SUMMARY OF PAR ATONIC MOVEMENTS 523 



This simple illustration will help us to understand many of the phenomena 

 of stimulation. Pressure on the button no more provides the energy for moving 

 the clapper of the bell than does contact in the case of a tendril, a shock in the 

 case of the leaf of Mimosa, or gravity or light in geotropic and heliotropic move- 

 ments respectively, carry out the work that is ultimately accomplished. It is 

 quite true that when geotropism was first studied, attempts were made to prove 

 that gravity was the actual source of the energy expended in the curving which 

 ensued. What in the case of geotropism took many years and much hard work 

 to demonstrate, was obvious on the face of it in the case of heliotropism. No one 

 dreamt of suggesting that sunlight pulled the stem towards it and pushed the 

 root away. Even the original explanation of heliotropic curvature given by 

 De Candolle, and which we have described as a mechanical explanation, is not 

 mechanical in the sense we now mean, for it always assumes a ' stimulating 

 effect ' of light. 



Looking first at heliotropism by way of illustration, the heliotropic curvature 

 or released response might be compared to the clanging of the bell in our mechanical 

 illustration. The bell's capacity may also be regarded as mechanical ; we can 

 readily understand that such an apparatus, on account of its structure, is able to 

 function in this way and in no other when an electric current affects it ; on the other 

 hand we do not know how it is that a plant curves when unilaterally illuminated, 

 although we must assume that this phenomenon is the necessary result of the 

 ' mechanical structure ' of the plant, just as a clanging noise is the necessary 

 consequence of the structure of an electric bell. That the active force in the 

 case of the plant is turgor or growth and in the case of the bell electricity has 

 nothing to no with the matter. The releasing force in the one case is pressure 

 on the key, in the other case sunlight ; with the sensitive apparatus in the 

 machine we are fully acquainted but we know nothing of it in the organism. 

 We can only say where the sensitive apparatus is in the plant, for we know that 

 in many instances it is closely associated with the reacting region and in others 

 that it is situated at some distance from it. Its structure apparently lies outside 

 the limits of microscopic investigation and hence we can do nothing more than 

 guess at its mode of operation. The same is true of all other stimulus phenoniena ; 

 as to the structure and mode of action of the sensitive apparatus we are quite in 

 the dark, although the conditions under which it carries out its functions have 

 been more or less accurately determined. 



Very many plants exhibit curvatures due to the influence of other stimuli, 

 which differ from heliotropic curvatures in no respect or only in trivial details. 

 Still, according to the nature of the stimulus, we have distinguished these move- 

 ments as geotropic, chemo tropic, thermotropic, and so on. These curvatures 

 owe their origin to all appearance to the same mechanical structure as the heho- 

 tropic movements, but we must assume that the perceptive apparatus must 

 have a different structure in each case, adapted to the reception of the particular 

 stimulus in question, just as a different kind of key will be required in the 

 electric bell apparatus according as pressure, electricity, light, &c., is the 

 releasing agent employed. 



Just as we meet with similar reactions in the plant accompanied by dis- 

 similar perception, so the converse, also, holds good, for we find dissimilar response 

 resulting from similar external influences. Thus the root responds positively, 

 but the stem negatively, to gravity ; again the same external factor may induce 

 curvature in one organ and torsion in another, and the same differential illu- 

 mination which induces curvatures may influence the symmetry of a plant, so as 

 to induce the formation of new organs on one side rather than another, as, for 

 instance, in the development of roots, roothairs, or sexual organs on the shaded 

 side. There are two possible suggestions which we might offer by way of 

 explanation of these phenomena, i. We might assume that the plant possesses 

 only one kind of perceptive organ by which it appreciates each individual 



