248 PHVTUX031Y. 



There remains nothing further, therefore, than to assume 

 a higher power in the sap-vessels, which being excited by the 

 sap and strengthened by hght, heat, and electricity, and 

 being also probably favoured by the elastic air, or by the ex- 

 halations which are produced in the spiral vessels, forces the 

 fluids upwards. The periodical change of the ascent of the 

 sap, without any stated change of the external influences, 

 must also lead to this idea, since the repeated motion of the 

 sap in our trees, during, perhaps, a similar temperature of 

 the atmosphere, and the similar repeated ascent of the sap in 

 tropical trees, and in those which are reared in hot-houses, 

 completely establish this opinion, (298.) 



377. 

 To confirm our conclusion respecting the higher life of 

 plants, we must further remark, that they are capable of re- 

 sisting the external influences of heat and cold, — although 

 many of the phenomena of this kind can partly be explained 

 by chemical effects. It is certain that plants in warm baths, 

 the temperature of which is from 150° to 180° of Fahr., and 

 that others on the brink of the margin of volcanoes, fjrow 

 briskly, where the air is warmed above the boiling ]X)int. 

 On the other hand, we see a great many trees in the po- 

 lar zone resist a cold which is from 20° to 25° of Fahr. 

 We see that the internal temperature of trees in winter is al- 

 ways higher than the temperature of the atmosphere, — so 

 much so, that this internal temperature of some trees seems 

 never to fall belov/ 52°, and never to rise above 75°. 



If we reflect on the manner in which plants and other living- 

 bodies withstand external cold and heat, we shall find that 

 this is a necessary consequence of their internal activity. By 

 means of this, evaporation must go on incessantly so long as 

 the plants are in leaf. In consequence of this function, the 

 heat that previously existed in a free state is employed in the 

 maintenance of the evaporation, and a diminution of the de- 

 gree of temperature must thus of necessity arise. To explain 

 the power by which plants resist cold, we might refer to the 

 production of an internal heat from the transition of fluid in- 



