54 



plants : it is productive of beneficial effects in carrying 

 off the noxious vapours, which may although unseen, and 

 guarded against, still float in the atmosphere ; and there 

 can be little doubt that another beneficial influence which 

 it exercises, results from the motion which is produced 

 by a body of air changing its position, which probably 

 promotes circulation, and increases the excitability of 

 the plants. 



Since therefore a change of the volume of the atmos- 

 phere in plant houses, is productive of benefit, and the ad- 

 mission of a large body of cold air, is at the same time so 

 decidedly objectionable, it is important, that in endeavour- 

 ing to secure the benefits of the practice, the injuries which 

 are liable to result, should if possible be avoided. The 

 regulation for the admistion of air, which is described in the 

 second chapter of this treatise, may be regarded as being 

 of some importance in this respect, as well as in the 

 provision which it includes, of supplying the heated air, 

 with a due proportion of moisture. 



Physiologists tell us, that plants derive a considerable 

 proportion of their food, directly from the atmosphere, by 

 a process similar to the inhaling of animals ; and that the 

 substances thus derived, are carbonic acid, ammonia, and 

 water, which contain the elements of organic matter in 

 considerable proportions. The influence of the atmos- 

 phere is exerted beneficially, by its constituents entering 

 into combinations with other matters, which are taken 

 into the system by the roots, and spread out and exposed 

 in the leaves : this exposure has so far the effect of alter- 

 ing the character of the substance carried up from the 

 roots, that it is no longer a body of crude juice, but is 

 undergoing a process of elaboration, and is being assimi- 

 lated with the superincumbent tissue of the plant. There 

 seems to be no reason why those particular gaseous bo- 

 dies which plants appropriate to themselves from the at- 

 mosphere, should not to a great extent be supplied to 



