Vegetable Physiology. 139 



But the roots of all plants, at the period of germination, must be in- 

 the same stratum, and of course be equally dependent upon it ; and 

 besides, the culture of the farmer turns up and mixes the various 

 layers of the soil together, so as to render them, in all probability,^ ho- 

 mogeneous. It is known also that plants of the same family, such as 

 clover (trefle) and lucerne do not prosper in succession, although their 

 roots are of very different lengths. 



The true explanation of the necessity of rotation, appears to be 

 founded on the fact stated by Brugmans, and more fully exposed by 

 De Candolle, that a certain portion of the juices which are absorbed 

 by the roots of plants, are, after the salutiferous portions have been 

 extracted by the vessels of the plant, again thrown out, by exudation, 

 from the roots and deposited in the soil. It is probably the exist- 

 ence of this exuded matter, which may be regarded, in some meas- 

 ure, as the excrement of the preceding crop of vegetables, that 

 proves injurious to a succeeding vegetation. It has been compared to 

 an attempt to feed animals upon their own excrements. The parti- 

 cles which have been deleterious to one tribe of plants cannot but 

 prove injurious to plants of the same kind, and probably to those of 

 some other kinds, while they may furnish nutriment to another order 

 of vegetables.* 



The author endeavored to subject these theoretic views to the test 

 of experiment. After various attempts to raise plants in pure sili- 

 ceous sand, pounded glass, washed sponge, white linen, &,c. he de- 

 cided upon pure rain water. After cleansing and washing the roots 

 thoroughly, he placed them in vials with a certain quantity of pure 

 water. After they had put forth leaves, expanded their flowers and 

 flourished for some time, he ascertained, by the evaporation of the 

 water, and the use of chemical reagents, that the water contained 

 matter which had exuded from the roots. He satisfied himself that 

 this is the fact with respect to nearly all the phanerogamous plants. 



Several plants of Chondrilla muralis, perfectly clean, were placed 

 with their roots in pure water. At the end of a week, the water was 

 yellowish and emitted an odor like opium, and had a bitter taste. 

 Sub-acetate and acetate of lead produced a brownish flocculent pre- 

 cipitate, and a solution of gelatine disturbed its transparency. As a 



* I have been assured, by farmers, of a fact somewhat analogous in relation to an- 

 imals. Hay, which has been left in the manger of a horse, or which has otherwise 

 received the impregnations of his breath, will not be touched by another horse, but 

 will be freely eaten by cows or sheep. — G. 



