18 VITAL ACTIONS. 



to injury from the presence of deleterious matter 

 in the earth ; and it is probable that, if in many 

 cases they reject it, it is because it does not acquire a 

 sufficient state of tenuity ; as in the case of certain 

 coloured infusions. 



36. But although this appears to be a general rule, 

 there are some exceptions of importance. If a Pea 

 and a grain of Wheat are placed side by side in earth 

 of the same kind, and made to grow under the same 

 circumstances, the Wheat plant will absorb silex in 

 solution from the earth, and the Pea will absorb 

 none ; whence it would seem that the Pea is unable 

 to receive a solution of flint into its system, and that, 

 consequently, it possesses what amounts, practically, 

 to a power of selection. In like manner, Dr. Dau- 

 beny has proved the Pelargoniums, Barley, and the 

 Winged Tea (Tetragonolobus) will not receive stron- 

 tian ; and it is mentioned in Saussure, that he could 

 not make Polygonum Persicaria absorb, by its roots, 

 a solution of acetate of lime, although it took up mu- 

 riate of soda (common salt) freely. 



37. It is a curious fact, that the poisonous sub- 

 stances which are fatal to man are equally so to 

 plants, and in nearly the same way. So that, by pre- 

 senting opium or arsenic, or any metallic or alkaline 

 poison, to its roots, a tree may be destroyed as readily 

 as a human being. 



38. The natural food of plants consists of carbon 

 in the state of carbonic acid, of nitrogen, certain 

 earths and salts, and water. The latter, if distilled, 

 has little power, by itself, of sustaining vegetable 



