Researches on Growth and Movement in Plants. 



381 



It will presently be shown how these fundamental effects of direct and 

 indirect stimulations are instrumental in bringing about various tropic 

 curvatures. 



C. Tropic and Nastic Movements. 



The diverse movements induced by external stimulus in different organs of 

 plants are extremely varied and complicated ; 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 dorsi ventral. Stimulus may act on one side or on all sides of 

 the organ. The response may or may not change with the mode of stimula- 

 tion. In the curving tendril under stimulus of unilateral contact, Fitting 

 finds a pronounced acceleration of growth on the convex side. "Although 

 the exact mode of production of these changes is uncertain, they are 

 undoubtedly the result of the contact stimulus."* Far more complicated are 

 the effects induced by light. Under unilateral stimulation of increasing 

 intensity, a radial organ exhibits a positive, a dia-phototropic and finally a 

 negative response. Strong sunlight brings about para-phototropic or midday 

 sleep movement, by which the apices of leaves or leaflets turn towards or 

 away from the source of illumination. The teleological argument advanced, 

 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 Carambola the movement is downwards, whichever side is 

 illuminated with strong light; in Mimosa leaflet the movement, under 

 similar circumstances, is precisely in the opposite direction. The photonastic 

 movement, apparently independent of the directive action of light, has come* 

 to be regarded as a phenomenon totally unrelated to phototropic reaction, and 

 due to a different kind of irritability and a different mode of response. So 

 very anomalous are these various effects that Pfeffer, after showing the 

 inadequacy of various theories that have been advanced, came to the 

 conclusion that " the precise character of the stimulatory action of light has 



yet to be determined When we say that an organ curves towards a 



source of illumination because of its phototropic irritability, we are simply 

 expressing an ascertained fact in a conveniently abbreviated form, without 

 explaining why such curvature is possible or how it is produced."f 



* Pfeffer, ' Physiology,' vol. 3, p. 58. 

 + Pfeffer, ' Physiology,' vol. 2, p. 74. 



2 H 2 



