16 INTRODUCTION 



although the apex of a root may be fully sensitive, no curvature can 

 take place if no further growth is possible, or if the curving region is 

 imbedded in a gypsum cast. The stimulus is perceived, but no response 

 can be exhibited. 



Every plant, and even every single organ, is sensitive to more than one 

 stimulus. In addition to the special functional irritability with which the 

 protoplasts of any organ may be endowed, a more general irritability must 

 also exist, by means of which the processes essential to life are carried on 

 and regulated. Both the general and the special irritability may be affected 

 simultaneously, and thus a multiple response be produced ; as when traction 

 applied during a geotropic curvature causes an increased thickness of the 

 cell-walls affected, or when an injury produces an increased respiratory 

 activity or causes streaming movements of the protoplasm in the cells of 

 the curving organ. Similarly, light may induce formative changes in the part 

 illuminated, in addition to the heliotropic curvature which may be induced. 



It is clear that a special manifestation of irritability for which a par- 

 ticular organ is especially adapted and intended may be brought into play 

 by many different stimuli. Thus either mechanical, chemical, or photical 

 stimulation may cause the leaflets of Mimosa pudica to fold together, but 

 the more specially adapted the organ or organism, the greater will be the 

 tendency to produce a particular response. It is therefore permissible to 

 speak of ' specific irritability' * when we intend to indicate such adaptive 

 preference. An organ or organism could not possibly continue to exist if 

 it responded to all stimuli only in one and the same way. 



Thermal, chemical, and photic stimuli may not only all cause in 

 a stem the same kind of curvature, but may simultaneously excite more general 

 irritabilities, and produce other results varying very widely in character. 

 Moreover it is self-evident that a plant which is attached to its substratum 

 can only move towards the source of stimulation by effecting a curvature, 

 as is the case, for example, when it bends towards the incident light. 



All organs in which a particular stimulus produces the same definite result 

 are said to be endowed with a special irritability, independently of whether 

 one or more of such irritabilities are present. It is only in this way that 

 we can understand why different plant organs react only to particular 

 stimuli, although they may all have the same powers of movement. Thus 

 one organ may be sensitive to geotropic, heliotropic, and hydrotropic stimuli, 

 another only to geotropic, while a third may only respond to heliotropic 

 stimulation. 



Irritability depends as little upon external differentiation as does life 

 itself. Even the lowest plants cannot be considered as inferior to animals, 



1 This applies not only to the sense in which the expression is used by Johannes Mitller, but 

 also to its usage in connexion with higher animals. 



