II. of the paper was 

 undertaken, and to Mr. H. F.MacMillau, Curator of tlie Botanic 

 Gardens at Peradeniya, for placing at my disposal the resources 

 of the Gardens in various ways. 



Summary (Part I.). 



1. By the aid of specially designed thermo-electric apparatus 

 the internal temperature of any leaf could be obtained to an 

 accuracy of • 5° C. 



2. In still air, with black bulb vacuum thermometer at from 

 55°-62° C. , the air temperatm-e in the shade being from 25°-28° C. 

 and the humidity about 70 per cent, of saturation, leaves, whether 

 thick and fleshy, or thinner and somewhat coriaceous, or quite 

 thin and pliable, when placed normal to the sun's rays, reach a 

 temperature of 16° C. above that of the sm^roiuiding air. They 

 attain, in fact, a temperatm'e of 40-43° C. — a temperature hitherto 

 often considered injurious to the functions carried on in the leaf. 



3. In the shade such leaves have an internal temperature 

 varying from 1 '5° C. below to 4° C. above that of the siuTOim.ding 

 air under different conditions. 



4. Though the thicknes.s of the leaf did not affect the tempera- 

 ture finally reached m the sunlight, yet it did affect the time 

 taken to reach that temperature. The thickest leaf used took ten 

 uiinvites to attain its steady temperatm-e after exposure, while 

 the thinnest leaf only took three minutes. 



