380 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY 



of 35 C. In the tropics fleshy leaves are quite commonly raised above 

 50 C. by prolonged insolation, and may be injuriously or even fatally 

 affected, for the cooling effect of the attendant rise of transpiration is limited 

 by the supply of water. Hence arises the common practice in tropical 

 plantations, especially when young (Coffee, Cocoa, &c.), of planting rapidly 

 growing shade-trees at intervals to ward off the midday sun ; and the 

 same effect is produced by the usual mode of planting Vanilla, whose 

 fleshy leaves are still more sensitive than those of such a plant as Hoya 

 carnosa^. Thin leaves, on the other hand, have relatively more surface 

 for radiation and transpiration, and in addition their gaseous exchanges 

 are much more rapid than those of fleshy leaves. 



Colour, hairiness, and position are all factors of considerable importance 

 in determining the rise of temperature due to insolation. Green and other 

 coloured leaves may absorb from fifty to ninety per cent, of the sun's rays 

 falling upon them, while when a leaf places its lamina at right angles 

 to the incident rays the absorption of heat is naturally greatly increased. 

 A thick covering of hairs largely intercepts the incident rays, and only 

 a small fraction of the heat absorbed by the hairs is transferred by con- 

 duction to the body of the leaf. The excretion of ethereal oils by lowering 

 the diathermanicity of the surrounding air may help to cut off the heating 

 effect of the sun's rays to a certain extent, but the amounts excreted even 

 by the most active plant are not sufficient to have much effect, in spite 

 of the extreme efficiency of these vapours for that purpose 2 . 



Cork and bark are bad conductors of heat, but a thin layer of cork 

 can be penetrated by sufficient radiant rays to raise the temperature of 

 a young branch considerably. Even when a thick layer of cork is present 

 the heat conducted inwards may make the sunny side of a tree 20 C. 

 warmer than the shaded one, and the centre of a tree exposed for some 

 time may rise to over 40 C. in temperature 3 . 



Contact with cold water naturally removes heat more rapidly than 

 contact with equally cold air, and in both cases the movement of the 

 medium accelerates the loss of heat, to which the effect of air-currents 

 in accelerating transpiration is to be added. Similarly, during cold clear 

 nights the uninterrupted radiation makes the plant colder than when the 

 sky is covered by clouds which hinder radiation. The temperature among 

 the grass of a meadow may be 6 or 8 C. lower than that of the air above 

 during night-time 4 , and in fact it is even possible for plants to be killed by 



1 Cf. Ewart, On the Effects of Tropical Insolation, Annals of Botany, 1897, Vol. xi, p. 444. 



8 Detto, Flora, 1903, p. 161 ; Volkens, Sitzungsb. d. Berlin. Akad., 1886, p. 78. 



3 Cf. Ihne, Bot. Centralbl., 1883, Bd. xv, p. 231 ; Miiller-Thurgau, Landw. Jahrb., 1886, Bd. xv, 

 p. 531 ; R. Hartig, Forstl. naturwiss. Zeitschrift, i892,Heft iii, pp. 10, 12 ; Prinz, Bot. Jahresb., 1894, 

 Bd. I, p. 226; Biisgen, Bau tmd Leben d. Waldbaume, 1897. 



* Boussingault, Agronom., China, agricoleet Physiol., 1861, T. n, p. 380; Tyndall, Fragments of 

 Science, 1879, Vo1 - x P- 9 5 Muller-Thurgau, 1. c., 1886, p. 557 ; Th. Hormen, Bot. Ztg., 1894, p. 277. 



