180 



ON THE MODE OF ACTION OF HEAT ON PLANTS. 



4. Temperatures below 0°, we repeat, are certainly useless for 

 all species of plants and for all their functions ; they produce no 

 effect whatever. But in thermometrical calculations we take 

 them for negative quantities, to be deducted from tempera- 

 tures above 0°. That is not considering- them as nothing ; it gives 

 them a real importance. We argue as if the plant receded 

 when temperature falls below 0°. It does not, however, recede. 

 The plant does not diminish like the column of mercury in the 

 thermometer ; it remains stationary. Thus all mean tempera- 

 tures in the data for which negative quantities are taken into 

 account are ill applied to facts in vegetation. We should 

 calculate them by substituting noughts for negative num- 

 bers ; whereas we have not in general before us meteoro- 

 logical tables containing sufficient details to enable us to make 

 the correction. 



5. Plants are almost always exposed to the direct rays of the 

 sun, and thermometrical observations, from whence the tem- 

 peratures of countries are deduced, are made in the shade. It is 

 well known that the heat of the sun's rays varies according to 

 season, geographical situation, height above the level of the sea, 

 and sundry local causes. Consequently, 10° of mean tempera- 

 ture in the shade during 10 days will correspond in one place 

 with one certain effect produced on plants exposed to the sun ; 

 in another place, or in another season, with some other greater 

 or lesser effect. 



My present object is to treat of the last of the above-men- 

 tioned circumstances, as being that which is the chief cause of 

 error resulting from the use of thermometrical means. The 

 question is not new ; but in the calculation of the direct action 

 of the sun's rays methods have been made use of which appear 

 to me but little applicable to plants, and I have endeavoured to 

 adopt a different one. It will, I trust, be admitted to be well 

 founded ; and if it presents some little difficulty in practice, if it 

 is merely sketched out in the trials I have made, it will at least 

 oblige one to reflect on the mode of action of heat on plants. 



Philosophers, who have wished to determine the solar action, 

 have always made use of thermometers exposed simultaneously 

 or successively to the sun and the shade. The differences have 

 been always considerable, and have borne relation to season and 

 geographical position ; but these differences have depended also 

 much upon the kind of thermometer, and the manner in which 

 the bulb receives the sun's rays and radiates during the night. 

 Sometimes the bulb has been covered with black wool, a sub- 

 stance which absorbs and radiates in a high degree ; sometimes 

 the thermometer has been left bare. Some have withdrawn it 

 from the influence of rain and dews, others have left it exposed 



