NUTRITION OF VEGETABLES. 21 



kept a fish in pure v;ater, till it grew too large for the vessel x© 



containing it, and by other similar experiments on different "' *^ 



animals. It would even seem, that food acts on the stomach p^^g ^^^^j ^^^ 



in a great measure as mould does on the roots of plants, on the stomach 



merely retaining water in such a state of division, as to fit it JJ^^Jfij jj^ ^^^ 



for absorption and assimilation. roots of plants? 



From what has been said it appears, that foreign matters Matter-; dis- 



dissolved in water only check the progress of vegetation ; but ^n^J^^^jol" To^'^'^ 



that the vital powers can sometimes surmount these obstacles, vegetation. 



appropriating only the pure water, that held these matters in 



solution. 



If experiments founded on lono: practice were still neces- Absence of so- 

 1 1 -1 • 1 1 /> luble matter 



sary to prove, that the soil is so much the more proper tor advantageous, 



vegetation in proportion as it is deprived of soluble foreign as appears 

 matter, I would mention the practice of paring and burning ^f p^rin^ and 

 wastes, used cliiefly in England. Lands thus treated remain burning j 

 in heart a long time ; the parts where the heaps of surface 

 mould were burned are most fertile ; and manure even ap- 

 pears to be injurious, by causing the wheat to run chiefly to 

 straw, with thin ears, and those of bad quality. 



This extraordinary effect of torrefaction on the soil ap- ^^^I'ch proba- 

 pears to me attributable to the combustion of those excre- strovint the ^ 

 mentitious matters, which are ejected by the roots of plants, matter excret- 

 When the soil is impregnated with these matters, which are roots of plums. 

 eminently injurious to vegetation, the perfect developement 

 of plants is prevented. This excretion from the roofs is evi- This excretion 

 dent from the surrounding earth, which becomes unctuous, f"ct^. '°^^ 

 and sometimes of a darker colour. In several of the euphor- 

 biuras and cicoraceous plants it is very perceptible, and 

 milky. It may be observed too, that roots, when they multi- 

 ply under water, become covered with a glairy matter, which 

 deserves to be examined ; and which no doubt the earth 

 would have absorbed, had the roots remained buried in it. 



It is to these excretions from the roots perhaps we must fre- ^"^ prob biy 



^, .,,-,,„ • 1 1 . the cause whv 



quently ascribe that kind of antipathy between certain some plants ' 



plants, which are never found together. Thus the thistle is particularly in- 

 injurious to oats, euphorbium and scabious to flax, elecam- {j^^ers 

 pane to carrots, fleabane and dnrnel to wheat, &:c. 



It would cei^ainly be wrong, to ascribe the fertility of land Effects of par- 

 pared and burned to the charcoal produced in this operation ; Jr^g and burn- 



r ing cannot be 



