BOTANY PART i 



other hand, place their flat surfaces at right angles to the incident 

 rays. The mode of reaction is not determined once and for all, but 

 can be profoundly modified. The tone of the plant is thus altered, 

 the change being brought about by either internal or external factors. 



The condition of receptiveness to stimuli in the plant is common to all 

 irritable movements and indeed all irritable phenomena. It largely depends on 

 external factors. The same factors that give rise to the stimulus may also 

 intensify or weaken the receptiveness. Other substances, such as the narcotics so 

 well known in animal physiology, may blunt the receptiveness. The stimulus 

 must give rise to definite changes in the plant ; the protoplasm must react to these 

 changes in such a way that the characteristic externally visible reaction ultimately 

 takes place. Between this result and the reception of the stimulus there doubt- 

 less intervene many and complicated processes which are at present but little 

 understood. The places where the stimulus is received and perceived are termed 

 sense organs or, better, organs of perception. There is particular reason to 

 distinguish organs of perception, when it can be shown that the place where a 

 stimulus is received is separated in space from the part where the movement is 

 effected. In such a case a conduction of the stimulus must take place. 



The power of perception or of sensation in the plant can be spoken of without 

 implying any subjective perception, will, or thought, as in the complicated human 

 psychology. This is unfortunately sometimes done by modern sensational writers. 

 The existence of a "soul " in the plant can neither be denied nor asserted ("). No 

 conclusion in this respect can be drawn from the fact that certain features of 

 stimulus movements take place in a similar way to our perceptions. These 

 regularities, which will be later referred to as showing a relation between the 

 intensity of the stimulus and the excitation (p. 347) are, however, of the'greatest 

 interest. 



Those movements which bring about a particular position with 

 regard to the direction of action of the stimulus may be grouped 

 together as MOVEMENTS OF ORIENTATION or TROPISMS. The other 

 movements of curvature, leading to the assumptions of definite posi- 

 tions with respect to the plant and not to the direction of the stimulus, 

 are termed NASTIC movements. 



(a) Tropisms 



In the movements of orientation we have to distinguish ortho- 

 tropous (parallelotropous) and plagiotropous organs. The former 

 place themselves in the direction of the stimulus and approach the 

 source of the stimulus (positive reaction) or move away from it 

 (negative reaction). Plagiotropous organs place themselves at right 

 angles to the direction of the stimulus or obliquely to its direction. 

 The mode of reaction of any particular organ may be changed by 

 external or internal factors. The movements of orientation are 

 distinguished as phototropic, geotropic, etc., according to the stimulus 

 bringing them about. 



The tropisms of attached plants correspond to the tactic movements of motile 



