FUNCTION.] HOOT EXCRETIONS. 183 



the extremity of their roots. In modern times more exact 

 inquiries into this subject have been made by Macaire, who, 

 in a paper in the Transactions of the Physical Society of 

 Geneva, has given an account of his experiments : He states 

 that Chondrilla muralis, and Cichoraceous plants in general, 

 secrete a matter analogous to opium ; Leguminous plants, a 

 substance similar to gum, with a little carbonate of lime ; 

 Grasses, a minute quantity of matter consisting of alkaline 

 and earthy muriates and carbonates, with very little gum ; 

 Papaveraceous plants, a matter analogous to opium; and 

 Euphorbias, a whitish yellow gum, and resinous matter of an 

 acrid taste. 



He also found that plants possess the power of freeing 

 themselves from matter that is deleterious to them, by means 

 of their roots. Acetate of lead is a well-known active vege- 

 table poison ; he took two bottles, one of which, A, was filled 

 with pure water, and the other, B, with water holding acetate 

 of lead in solution. He placed a plant of Mercurialis annua 

 with half its roots plunged in A, and the other half in B. 

 After a short time the water in the bottle A contained a 

 notable proportion of acetate of lead, which must have been 

 carried into the system by the roots in bottle B, and thrown 

 off again by those in bottle A. He also states that various 

 plants which had lain several days in water charged with 

 lime, or acetate of lead, or nitrate of silver, or common salt, 

 in small quantity, having been carefully washed and placed 

 in pure water, gave back from their roots the deleterious 

 matter they had absorbed. 



At one time these supposed facts were expected to throw 

 light upon what is called the rotation of crops : and some 

 writers have gone so far as to reason in detail upon them. 

 But other experiments, like those of Macaire, only performed 

 with greater precautions, have not led to the same result ; 

 and root secretions are now regarded as unimportant, if not 

 altogether apocryphal, except in cases where the roots are 

 wounded. 



M. Payen has ascertained (Ann. des Sc., n. s., iii. 18.) that 

 the roots of plants contain a large proportion of azotised 



