190 8- 1909.] The Sense- Organs of Plants. 165 



a particular region — the receptive spot — is specialised for the 

 reception of this stimulus. That growth -movements of the 

 ovum, equally with the locomotor movements of unicellular 

 organisms, are influenced by chemical change in the environ- 

 ment, is evidenced by the experiments of Loeb and others 

 on the " chemical fertilisation " of certain animal ova. 



Once initiated, growth goes on automatically, but, as in the 

 case of locomotor movements, it is powerfully influenced in 

 plants by such environmental changes — stimuli — as varia- 

 tions in the direction and intensity of light, contact, and so 

 forth. It is mainly for the perception of such stimuli affecting 

 growth that sense-organs are specialised, but not altogether, 

 for sense-organs occur in organs which have ceased to grow, 

 and which, in response to external stimuli, exhibit movements 

 in space effected by special contrivances — motile leaf pulvini, 

 for instance — into which growth does not enter. 



Haberlandt (' Sinnesorgane in Pflanzenreich,' 1906, p. 9) 

 defines a sense-organ as "any contrivance serving for the 

 reception of external stimuli, and showing a more or less far- 

 reaching relationship between structure and function from 

 this point of view." Pfeffer (' Physiology of Plants,' iii. 

 67) writes: "Various special irritabilities may reside in 

 cells and tissues which differ in no anatomical features from 

 ordinary indifferent cells and tissues. All cells and organs 

 capable of perceiving stimuli may be termed sense-organs, 

 whether they show any special anatomical structure or not." 



On Pfeffer's more comprehensive definition a considerable 

 list of sense-organs can be made out for plants. In contrast 

 to those of animals, plant sense-organs are of simpler structure : 

 the same organ may serve for the perception of more than one 

 kind of stimulus ; there is no special co-ordinating mechanism 

 of the nature of a central nervous system between sense-organ 

 and motor-organ ; the induced movements of plants are there- 

 fore somewhat of the nature of reflexes. The stimulus is 

 conveyed from the perceptive organ to the motor area in 

 various ways, — sometimes by intercommunicating protoplasmic 

 fibrillse, sometimes by hydrostatic pulsations along special 

 channels : the former method is found in growing organs, the 

 latter in adult organs. 



It can be experimentally shown that plants possess organs 



VOL. VI. M 



