•50 VEGETATION. 



rendering it more friable, or in hastening the putrefac- 

 tion of noxious plants ; or, it is owing to the abstrac- 

 tion and accumulation of oxygen. And in the case of 

 plowing and trenching, it is owing to the increased fa- 

 cility with which the roots can penetrate to a proper 

 depth. 



But it often happens, that the soil can no longer be 

 ameliorated by any of the foregoing means, and in this 

 case, there must be a direct and actual application made 

 to it, of such substances as are fitted to restore its fer- 

 tility. And hence the indispensable necessity of ma- 

 nures, which consist chiefly of animal and vegetable 

 remains, that are buried and finally decomposed in the 

 soil, from which they are afterwards absorbed by the 

 root of the plant, in a state of solution." 



7. HEAT. — Besides the qualities of the earth, there 

 are other agents, which maintain an unbounded influ- 

 ence over the vegetation of plants. One of these is 

 heat, and without it no plants have been made to grow, 

 though some require a greater degree of it than others. 

 They all require sufficient while vegetating, to pre- 

 vent the congelation of their fluids, and the tempera- 

 ture at which this takes place, their freezing point de- 

 termines the extent of their power to resist the opera- 

 tion of cold. The vital energy of equinoctial plants, is 

 not calculated to resist its agency, and therefore they 

 cannot endure the temperature of freezing water. 



They require to be protected during the winter, 

 and if originally brought from the southern hemisphere, 

 they ought to be exposed to a good degree of artificial 

 heat, or they lose their native beauty and fragrance. 

 But the vegetables of Polar regions possess an aston- 

 ishing power of resisting the operation of cold, which 

 enables them to brave the inclemency of every season. 



