^28 rHYTOCHEMY. 



the contraction in the whole body, and raise its entire irri- 

 tability, we must be sensible, that this is the time when the 

 expenditure of sap must be the greatest, although we should 

 not take into consideration the fresh exhalation from tlie blos- 

 soms, or the evolution of azote ^nd hydrogen from them. 



In fact, all our experience, both on a great and on a small 

 scale, confirms this remark. In horticulture, it is known, that a 

 plant requires the most powerful irrigation, when it puts on 

 its blossoms and fruit. In agriculture, it is kno^\^l, that it 

 makes an important difference whether the juicy or the ripen- 

 ed stalk remains in the ground. The latter exhausts it so 

 much the more powerfully, as the dead roots are no longer 

 able to give out again the superfluous sap. It is certain, 

 that from this cause Flax in particular exhausts the soil so 

 much, that no winter crop at least can be obtained after it. 

 It even seems to be true, that the richer the crop of grain is, 

 and the more nutritious its parts, the more is the soil exhaust- 

 ed by it ; not that the moisture of the earth passes directly in- 

 to the fruit, but because a greater expense of power, and a 

 more lively elasticity in all the parts, is required for the for- 

 mation of the nutritious parts of the fruit, so that the ab- 

 sorption of fluid sap must also be brisker. Einhoff's calcula- 

 tions teach us, that among common plants, French Beans con- 

 tain the greatest quantity of nutritious matter, namely 85 

 per cent. ; Wheat contains 78, Pease 75 J, Lentils 74, Rye 

 70, Barley Qo, and Oats 58. From this we may judge of 

 the exhausting power which these plants exert upon the 

 soil. 



At the same time, it cannot be denied, that plants with 

 strong and deep-seated roots, or with juicy tubercles, take a 

 great quantity of nourishment from the ground, even when 

 they are not in flower. Among these we may reckon Beet 

 and Potatoes. Recent experiments have taught us, how 

 much the former of these roots contributes to the manuring 

 of the land, when it is suffered to remain in it, and when, 

 in the end of harvest, the sheep are enclosed in a field of this 

 plant after its leaves are shed, and time is allowed them to 

 gnaw its roots, and to manure the field. These putrefying 



