14 



THE PLANT 



to the excessive elongation of the parenchyma cells, but the, cause of the 

 latter is far from apparent. It is generally assumed to be due to a lack of 

 the tonic action of sunlight, which brings about a retardation of growth in 



sun plants. The evidence in 

 favor of this view is not con- 

 clusive, and it seems probable 

 at least that the elongation of 

 the parenchyma cells takes 

 place under conditions which 

 favor the mechanical stretch- 

 ing of the cell wall, but inhibit 

 the proper growth of the wall 

 by intussusception. It is 

 hardly necessary to state that 

 the reduced photosynthetic ac- 

 tivity of shade plants favors 

 such an explanation. What- 

 ever the cause, the advantage 

 that results from the elongation 

 of the internodes is apparent. 

 Leaves interfere less with the 

 illumination of those below 

 them, and the leaves of the 

 branches are carried away 

 from the stem in such a way as 

 to give the plant the best possi- 

 ble exposure for its aggregate 

 leaf surface. 



179. Modification of the 

 epidermis. The development 

 of epidermal chloroplasts in 

 diffuse light is the only change 

 which is due to the direct effect 

 of light. This does not often 

 occur in the shade ecads of 

 sun species, but chloroplasts 

 are regularly present in the 

 epidermis of woodland ferns 

 and of submerged plants. The 

 slight development of hairs in sciophilous plants is an advantage, but it 

 must be referred to the factors that determine water loss. The significance 



Fig. 40, Isophotophyll of Allionia linearis, 

 showing diphotic ecads: 1, light 1; 2, light .012; 

 3, light .003. . X 130. 



