in their Structure and Functions, ^ l|^l 



specific effects upon the brain and general system through 

 the sentient extremities of the nerves, and through these 

 only; and that, when introduced into the current of circu- 

 lation in any way, their effects result from the impression 

 made upon the sensible structure of the blood-vessels, and 

 not from their direct application to the brain itself. Since, 

 then, it is proved, that certain poisons can act upon the ani- 

 mal body in no other way than by affecting the functions of 

 the nervous system ; and since it is further proved that they 

 occasion their specific effects upon the general frame through 

 the medium of the nerves, and through these alone ; and since 

 these poisonous agents produce the same injurious effects 

 upon vegetables that they produce upon animals ; I think we 

 have a right to infer that plants not only possess a nervous 

 system, but that they possess one very much resembling that 

 which exists in the animal body. Again, it is well known 

 that both plants and animals have the faculty of preserving 

 a certain degree of temperature, let that of the medium in 

 which they are placed be what it may. For instance, the 

 temperature of the interior of the stem of a tree will seldom 

 sink below 56^, although that of the atmosphere be not higher 

 than 20°. The human body never has its temperature re- 

 duced below 98° or 96°, not even if surrounded by an atmo- 

 sphere cold enough to effect the freezing of quicksilver. Now, 

 there can be no doubt that the heat of vegetables is produced, 

 in a great measure, by various chemical processes going on 

 within their different organs : yet it is very clear that it must 

 arise also from other causes ; for it continues to be generated, 

 though in a less degree, even in winter, when every chemical 

 action within the plant is almost entirely suspended. Some 

 have supposed that at this season it is transmitted, through 

 the roots, from the earth in which the plant is growing. But 

 if this were true, how does it come to pass that we sometimes 

 find the water immediately surrounding the roots and their 

 spongioles in a frozen state, while that within them and within 

 the stem is quite fluid ? and how comes it to pass that plants 

 situated on the side of rocks, whose roots, from the deficiency 

 of soil, are almost as much exposed as their branches, possess 

 as much warmth within the interior of their stem, when the 

 thermometer stands at 30° below zero, as those whose roots 

 are deeply buried in the earth? Animal heat, like that of 

 plants, likewise depends very much upon a chemical process, 

 viz. the combination of the oxygen of the air with the carbon 

 of the blood, forming carbonic acid. It is not, however, 

 derived entirely from this source. The experiments of Brodie 

 and of Sir E, Home show that it is to a considerable extent 



