PLATE 12 
A. —Oenothera biennis. Plants at left were exposed to the full daylight period of 
the spring months. The individual at right received only io hours of illumination, 
beginning March 19. It is seen that the short-day conditions caused a decided weaken¬ 
ing in power to elongate the primary axis, resulting in development of numerous basal 
shoots. In this case dominance of the apical bud is lost, a characteristic response 
to a suboptimal light period for elongation of the stem. Photographed June 21. 
B. —Sorrel (Rumex acetosella). Plants at left received electric illumination from 
sunset till midnight, in addition to natural daylight in the greenhouse, during the 
winter months. These plants developed numerous aerial flowering stems and in 
general showed the typical summer behavior of this species. The plants at right, 
which received only natural illumination, were unable to develop aerial stems but 
formed thick rosettes of leaves. In reality, however, these latter plants were more 
active than appearances indicate, for a very large number of underground stems were 
developed. This behavior of sorrel furnishes further demonstration of the fact that 
the position, character, and extent of branching are subject to regulation by the length 
of day. The leaf-rosette form of activity during the fall, winter, and early spring 
is typical of many plants. Photographed March 25. 
