246 



PHYSIOLOGY OF GROWTH AND C0NFIGUIL4.TI0N 



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 sUghtly 

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

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

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

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

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

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

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





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Fig. 126. — Leaf-mosaic of Hedera. {From Gager.) 



Verbascum tkapsus (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 

 Hedera 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 inchned toward the east, at midday the blades take a nearly 

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



