POSITIVE HELIOTROPISM 58 1 



rate of growth under shade as the active factor in growth- 

 curvature, elongation on the lighted side being retarded, where- 

 as in the case of positive light-curvatures the motive-power 

 really lies in the active responsive contraction of the lighted 

 side, the expelled water from which, reaching the opposite, 

 may further cause an increase of growth above the normal. 

 This fact, that it is the active contraction of the lighted, and 

 not the passive growth of the unlighted, side that is actually 

 the efficient cause of heliotropic curvature, will be made clear 

 by taking an extreme case in which there is no growth. 

 Here, if heliotropic curvature had been due simply to differ- 

 ential growth, the occurrence of curvature would have been 

 an impossibility. But heliotropic curvatures are observed in 

 organs which have come to growth-standstill. Again, grass 

 haulms, in which growth is arrested, exhibit curvatures due 

 to the contraction of the lighted, and the renewal of growth 

 (due to the increased turgescence caused by expelled water) 

 on the unlighted, side, in precisely the same manner as they 

 were observed to do under the action of gravitational 

 stimulus. 



I shall now proceed to describe some typical experiments 

 on heliotropic effects as induced by the unilateral application 

 of light. And since it has been shown that the responses of 

 growing organs are not essentially different from those of 

 pulvinated organs, it will be helpful to begin by observing the 

 effect of the unilateral application of light on the latter, 

 especially as these have the advantage of showing relatively 

 rapid reactions. In order to obtain a pulvinus which 

 approximates in character to that of a radial organ, a speci- 

 men must be selected in which the difference of excitability, 

 as between the upper and lower halves, is as small as possible. 

 This may be found in the pulvinus of the large terminal 

 leaflet of Desmodium. 



Definition of terms ' positive' and ' negative.' — It will 

 be demonstrated, in this and succeeding chapters, that how- 

 ever various in type may be the responsive movements 

 induced by light, they are not due to the different specific 



