CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 749 



In the case of foliage-leaves and other dorsiventral members 

 which cannot execute movements of variation, the light-position is 

 assumed in the course of development, and is fixed. Since it 

 cannot be altered in relation to variations in the intensity of the 

 incident rajs, the position assumed is determined by the most 

 frequent direction of incidence of the rays of suitable intensity. 

 For instance, the fixed light-position of the foliage- leaves of plants 

 growing free in the open, is usually not such that the ventral 

 surface is horizontal, facing the zenith ; but such that it is directed 

 towards that quarter of the sky from which, not the brightest 

 sunlight, but the brightest diffuse daylight, falls perpendicularly 

 upon it. In fact, it is not unusual to find that the fixed light- 

 position of leaves, when the light is of high average intensity, is 

 such that the surfaces are vertical, so that the margin is pre- 

 sented to the zenith. Under these circumstances both surfaces 

 are equally exposed to light, and the structure of the leaf becomes 

 more or less isobilateral (see pp. 164, 686). 



A most remarkable instance of a fixed light-position with vertical surfaces, 

 is afforded by the so called " Compass -plants," Silphium laciniatum, and 

 Lactuca Scariola. The surfaces of the leaves of these plants are not only 

 vertical when fully exposed to bright light, but they place themselves in a 

 vertical plane which more or less nearly coincides with the meridian of the 

 locality, the surfaces of the leaves facing east and west. It is this relation of 

 the fixed light-position of the leaves to the points of the compass which sug- 

 gested the name " Compass-plants." 



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. 479), 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 direction 

 of the incident rays ; in the case of radial members the important 

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

 rays. 



