1837.'] 



Extra-Tropical Plants within the Tropics, 



41 



November. On the 4th December they germinated, and on the 10th of 

 January he planted them out in trenches. During the time they were 

 in the bed he daily moistened them with water that had been exposed 

 for several hours to the heat of the sun: and, on the 11th May, a 

 bundle of red celery, sent to the Society with his communication, is 

 recorded " very fine considering the late season of growth." 



Can the opposite resultsW these experiments be referred to any fixed 

 principle, governed by the laws of organic life, which, if discovered, 

 could be successfully applied to the introduction of foreign plants 

 into tropical climates ? 



I think they can, but, to explain the principle, I must seek the aid of 

 animal physiology, from the laws of vitality being better understood, 

 and more cognizable by the senses, in animals than in vegetables. 



Animals are endowed with a principle of life, only known to us by 

 its effects, but the laws of which have been carefully studied. It has 

 received the name of Excitability or Irritability (I prefer the former 

 term) from its property of being acted on, and excited, by the applica- 

 tion of external agents : the agents are called stimulants ; and the 

 action excitement. Excitability increases in proportion as we reduce 

 excitement by the abstraction of stimuli, and diminishes with their ap- 

 plication : hence a person famished with hunger has his excitability so 

 increased, that the stimulus of the mildest food induces violent excite- 

 ment, and a person frozen will, from the same cause, be destroyed by 

 heat of a moderately warm room. On the other hand, a person in the 

 habit of maintaining considerable excitement by the habitual use of 

 stimulants, Cannot bear their privation so well or for so long a time as 

 one habitually temperate ; nor does the same quantity of stimuli pro- 

 duce in each an equal degree of excitement; on the contrary, the quan- 

 tity that would produce high fever and rapid exhaustion in the one, is 

 barely sufficient to preserve health in the other. Excitement is the 

 process by which excitability is reduced, and kept within the 

 bounds consistent with the health and well being of the animal ; and 

 by the graduated use of stimulants, we can so modify the susceptibi- 

 lities of this vital principle, that a considerable degree of stimulation 

 becomes necessary for the maintenance of the organic actions, by 

 which the functions of life, assimilation, circulation and transpiration, 

 are carried on : pr, by their abstraction we can raise the susceptibility 

 so high that a slight stimulus will cause great excitement. Excitabi- 

 lity is then equivalent to privation of stimuli, Excitement to their ex- 

 cess. Excitability being high, a slight stimulus produces violent 

 action, if low, strong stimulants produce little excitement. Let us now 

 apply these principles of animal to vegetable life, and see whether they 

 afford us a satisfactory solution of the question proposed, and prove the 



