100 A FEW OBSERVATIONS ON THE PHYSIOLOGY OF PLANTS. 



serves to afford to the root of the plant mechanical support, it acts as a 

 medium for the conveyance of moisture, and is the source of certain 

 fixed elements, which are found essential to the perfection of vegetable 

 growth. The presence of water is absolutely necessary, for without 

 it, plants neither grow nor live ; a definite quantity seems important ; 

 this quantity, however, is less than is generally supposed. The atmo- 

 sphere is the source whence plants derive their chief supply of food, 

 which exists there in different forms. Thus, aqueous vapour occurs 

 in the air in very large quantities, and descending in the form of rain, 

 mist, &c, supplies the plant with moisture ; dew, also, another form 

 in which water is deposited from the air, exerts a powerful effect on 

 vegetation ; how far the water held in solution in the air is directly ap- 

 propriated by plants, has not been satisfactorily determined. The car- 

 bonic acid of the air is the chief source of the carbon of plants ; their 

 azote or nitrogen, also, is principally obtained from the air, where it 

 exists in different states, namely, in the form of ammonia, nitric acid, 

 &c, and also in the uncombined states. Other substances, also, 

 which affect vegetation, are found in the air, as common salt, parti- 

 cularly near the sea ; and as it is highly probable that all the solid 

 elements of the earth exist in very small quantities in the air in a state 

 of vapour, it is possible that some of them may exert an influence on 

 vegetation with which we are totally unacquainted. The influence of 

 light, heat, and electricity, upon the vitality of plants is well known 

 to be indispensable for their healthful growth ; it is also probable 

 that the action of such on the soil and on manures is highly important, 

 though the question of their peculiar mode of action is involved in 

 much doubt and obscurity. Some of the effects of light on vegetation 

 are well known ; for instance, it is the cause of colour in plants j 

 those grown in the dark being blanched or colourless. The effect of 

 light over the vital or chemical actions taking place within the sub- 

 stance of plants, is strikingly shown by the fact that, when exposed to 

 its influence, they exhale from their leaves, &c, oxygen gas, whereas 

 in the dark they exhale carbonic acid gas. The scientific views at 

 present entertained about the growth of plants, are by no means of 

 that settled character which would induce the horticulturist to rely on 

 them as true guides in practice ; indeed, it may be observed, that dif- 

 ferent, and even opposite views, are now held upon some of the first 

 principles of vegetable physiology, and this by men of the greatest 



