The Light Relation 



417 



important. In these cells the light may be focused in some 

 basal region of the pro- 

 toplasm, and through 

 the unequal illumination 

 the stimulus to orienta- 

 tion is supposed to be 

 given. It has been dem- 

 onstrated photograph- 

 ically that these cells 

 focus the rays, but since 

 such cells occur under a 

 variety of conditions, 

 and for many other rea- 

 sons, they are not posi- 



FIG. 113. Epidermal modifications which 

 focus light rays ; Berberis (a) , Rhodo- 

 dendron (b), and Prunus Lauro-cerasus 

 (c). [After Haberlandt.] 



tively connected with 



this form of irritability. 



Important in the orientation is the direct or indirect 



sensitiveness of the petiole. Wager believes " that the 



perception of light is bound up with its absorption by the 



chlorophyll grains, in which case the palisade cells would 



be the percipient cells." 



253. Diverse requirements. From casual observa- 

 tion of plant habitats, it may be noted that there is great 

 diversity in the light intensity under which different species 

 grow to maturity. Many plants reach perfection only 

 when exposed, but others develop more vigorously under 

 the partial shade of the forest or thicket. Exposed and 

 shaded situations usually differ with respect to other en- 

 vironmental factors, such as humidity and evaporation ; 

 and in a careful study of habitats it is necessary to measure 

 and to attempt an evaluation of all factors. 



2E 



