276 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



Among plants that are especially sensitive to these differences in light in- 

 tensity, on the two opposite sides, may be mentioned Vicia sativa. If etiolated 

 seedlings of this plant are placed between two sources of light differing so slightly 

 that the difference cannot be detected by ordinary photometric methods, the 

 seedlings always bend promptly toward the source of the more intense light. 

 Phototropic bending is often difficult to observe in plants growing in sunny 

 places in the open, such as Cichorium intybus, Verbena officinalis, Sisymbrium 

 strictissimum, Achillea millefolium (yarrow). If such plants are grown in 

 weaker light, however, the light reaction becomes apparent. The stems of 

 Dipsacus (teasel) and Equisetum are but slightly phototropic and those of 







Fig. 126. — Leaf-mosaic of Boston ivy. (From Gager.) 



Verbascum thapsus (mullein) and V. phlomoides do not exhibit phototropism 

 at all. 



Phototropic responses occur very commonly in leaves, these organs tending 

 to assume such positions that they do not shade one another. Observed from 

 above, such an arrangement of leaves appears like a mosaic, as in the case of the 

 ivy leaves shown in Fig. 126. In this case, the lobes of one leaf approxi- 

 mately fill the indentations of others, so that a closely fitting arrangement 

 results. 



Many leaves bend so as to place the blades at right angles to the direction 

 of strongest illumination (Fig. 127). Shortly after sunrise the upper surfaces 

 of these leaves are inclined toward the east, at midday the blades take a nearly 

 horizontal position, and in the evening they are turned toward the west. In 



