48 THE VITAL FUNCTIONS. 



French bean (Phaseoliis vulgaris) during the night 

 and the day, he found it to be much more con- 

 siderable at night ; an effect which it is natural to 

 ascribe to the interruption in the action of the 

 leaves when they are deprived of light, and when 

 the corresponding 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 exudation, 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 experi- 

 ments on the Mercurialis cmtma, the Senecio vul- 

 garis, and Brassica campcstris, 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 vessel. After some 

 days, during which th^ plants continued to vegetate 

 tolerably well, the water in tl\e latter vessel being 

 examined, was found to contain a very perceptible 

 quantity of the acetate of lead. The experiment 

 was varied by first allowing the plant to remain 

 with its roots immersed in a similar solution, and 

 then removing it, (after careful washing, in order 

 to free the roots from any portion of the salt that 



