216 PART III. PHYSIOLOGY. [ 48 



exposed to light, and the structure of the leaf becomes more or 

 less isobilateral (see p. 114). 



The fact that the ultimate position of dorsiventral leaves is 

 mainly determined by light, is demonstrated by removing them 

 whilst still growing, and therefore capable of a change of posi- 

 tion from its influence. In darkness these leaves take up an 

 altogether different position (see Fig. 131), becoming curved in 

 various ways ; when again exposed to light they resume their 

 previous diaheliotropic position. 



With reference now to radial members, it may be generally 

 stated that the essential feature of their response to the directive 

 influence of light is that they tend to place their long axes in the 

 direction of incidence of the brightest light falling upon them. 

 Whereas in the case of dorsiventral members the important point 

 is the relation of the morphologically upper surface to the direc- 

 tion of the incident rays ; in the case of radial members the im- 

 portant point is the relation of the long axis to the direction of the 

 incident rays. 



An exact coincidence between the direction of the long axis of the mem- 

 ber and that of the incident rays is, however, not always attained in 

 .nature, on account of the antagonistic action of other directive influences. 

 This point is more fully discussed on p. 222. 



It must be mentioned that, inasmuch as there are no radial 

 members which are both heliotropically irritable and capable of 

 performing movements of variation, all that is here said refers to 

 growing radial members. 



In illustration, the case of a radial member which has been 

 grown in the dark may be taken, and it may be assumed to be 

 vertical. Light is allowed to fall upon it from one side ; the effect 

 is a gradual curvature of the member, as it continues to grow, so 

 that its long axis comes to coincide more or less nearly with the 

 direction of the incident rays. 



But the curvature may be in one of two directions ; it may be 

 .either such that the apex of the member comes to point towards 

 the source of light, or such that it points in the opposite direction. 

 When the former is the case the member is said to be positively 

 heliotropic ; when the latter, it is said to be negatively heliotropic. 



The nature of the curvature, whether positive or negative, 

 depends upon the specific irritability of the member. Thus, gene- 

 rally speaking, primary shoots, including such forms as the stems 



