£48 



PjaYTONOMY 



There remains nothing further, therefore, than to assume 

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

 sap and strengthened by light, 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 e:?j:ternal 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 iti those which are reared in hot-houseS| 

 completely establish this opinion, (298.) 



377. 



To confirm our conclusion respecting the higher life q£ 

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

 sisting the external influences of heat and cold, — althougb 

 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, grow 

 briskly, where the air is warmed ^bove the boiling point. 

 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 a,tmQspherp, — so 

 much so, that this internal temperature of some trees seen^s 

 never to fall below 52°, and never to rise above 75°. 



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

 bodies withstand external cold ^nd 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- 



