44 VEGETATION. 



up "by the plant in proportion to their degree of solu- 

 bility in water and to the quantity in which they exist 

 in the soil. Now one of the most important of these 

 substances is vegetable extract. 



When plants have attained to the maturity of their 

 species, the principles of decay begin gradually to op- 

 erate upon them, till they at length die and are con- 

 verted into the dust from whence they sprang, thus 

 resembling the animal to whom they afford support. 

 The substance to which they are finally converted has 

 been denominated vegetable mould.* And this, as might 

 be expected, constitutes a considerable proportion of 

 the soil. The chance then is, that it is again convert- 

 ed into vegetable nourishment, and again enters the 

 plant. But it cannot wholly enter the plant because it 

 is not wholly soluble in water. Fart of it, however, is 

 soluble and consequently capable of being absorbed by 

 the root, and that is the substance which has been de- 

 nominated extract. But the quantity of extract that 

 may be separated from pure mould formed by nature 

 upon the surface of the globe is not in general very 

 considerable. And yet it frequently seems to be more 

 than sufficient for the purposes of vegetation ; for a 

 mould containing an excessive quantity was found by 

 Saussure to be less fertile than that which contains an 

 ordinary share. But if the quantity of extract must 

 not be too much, neither must it be too little. Plants 

 that were put to vegetate in mould deprived of its ex- 

 tract, as far as repeated decoctions could deprive it, 

 were found to be much less vigorous and luxuriant 

 than plants vegetating in mould not deprived of its ex- 

 tract ; and yet the only perceptible difference between 

 them is, that the former can imbibe and retain a much 

 greater quantity of water than the latter. 



