748 TART IV. — THE PHYSIOLOGY OF PLANTS. 



Another important case is the change of position ot* the chlorophyll- 

 corpuscles in the cells (see p. 686). 



Motile cellular members, whether mature or growing, are, as a 

 rule, sensitive to the directive influence of the incident rays of 

 light. Among mature motile members, foliage-leaves are those 

 which most markedly respond to the directive or heliotropic influ- 

 ence of light ; among growing members, it is more especially stems 

 and leaves which are sensitive, bnt roots have in many cases been 

 found to be so. All these irritable members take up a definite 

 position, the light-position (p. 688), 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. 



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

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

 !the niorphologically upper (ventral) 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. 



It must, however, be borne in mind that the position of dorsiventral mem- 

 bers with regard to light may not, in nature, always be found to be exactly that 

 described above, because other directive influences are in operation which 

 interfere with the effect of light. When, by appropriate arrangements, the 

 effect of the other influences is removed, and light alone is acting, the truth of 

 the above statement will be demonstrated. 



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. 689), 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 ( para heliotropism, see p. 689). 



