44 THE VITAL FUNCTIONS. 



responding absorption by the roots is also suspended. This 

 was confirmed by the result of some experiments he made 

 on the same plants by placing them, during day time, in the 

 dark, under which circumstances the excretion from the 

 roots was found to be immediately much augmented: but, 

 even when exposed to the light, there is always some exu- 

 dation, though in small quantity, going on from the roots. 



That plants are able to free themselves, by means of this 

 excretory process, from noxious materials, which they may 

 happen to have imbibed through the roots, was also, proved 

 by another set of experiments on the Mercurialis annua, 

 the Senecio vulgaris, and Brassica campestris, or common 

 cabbage. The roots of each specimen, after being thoroughly 

 washed and cleaned, were separated into two bunches, one 

 of which was put into a diluted solution of acetate of lead, 

 and the other into pure water, contained in a separate ves- 

 sel. After some days, during which the plants continued 

 to vegetate tolerably well, the water in the latter vessel 

 being examined, was found to contain -a very perceptible 

 quantity of the acetate of lead. The experiment was va- 

 ried by first allowing the plant to remain with its roots 

 immersed in a similar solution, and then removing it, 

 after carefully washing, in order to free the roots from 

 any portion of the salt that might have adhered to their 

 surface, into a vessel with rain water; after two days, 

 distinct traces of the acetate of lead were afforded by the 

 water. Similar experiments were made with lime-water 

 and with a solution of common salt, instead of the ace- 

 tate of lead, and were attended with the like results. De 

 Candolle has ascertained, that certain maritime plants which 

 yield soda, and which flourish in situations very distant 

 from the coast, provided they occasionally receive breezes 

 from the sea, communicate a saline impregnation to the 

 soil in their immediate vicinity, derived from the salt which 

 they doubtless had imbibed by the leaves. 



Although the materials which are thus excreted by the 

 roots are noxious to the plant which rejects them, and would 

 consequently be injurious to other individuals of the same 



