48] CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 215 



position, the light-position (p. 173), with reference to the direction 

 of incidence of the rays of light. Members capable of performing 

 movements of variation can, if necessary, change their light- 

 position, whereas the light-position of other members can only be 

 changed so long as they are growing. 



The particular position which the member assumes under the 

 heliotropic influence of light, depends upon its organisation. Three 

 classes of members, namely the dorsiventral, the isobilateral, and 

 the radial, have therefore to be considered. 



It may be generally stated of dorsiventral members, that, for a 

 certain mean intensity of light, their light-position is such that 

 the morphologically upper surface is directed towards the source 

 of light, and lies in a plane perpendicular to the direction of 

 incidence of the rays : that is, they are diaheliotropic. 



The case of motile foliage-leaves may be taken first in illustra- 

 tion, such as those of the Sensitive Plant, Robinia, Scarlet Runner, 

 etc. When these leaves are exposed to light of sufficient intensity 

 to cause them to assume the diurnal position (see p. 174), their 

 upper (ventral) surfaces are at right angles to the direction of 

 incidence of the rays. If, on the one hand, the light to which 

 they are exposed becomes less intense than this, they will manifest 

 no sensibility to its direction of incidence, but will merely assume 

 the nocturnal position. If, on the other hand, the light becomes 

 more intense, then the leaves will alter their position so that the 

 blades will present their edge, instead of their ventral surface, to 

 the incident rays (paraheliotropism, see p. 174). 



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 rays, the position assumed is determined by the most 

 frequent direction of incidence of the rays of suitable intensity. 

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

 growing free in the open, is usually not such that the upper sur- 

 face 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 presented 

 to the zenith. Under these circumstances both surfaces are equally 



