8 Hints on Vegetation. 



phical speculation, may yet be found productive of important practi- 

 cal results. 



1. Of the Agents which are necessary or useful to Vegetation. 



1. Earth. It is sufficiently obvious, that a soil is necessary, not only 

 for the purpose of enabling plants to grow steady and upright, and 

 thus to receive whatever nourishment the air affords, but also for 

 containing in it, the most essential parts of the food on which they sub- 

 sist, which they extract from the soil by their roots. 



Many aquatic plants indeed grow in water, but the greater part of 

 them have their roots in earth. Marine plants grow upon bare rocks, 

 or stones ; but then it is well known, that they are fixed to them, and 

 that they derive their food from the sea-water in which they live. 

 Some authors have contended, that plants are actually nourished by 

 the earth alone ; and Tull, in his Treatise on Horsehoeing, published 

 in 1733, advanced the doctrine, that minute earthy particles supply 

 the whole nourishment of the vegetable world *. But such an idea is 

 sufficiently refuted by various experiments, made with great care, and, 

 in particular, by the small portion of earth that is found in plants ; so 

 small indeed, that the very water which they imbibe, from the par- 

 ticles of earth which it contains, can account for it. It is only those 

 particles of earth, held in solution, which plants can imbibe, and the 

 earth itself is, in no other respect requisite for the growth of plants, 

 than as a laboratory to prepare their food, and as a bed in which their 

 roots may range, and procure their sustenance, Duhamel thought, that 

 by dividing the soil as completely as possible, any number of crops 

 might be raised in succession from the same land ; but he was soon 

 convinced, that his opinion was erroneous, and that no single material 

 afforded the food of plants. 



2. Air. This agent is another essential requisite to the growth 

 of plants, and is as necessary for their healthiness and existence, as 

 for that of man. Pure air is likewise required, to bring various plants 

 to perfection ; for there are many fruit trees, and vegetables, that will 

 not grow in the contaminated air of London, or its immediate neigh- 

 bourhood, but will thrive at some miles distance. Some respectable 

 philosophers indeed, have entertained an idea, that atmospheric air is 

 the sole, or at least the principal food of plants. It is contended, that 

 trees, whose branches are extended in the air, were not furnished with 

 foliage for ornament merely, and that their leaves are analogous to 

 the lungs of animals, and act in the same way. The advantages of fal- 

 lowing also, according to some, prove, that the earth extracts nourish- 

 ing particles from the air, and is fertilised by its influence. Such doc- 



* M. Giobert's experiments are decisive as to this point, for having mixed 

 pure earth of alum, silex, calcareous earth and magnesia in various propor- 

 tions, and moistened them with water, he found that no grain would grow in 

 them ; but when they were moistened with water /rtfwt a dunghill, corn grew in 

 them prosperously. Hence the necessity of the carbonaceous principle, for the 

 purposes of successful vegetation, is apparent. See Kirwan's Essay on Ma- 

 nures and the Food of Plants, p. 40. 



