CHAP. I. VITAL PROPERTIES AND STIMULANTS. lp 



not appear to have produced any derangement or disor- 

 ganisation in their tissue. But it has been argued that, 

 as these latter poisons act upon the nervous system of 

 animals, we may suspect something analogous to this 

 system to exist in vegetables also. A long list has 

 been given of substances which act as poisons on 

 plants ; and it has been ascertained that very nearly 

 all such as are deleterious to animal are so likewise to ve- 

 getable life, and many others besides, which animals may 

 take with impunity. Some of those which it is necessary 

 to administer in large quantities in order to produce 

 death in animals, are sufficiently powerful to kill plants 

 when given in very small doses as alcohol, ethers, 

 and oils ; whilst on the other hand, the oxides of lead 

 and zinc, which poison animals when administered in 

 small portions, produce little or no effect on plants, 

 probably because they are incapable of being absorbed 

 by the spongioles. Most vegetable extracts and ex- 

 cretions act as poisons on all plants (even upon those 

 from which they were obtained) when they are imbibed 

 by the roots. Gases diffused in water are harmless. 

 Many salts are highly noxious, but most of the salts of 

 lime produce no effect. Fortunately for the permanence 

 of vegetation on the surface of the earth, the natural 

 poisons are not very generally diffused in places where 

 plants are likely to grow. 



(151.) Periodicity. In tracing the various ana- 

 logies which exist between the phenomena of animal 

 and vegetable life, we find a remarkable example in 

 what may be termed the individual temperament, or 

 idiosyncracy of a living organic being. Besides that 

 general resemblance between the manner in which the 

 same functions are performed by all individuals of the 

 same species, there are certain modifications in the re- 

 sults which are peculiar to particular individuals, and 

 which must be attributed to some peculiarity in their 

 temperament. This is remarkably exhibited in the 

 differences observable among separate individuals of 

 the same species, as regards their periods of leafing 



