514 EXPERIMENTS ON THE ROOTS OF PLANTS. 



expresses himself, p. 167- " M. Brugmans, having placed some plants 

 in dry sand, saw some small drops of water exude from the extremity 

 of the radicles." And further on, in p. 91 : " In fine, the roots 

 themselves in some plants present particular secretions ; this may be 

 observed in the Carduus arvensis, the Inula Helenium, the Scabiosa 

 arvensis, several Euphorbias, and several of the Succories. It appears 

 that these secretions of the roots are only parts of the juices, which not 

 having served for nourishment, are rejected when they arrive at the 

 inferior parts of the vessels. Perhaps this^phenomenon, which is not 

 easily perceived, is common to a great number of plants. MM. 

 Plenck and Humboldt conceived the ingenious idea of seeking from 

 this fact the cause of certain habits of plants. , Thus, we know that 

 the thistle is injurious to oats, the Euphorbia and Scabiosa to flax, the 

 Inula betulina to the carrot, the Erigeron acre and tares to wheat, &c. 

 Perhaps the roots of these plants give out a matter which is hurtful to 

 the vegetation of others. On the contrary, if the Ly thrum salicaria 

 grows freely near the willow, and the branching Orobanche near the 

 hemp, is it not because the secretions from the roots of these plants 

 are beneficial to the vegetation of the others ?" 



Extending these ideas still further, and applying them to the theory 

 of the rotation of crops, both in his public lectures and in his Vegetable 

 Physiology, M. De Candolle admits, that every plant, in ejecting all 

 the moisture that extends to the roots, cannot fail to eject also such 

 particles as do not contribute to nourishment. Thus, when the sap 

 has been spread by circulation throughout the vegetable, elaborated and 

 deprived of a great quantity of water by the leaves, and then rede- 

 scending has furnished to the organs all the nourishment it contained; 

 there must be a residue of particles Avhich cannot assimilate with the 

 vegetable, being improper for its nourishment. M. De Candolle 

 asserts that these particles, after having traversed the whole system 

 without alteration, return to the earth by the roots, and thus render it 

 less proper to sustain a second crop of the same family of vegetables, 

 by accumulating soluble substances that cannot assimilate with it j in 

 like manner, he observes, as no animal whatever can be sustained by its 

 own excrement *. Besides, it may also follow that the action even of 

 the organs of a vegetable converts the mixed particles into substances 

 deleterious to the plant which produces it, or to others, and that a 

 portion of this poison be also rejected by the roots. Some refinements 



* Yet, the ostrich and cassowary always devour what falls from them. E0. 



