186 



ON THE MODE OF ACTION OF HEAT ON PLANTS. 



nately insects, and a rather high flood, destroyed the greater part 

 of the plants raised in the shade. Perhaps I may be enabled 

 hereafter to publish some of the observations of M. Moquin, when 

 I give the whole of my own, in a work on botanical geography, 

 upon which I have for some time been engaged. 



The advantage of the proposed method is the obtaining a mea- 

 sure of the solar action on plants, by means of the plants them- 

 selves, and the recording the effect observed in ordinary degrees 

 of the thermometer. The plants raised in the shade form the 

 connecting link, the means of reduction, as it were, between the 

 plants raised in the sun and the ordinary thermometer kept in the 

 shade. This is certainly much better than exposing the thermo- 

 meter to the sun, for in the latter case, notwithstanding all the 

 different processes imagined, no one can say that the thermometer 

 receives the influence of the sun in the same way as the plants. 



It would appear desirable that experiments analogous to my 

 own should be made in countries differing from each other in 

 elevation and in latitude, as well as in nebulosity or clearness of 

 atmosphere. We should then obtain a knowledge of how much 

 should be added in each locality to the monthly means for the 

 solar heat, which is neglected in the ordinary observations made 

 in the shade. I am persuaded that a comparison made, for in- 

 stance, between England* and Eastern Europe, would show that 

 the thermometrical mean temperatures, observed in the shade, do 

 not fairly express the comparative nature of the climates in an 

 agricultural point of view. As essays of this kind are multiplied, 

 it will be ascertained which species of plants are best adapted to 

 the showing clearly the effects of temperature. It may also be 

 found that thermometers covered with wool, or with sand, or 

 those exposed in some other manner to the sun's rays, will give 

 results the nearest to those obtained from the plants them- 

 selves. In that case such thermometers may be used without 

 scruple, which would be more convenient for observations on 

 vegetables. 



I will not conclude without stating that these experiments have 

 considerably modified my views of the mode of action of external 

 circumstances, of heat more especially, upon plants. I have some- 

 times, with many physiologists, committed the error of regarding 

 the plant as a species of thermometer. It is a faulty comparison, 

 which leads into error. I repeat it, the low ering of the temper- 

 ature does not destroy in the plant the effect w hich a rise had 



* In any observations made in England or in the United States it is 

 evident that 32 degrees must be deducted from all quantities noted in degrees 

 of Fahrenheit. The use of this thermometer is a great bar to the ready 

 comprehension of facts in vegetation observed by persons not well instructed. 



