BOOTS CAN SELECT THEIR FOOD. 27 



in a sufficiently attenuated form. Their feeding property- 

 depends upon the mere hygrometrical force of their tissue, set 

 in action in a peculiar manner by the vital principle ; this 

 force must be supposed to depend upon the action of capillary 

 tubes, of which every part of a vegetable membrane must, of 

 necessity, consist, although they are, in all cases, invisible to 

 the eye, even aided by the most powerful microscopes. 

 Whatever matter is presented to such a set of tubes will, we 

 must suppose, be attracted through them, provided its 

 molecules are sufficiently minute ; and, as we have no reason 

 to believe that there is, in general, any difference in the size of 

 the molecules of either gaseous matter or fluids consisting prin- 

 cipally of water, it will follow that one form of such matters 

 will be absorbed by the roots of plants as readily as another. 

 For this reason, plants are peculiarly liable to injury from the 

 presence of deleterious substances 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. 



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 abundance of silex in solution from the earth, and the 

 Pea will absorb little or 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. Daubeny has proved 

 that Pelargoniums, Barley, and the Winged Pea (Tetragono- 

 lobus) wUl - not receive strontian ; and it is mentioned by 

 Saussure, that he could not make Polygonum Persicaria 

 absorb, by its roots, a solution of acetate of lime, although it 

 took up muriate of soda (common salt) freely. 



It is a curious fact that the poisonous substances which are 

 fatal to man are equally so to plants, and in nearly the same, 

 way. So that, by presenting opium or arsenic, or any metallic 

 or alkaline poison, to its roots, a tree may be destroyed as 

 readily as a human being. 



