12 CHYMISTRY APPLIED TO AGRICULTURE. 



It is by this, that transpiration and the absorption of 

 nourishment from the atmosphere are carried on. If we 

 should reverse the order of things, and present the under 

 surface of a leaf to the rays of the sun, we should very 

 soon see that it would make great efforts to resume its 

 natural position. 



When a plant is dead, or rather when an annual plant 

 has fulfilled its destiny, giving assurance of its reproduc- 

 tion by the formation of its fruit, the action of heat and of 

 the other chymical agents is no longer modified by any of 

 the causes of which I have just spoken, and the plant re- 

 ceives their impression in an absolute and unmodified 

 manner. • When the temperature of the atmosphere sinks 

 below a certain point, the fluids in plants become con- 

 densed, the movement of the juices is retarded, the 

 activity of their organs languishes, and is at length sus- 

 pended, until restored by the return of heat. The action 

 of the atmosphere upon plants, when deprived of its due 

 proportion of heat, is however modified by the emission 

 or disengagement of caloric, which is always given out 

 when liquids are condensed, or solids contracted; and 

 this occasions the temperature of plants, during the winter, 

 to be always a little higher than that of the atmosphere. 



It sometimes happens that the temperature of the at- 

 mosphere sinks so low, as to produce fatal effects upon 

 plants by freezing their sap, and thus occasioning their 

 death. This effect does not always depend upon the in- 

 tensity or degree of cold to which they are exposed, but 

 upon particular circumstances. I have seen olive trees 

 resist a temperature of 22° .2 Fahrenheit, and perish from 

 that of 28°. 6, because in the last case the snow, which 

 had collected upon the branches of the trees during a 

 night, was dissolved the following day by the heat of the 

 sun, and the wet tree was exposed during the succeeding 

 night to the action of 28°. 6. There is nothing more dan- 

 gerous for corn and grasses, than those frosts which follow 

 immediately after a thaw, because the still wet plants, not 

 being deeply rooted in the ground pulverized by the frost, 

 have no means of defending themselves from the effects of 

 the cold. 



3. Sennebier was the first to admit that the influence of 

 light was hurtful to the germination of seeds. Ingen- 

 houz confirmed this opinion by actual experiment; but 

 M. de Saussure, who caused grains to germinate under 



