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Mr Smith, Physiology of Plants in the Tropics. 
Physiology of Plants in the Tropics. By A. M. Smith, B.A., 
Emmanuel College. (Communicated by Mr F. F. Blackman.) 
[Read 20 May 1907.] 
(a) The Internal Temperature of Leaves under Tropical 
Insolation. 
The temperature was obtained by a thermo-electric apparatus 
reading to *5° C. 
In still air, with Black Bulb Vacuum Thermometer at from 
55° — 62° C., the air temperature in the shade being from 25° — 28° C. 
and the humidity about 70°/ o , leaves whether thick and fleshy 
( Tradescantia discolor ) or thinner and somewhat coriaceous 
Magnolia sp., Aniherstia nobilis ) or quite thin and pliable 
(. Acalypha sp.), when placed normal to the sun’s rays, reach 
a temperature of 15° C. above the surrounding air, i.e. 40° — 43° C., 
a temperature hitherto often considered injurious to the functions 
carried on in the leaf. 
In the shade the internal temperature varies from 1'5° below 
to 4° above that of the surrounding air under different conditions. 
Of factors tending to reduce this high temperature the movement 
of the air is the most important. Breezes of various strengths 
reduced the temperature in the sun by amounts varying from 
2° — 10° C. 
An attempt was made to estimate the magnitude of the 
cooling due to transpiration. Two leaves with stomata outwards 
were consistently lower in temperature than two with their 
stomatal surfaces facing each other. The difference was on an 
average 2 5° C. 
Investigations into leaves with red or red-brown colouring 
matters showed that the red pigment raised the internal 
temperature of the leaf from 2° — 4° C. above similar leaves which 
were white or nearly so. 
Young coloured leaves of thin texture would probably be 
cooler than mature green leaves of the same species owing to 
more rapid loss of water. The presence of the red pigment, 
however, causes their temperature to be almost equal to or even 
higher than that of the mature leaf in different cases. 
(/3) Periodicity of growth in Ceylon. 
Monthly observations on the time of growth of certain 
evergreen trees with young foliage coloured red or brown showed 
