SECT. II PHYSIOLOGY 301 



this result and the reception of the stimulus tliere doubtless 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. The power of perception or of sensation in the plant can be 

 sjjoken of without implying any subjective perception, will, or thought, as in the 

 complicated human psychology. This is unfortunately sometimes done by modern 

 sensational writers. There is reason to distinguish organs of perception, especially 

 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 ("■*). 



1. Directive Movements (Movements of Orientation) 



Whether the directive movements are expressed in free changes of 

 position or in curvatures, we have to distinguish orthotropous (parallelo- 

 tropous) 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). Plagio- 

 tropous organs place themselves at right angles to the direction 

 of the stimulus (diatropic organs) or obliquely to its direction 

 (klinotropic organs). 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., and as 

 phototactic, geotactic, etc., according to the stimulus bringing them 

 about. 



(a) Stimulus of Light 



Phototaxis (■'^). Phototactic movements may be best observed 

 when a glass vessel containing water in which are Volvocineae, 

 Chlamydomonadinac, or swarm-spores of Algae is exposed to one-sided 

 illumination from a Avindow. After a short time the uniform green 

 tint of the water disappears since the motile organisms have all 

 accumulated at the better-illuminated side of the vessel. If the latter is 

 turned through an angle of 180'' the Algae hasten to the side Avhich is 

 now illuminated. If, however, a stronger light, such as direct sunlight, 

 is allowed to fall on the vessel the same organisms which have reacted 

 till now positively become negatively phototactic and swim away from 

 the source of light. 



The change from a positive to a negative reaction involves the passing through 

 an indifferent stage in which no tactic movement results. This stage of indifference 

 does not lie at the same intensity of the light for all the individuals of a species. 

 Exceptionally also changes in the reaction of single individuals occur, the causes 

 of which are only partially recognisable. In some organisms, such as the jilasmodia 

 of Myxomycetes, we find a negative reaction even to a light of low intensity. 

 There are also colourless organisms which have a positive phototactic reaction. 



In the case of many swarm-spores and some Volvocineae phototaxis has the effect 

 of causing when light falls from one side, a rotation of the organism, bringing it into 



