THE NATURE OF IRRITABILITY 17 



either as regards their sensitiveness, or the varied nature of their different! 

 irritabilities. Man, for example, is unable directly to perceive the ultra- } 

 violet rays which many plants readily detect, and to which they markedly 

 respond. 



The excitation of any given irritability by a stimulus may produce 

 either a transitory or a permanent effect. In the former case, as is well 

 illustrated by the sensitive leaflets of Mimosa, as the effect of the stimulus 

 passes away, the self-regulatory mechanism induces the organ to return to 

 its original position of equilibrium ; whereas in the latter case a change in 

 the external or internal conditions produces some permanent adaptive 

 alteration, calling forth a new condition of equilibrium suited to the changed 

 conditions 1 . Thus, when a heliotropic curvature is produced, if the direction 

 of the incident light and other external conditions remain constant, the 

 stem adopts the new direction of growth, and as the mechanical tissues 

 differentiate the curvature is rendered permanent. 



Similarly, a continuous stimulus produces a continuous effect when a rise 

 of temperature causes the recommencement of growth in a plant which has 

 been in a condition of cold rigor, or increases the rapidity of growth where 

 previously it was slow. The rise of temperature acts only indirectly as 

 a stimulus, arousing or accelerating the processes of growth, which the plant 

 itself regulates, and which are possible only when a supply of energy is 

 available. The direct action of a rise of temperature is shown in the 

 case of a fire, when the heating of the coal to combustion-point incites 

 a renewed liberation of energy ; but a marked direct action of heat can, 

 however, hardly be possible in a plant, considering how low is the 

 highest temperature to which it can respond. 



The continued stimulating action of warmth is as essential for the 

 maintenance of the life of the plant as a continued light stimulus is 

 necessary to induce and maintain a heliotropic curvature. A progressive 

 diminution in the intensity of light produces a decreasing and finally im- 

 perceptible effect, and similarly as the temperature is lowered, so also does 

 the vital activity which is dependent upon it diminish and ultimately cease. 



The action of a stimulus is perceptible only during the transition from 

 the previous condition of equilibrium to the new one, and the plant may 

 remain stationary in this new condition of equilibrium, although the stimulus 

 continues to act. Indeed the stimulating action of a certain temperature, 

 and of other agencies as well, is an absolute necessity for the continuance of 

 life. This view was first correctly enunciated by Johannes Muller 2 ; and 

 the lack of recognition it has received may be taken as a sign that a clear 



1 Transition stages exist between these two types. 



2 J. Miiller, Handbuch d. Physiologic d. Menschen, 1844, 4- Anfl., p. 28. [John Brown (Elements 

 of Medicine, London, 1788) regarded life as the result of an aggregate of stimulatory reactions.] 



