XXIII.— ON TROPIC MOVEMENTS 



By 



Sir J. C. BosE. 



The diverse movements induced by external stimuli in 

 different organs of plants are extremely varied and com- 

 plicated. The forces in operation are manifold — the 

 influence of changing temperature, the stimulus of contact, 

 of electric current, of gravity, and of light visible and 

 invisible. They act on organs which exhibit all degrees 

 of physiological differentiation, from the radial to the 

 dorsiventral. An identical stimulus may sometimes induce 

 one effect, and at other times, the precisely opposite. Thus 

 under unilateral stimulation of light of increasing intens- 

 ity, a radial organ exhibits a positive, a dia-phototropie, 

 and finally a negative response. Strong sunlight brings 

 about para-heliotropic or ' midday sleep ' movement, by which 

 the apices of leaves or leaflets turn towards or away from 

 the source of illumination. The teleological argument ad- 

 vanced, that in this position the plant is protected from 

 excessive transpiration, does not hold good universally ; for 

 under the same reaction, the leaflets of Cassia montana 

 assume positions by which the plant risks fatal loss of 

 water. In Averrhoa caramhola the movement is down- 

 wards, whichever side is illuminated with strong light ; 

 in Mimosa leaflet the movement, under similar circum- 

 stances is precisely in the opposite direction. The photo- 

 nastic movement, apparently independent of the directive 

 action of light, has come to be regarded as a phenomenon 

 unrelated to phototropic reaction, and due to a different 



