82 EXPERIMENTS OF DE SAUSSUHE. 



v^hen the sap descends, such as are unfit to contribute to their support, 

 or would be liurtful to them if not rejected from their system. He further 

 supposes that, after a time, the soil in which a certain kind of plant 

 "rows becomes so loaded with this rejected matter, that the same plant 

 refuses any longer to flourish in it. And, thirdly, that though injurious 

 to the plant from which it has been derived, this rejected matter may be- 

 wholesome food to plants of a different order, and hence the advantage to 

 be derived from a rotation of crops. 



There seems no good reason to doubt that the roots of plants do at 

 times — it may be constantly — reject organic substances from their roofs. 

 The acetic acid given off during germination, and the same acid found 

 by Braconnot in remarkable quantity in the soil in which the poppy 

 {papaver somniferum) has grown — may be regarded as sufficient evi- 

 dence of the fact — but the quantity of such organic matter hitherto de- 

 tected among what may be safely viewed as the real excretions of plants, 

 seems by far too small to account for the remarkable natural results at- 

 tendant upon a rotation of crops. 



Tlie consideration of these results, as well as of the general theory of 

 such a rotation, will form a distinct topic of consideration in a subsequent 

 part of these lectures. I shall, therefore, only mention one or two facts 

 which seem to me capable of explanation only on the supposition that 

 the roots of plants are endowed with the power of rejecting, and that 

 they do constantly reject, when the sap returns from the leaf, some of 

 the substances which they had previously taken up from the soil. 



1°. De Saussure made numerous experiments on the quantity of ash 

 percent, left by the same plant at different periods of its growth. Among 

 other results obtained by him, it appeared — 



A. That the quantity of incombustible or inorganic matter in the dif- 

 ferent parts of the plant was different at different periods of the year. 

 Thus the dry leaves of the horse chestnut, gathered in May, left 7-2 per 

 cent., towards the end of July 8-4 per cent., and in the end of Septem- 

 ber 8-6 per cent, of ash; the dry leaves of the hazel in June left 6-2, 

 and in September 7 per cent. ; and those of the poplar (populus nigra) 

 in May 6-6, and in September 9-3 per cent, of ash. These results are 

 easily explained on the supposition that the roots continued to absorb 

 and send up to the leaves during the whole summer the saline and 

 earthy substances of which the ash consisted. But — 



B. He observed also that the quantity of the inorganic substances in 

 — or the ash left by — the entire plant, diminished as it approached to 

 maturity. Thus the dry plants of the vetch, of the golden rod {solida- 

 go vulgaris), of the turnsol {helianthus animus), and of wheat, left res- 

 pectively of ash, at three different periods of their growth, [Davy's 

 Agricultural Chemistry, Lecture HI,] — 



Before flowering. 

 per cent. 



Vetch 15 



Golden rod ... 9-2 



Turnsol .... 14-7 



Wheat .... 7-9 



This diminution in the proportion of ash, might arise either from an 



increase in the absolute quantity of vegetable matter in the plants ac- 



