The effect of diffuse light in causing stems to elongate, though known for a long time, is still unexplained. The old explanation that the plant stretches up to obtain more light seems to be based upon nothing more than the coincidence that the light comes from the direction toward which the stem grows. Later researches have shown that the stretching of the stem is due 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 conclusive, and it seems probable at least that the elongation of the parenchyma cells takes place under conditions which favor the mechanical stretching of the cell wall, but inhibit the proper growth of the wall by intussusception. It is hardly necessary to state that the reduced photosynthetic activity of shade plants favors such an explanation. Whatever 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 possible exposure for its aggregate leaf surface. [Illustration: Fig. 40. Isophotophyll of _Allionia linearis_, showing diphotic ecads: 1, light 1; 2, light .012; 3, light .003. × 130.] [Illustration: Fig. 41. Isophotophyll of _Helianthus pumilus_, showing isophotic ecad: 1, sun leaf; 2, shade leaf (light .012). × 130.]