OF THE ATMOSPHERE OF HOT-HOUSES. 299 



suggests itself, whether, in the atmosphere of hot-houses gener- 

 ally, these essentials to the growth of plants be suitably provided. 

 By chemical research, we find that nitrogen forms only a 

 small portion of plants, but it is never entirely absent from any 

 part of them ; even when it is not found in any particular organ, 

 it is found to be present in the fluids that pervade it. Many 

 experiments have been instituted, with the view of ascertaining 

 expressly, by what particular organs nitrogen entered into the 

 plant, and in what form it enters. Indeed, this is a question 

 which at present occupies much attention. It is well known 

 that the leaves of plants absorb gaseous elements largely from 

 the atmosphere, both free and in a combined state, and we might, 

 therefore, expect that some of the nitrogen of the air would, by 

 this channel, be admitted into their circulation. This view, 

 however, is not confirmed by any of the experiments heretofore 

 made, with the view of investigating the action and functions of 

 the leaves. We are not at liberty to assume, therefore, that 

 any of the nitrogen which plants contain, has in this way been 

 derived directly from the atmosphere. It may be the case, but 

 it is not yet proved. There is little doubt, however, that nitro- 

 gen enters the roots of plants, in a state of solution ; but the 

 quantity they thus absorb is uncertain ; it is supposed to be 

 small, and must be variable. Therefore, by whatever organs it 

 finds an entrance into plants, and in whatever quantity it may 

 be present, the question still remains, that it is the ammonia of 

 the atmosphere that chiefly furnishes nitrogen to plants. 



3. In a former part of this treatise, while treating on the 

 subject of heating, by means of fermenting manure, we have 

 alluded to the extraordinary effects of ammonia upon plants. It 

 is unnecessary^ at present, to recapitulate what has already been 

 said on that interesting point. It has, we think, been clearly 

 established, that the difference between a hot-bed of manure, and 

 that heated by any other means, does not lie. in the quality of 

 the heat generated ; as we know full well that a hot-bed of 

 manure, warmed beyond a certain point, will burn the roots 

 of plants as quickly as one heated by any other method to the 

 same temperature ; nor does it consist in any life-giving proper- 

 §6 



