176 The Influence of the Soil 



substance ; those of leguminous plants, a gum ; those of gra- 

 mineous plants, a gummy substance containing salts. Upon 

 plunging one of the roots of a Mercurialis in acetate of lead, 

 whilst the other was steeped in distilled water, he very soon 

 discovered in the latter, manifest traces of the presence of this 

 salt of lead. These experiments are however subject to two 

 very strong objections : — 1st, it is impossible that in freeing 

 the roots from the earth which surrounded them, some entan- 

 gled filaments should not have been broken ; in which case 

 the juices contained in the vessels themselves would have es- 

 caped, and it is not surprising that the writer should have 

 found them : 2ndly, the transmission of the acetate of lead 

 through the roots, is easily explained by the mere capillary 

 action of the root itself. To provide against these inconveni- 

 ences, I have selected plants whose roots do not adhere to the 

 soil, as, for example, Lemna minor. Three grains of acetate 

 of lead were dissolved in four ounces of pure water, and about 

 forty small plants of the Lemna placed in the solution : on 

 the eighth day many of them had become yellow. These 

 were separated from the others, and frequently washed in pure 

 water, and afterwards in distilled water, until the latter ceased 

 to indicate the presence of the acetate of lead. They were 

 then placed in a glass containing two oimces of distilled wa- 

 ter, but the water did not give the least sign of the presence 

 of the salt ; nevertheless, it had penetrated the tissue of the 

 plants, for when hydrosulphate of ammonia was applied to 

 them, they became entirely black. A microscopic examina- 

 tion farther shewed, that this blackness was communicated 

 even to the membrane of the cellules ; besides which, it had 

 coloured all the cells of the lower surface, while those of the 

 upper one were only partially affected ; which leads me to 

 suppose that even the frond of the plant had absorbed the poi- 

 son, by the surface in contact with it. 



Wishing to make a contrary experiment, I placed some 

 plants of Lemna in a solution of hydrosulphate of ammonia, 

 and then removed them into water containing acetate of lead 

 in solution ; the liquid was not at all disturbed by the process : 

 and here my experiments terminated. Those of M. Daubeny 

 made with different substances, have led to analogous results. 

 It follows from these experiments that plants do, to a certain 

 degree, select the substances with which they find themselves 

 in contact ; but that those substances which are absorbed, are 

 so, simply in the direct ratio of their quantity. Thus we must 

 admit that water, air, and earth, exercise a real and sensible 

 influence upon the character of vegetable life. Hence arises 

 the presence of certain species in localities whence others are 



