22 INTRODUCTION 



gradually. The effect of a blow is different from that of a slowly in- 

 creasing pressure, not only from a mechanical point of view, but also 

 in respect to its stimulatory action : thus a steel band may break when 

 suddenly bent, although it may undergo a very marked curvature without 

 breaking when the pressure upon it is gradually increased. 



It is not surprising that the mechanical or stimulating effect of a high 

 temperature, want of nourishment, &c., may be resisted for a short time, 

 although a more prolonged action may produce injury or death. Indeed 

 death inevitably supervenes, if the conditions are such that growth and 

 increase are impossible. 



As has been already pointed out, every vital manifestation is dependent 

 upon the specific nature of the living organism, and it is the inherent 

 characteristics and peculiarities of the organism which determine whether 

 any response to a given stimulus is possible, and the character that this 

 shall assume. In the same way, results which have been primarily induced 

 by external influences are finally determined by the inherent internal 

 properties of the organism, independently of the fact that the interactions 

 between the external and internal conditions may either have been merely 

 mechanical or of the nature of co-ordinating stimuli. 



A particular combination of external conditions is essential for the 

 continued existence and for the vital activity of the plant. Within these 

 physiological limits, all external conditions influence every activity of which 

 the plant is capable in a manner corresponding to their intensity and 

 duration. The same general relationships exist between the causes of 

 a given change and the nature of that change, in every substance, living 

 or dead : thus it is the inherent properties of a piece of iron, or of 

 a machine, which determine whether a physical or chemical agency will 

 produce any effect upon them, and what the action will be. Hence any 

 alteration in the result obtained is an infallible sign that, in some way 

 or other, an alteration of the original conditions of equilibrium has taken 

 place. 



In living organisms, and also in the developmental history of our 

 planet, much less has been achieved by sudden, and hence more con- 

 spicuous, actions or changes, than by more gradual processes which, 

 although imperceptible at any given moment, are nevertheless able by their 

 continued action to produce final results of altogether disproportionate 

 magnitude. 



Cases in which the reaction is rapid or instantaneous, as when the 

 leaflets of the sensitive plant fold together, are of more special character, 

 and such reactions are of less general importance than the numerous slow 

 and gradual regulatory reactions which are constantly occurring. Since 

 stimuli are responsible for the production of the necessary supply of energy, 

 usually solely by calling the self-regulatory mechanism into action, and as 



