COMPOSITION OF PLANTS. 



227 



350. 



The attraction of sap from the earth, differs in the same 

 plants in the different periods of their growth. In the first 

 periods, before the plant has blossomed, a greater quantity of 

 nourishment is taken up, the more the plant is covered with 

 leaves; because we have already seen (321.) that the evaporation 

 promoted by the contracting power and irritability which the 

 plant maintains, is a very important function, and one which 

 must therefore strengthen its absorbing power. But we must 

 take into account, that, by means of the more powerful fron- 

 descence, the ground is more shaded, and carbonic acid is de- 

 posited in greater quantity. Perhaps we ought also to take 

 into account the evacuation of superfluous sap from the roots, 

 (285.) It is certain that any species of grain succeeds so 

 much the better among the stubble of a previous crop suitable 

 to it, as, for instance. Wheat after Clover or Pulse, the more 

 luxuriant that previous crop had been, which must be as- 

 cribed to the shadowing of the soil ; so that clover in general 

 promotes the fertility of the soil, the more luxuriantly it 

 springs. It is possible that this proceeds from the more live- 

 ly attraction of sap from the soil, by means of which exactly 

 such a quantity of the latter crop is called up, as is best for 

 the crop of grain which is to succeed it. We must also take 

 into account the extractive matter which a plant imparts to the 

 soil by its decay ; in comparison with which, the portion of 

 matter which the plant takes from the soil is scarcely worth 

 reckoning. At least a strong and rank fallow may always be 

 regarded as a manure. The thick grass sward, which is 

 produced on the soil after a long rest, increases its fertility in 

 an uncommon degree. 



It is an undeniable fact, however, that a plant draws the 

 greatest quantity of nourishment from the soil, at the time 

 when it blossoms and forms its fruit. On a superficial con- 

 sideration, we might have supposed the reverse. We might 

 have thought, that as the period of growth ends with the 

 blossom, a more powerful absorption could not take place 

 than formerly ; but when we reflect, that the blossoming and 

 setting of the fruit are entirely new operations which increase 



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