ON THE MODE OF ACTION OF HEAT ON PLANTS. 



181 



to these causes of cooling. The series of observations made in 

 the garden of the Horticultural Society of London * has been 

 with thermometers covered with black wool: the one in the sun, 

 the other in the shade, compared with ordinary thermometers in 

 the shade. M. de Gasparin,f wishing to place the thermometers 

 more in the situation in which plants themselves are, or at least 

 the upper roots of plants, covered the bulbs with a millimeter 

 (about 3V °f an inch) of earth. By all these processes the 

 monthly mean temperatures have been greater in the sun than 

 in the shade by 4° at the most near London, by 15° at the most 

 at Orange : these figures, however, depend much upon the 

 methods employed in each case. 



It appears to me useless to discuss which of the above two 

 apparatus is the best. I consider them both as bad when applied 

 to vegetable life. Nobody, indeed, can imagine that the sur- 

 faces of branches and leaves are warmed in the sun or radiate in 

 the shade like this or that thermometer. They are solid bodies, 

 into which heat penetrates slowly, and we compare them with 

 liquid mercury in which the heated molecules are set in motion ! 

 The surfaces are green, with a mixture sometimes of brown, 

 yellow, or other colours, and we compare them with uniform 

 surfaces, sometimes very different from green ! Shining leaves 

 often reflect a portion of the light, and we compare them with 

 the round bulb of a glass thermometer, or to black wool, which 

 reflects no light at all ! In the plant the cold of the night does 

 not cause it to withdraw within itself the leaves and flowers 

 whicli have been formed during the day ; alternations destroy 

 nothing; yet we compare it with a thermometer where the fall 

 of the mercury is calculated in reduction of its rise ! Lastly, 

 all physiologists know that the chemical properties of solar rays 

 have an immense effect on vegetable tissue ; for it is their 

 chemical action (independently of heat) which causes the carbonic 

 acid gas to decompose, and a great deal of water to evaporate by 

 the opening of the stomates. A ray of light, almost without 

 heat, certainly has its influence. It is, therefore, very useful 

 to have some measure at once of the calorific and of the chemical 

 effects of the sun's rays. 



I conclude that the only logical method of measuring the effect 

 of the surfs rays upon plants is by the observation of the vege- 

 tables themselves : that is to say, the comparison of their develop- 

 ment, firstly \ in the shade and in the sun ; and, secondly, under 



* Published in the Transactions of the Society. Mr. Dove has calculated 

 the means from 1826 to 1840 monthly, converting the degrees of Fahrenheit 

 into the Centigrade scale. See Dove, Ueber den Zusammenhang der Atmos- 

 phare mit der Entwickelung der Warmeveranderungen der Pfianzen, Berlin, 

 1846. 



t Cours d' Agriculture, vol. ii. p. 72. 



