EFFECTS OF PLANTS UPON THE SOIL. 93 



year round ; so that they constantly absorb water and ca*-- 

 bonic acid, and assimilate their constituent principles. 



The soil is always exhausted, in a greater or less degree, 

 by the plants it produces ; and much more by those that are 

 annual, than by those that are perennial. Air and water 

 alone do not afford a sufficient degree of nourishment to 

 plants, for when they have been made to grow in well washed 

 sand, watered with distilled water, though they have flow- 

 ered, their fruits did not arrive at maturity. Experiments to 

 this effect have been made by Messrs. Giobert, Hassenfratz, 

 de Saussure, &lc. 



Those annual plants which transpire most, generally 

 exhaust the soil in the greatest degree. Peas, beans, and 

 buckwheat, though they have succulent stalks, exhaust it 

 least, because they transpire but little. 



When annual plants are cut at the time of flowering, 

 they do not exhaust the soil, as their succulent roots furnish 

 materials for replacing the loss occasioned by their growth ; 

 but after having produced their fruits, the soil derives but 

 little advantage from the dry fibres which are the only re- 

 mains of their stalks and roots. 



During fructification, plants absorb but little nourish- 

 ment from the soil ; the supply necessary to the formation 

 of the seed is furnished by those juices which already 

 exist in the roots and stalks, and this occasions them to 

 become dry and exhausted, so that, when the fruit is per- 

 fected, the roots and stalks consist only of woody fibre. 

 It is necessary that this fact should be known, in order 

 that too late mowing of meadows, whether natural or ar- 

 tificial, may be avoided. The most favorable period for 

 cutting grass is that of its flowering ; if the operation be 

 postponed till the seed is formed, two great disadvantages 

 will arise ; the first is, that the fodder obtained will have 

 parted with the greater portion of its nutritive qualities ; and 

 the second, that the plants, having fulfilled all the laws of 

 their nature, by providing for their reproduction, cannot 

 flourish again with vigor during the same year. In sup- 

 port of this doctrine, I will mention one well-known fact, 

 which is, that meadows mown before fructification af- 

 ford the most abundant harvests, and the greatest num- 

 ber of them, as they may be mown several times in a year. 

 The perennial plants which serve as fodder, may by this 

 means be preserved for several years in a state of repro- 

 duction, but if mown afi;er the formation of seed, the 

 plants are weakened and the reproduction is lessened. 



