[Chap. XXIII PLANT BEHAVIOR RELATED TO OSMOSIS 215 



of their deflection toward the lower side. Similarly, in many plants 

 they accumulate more abundantly on the shaded side of stems than on 

 the side exposed to the more intense light. As a result, the cells on the 

 shaded side enlarge more readily; this side of the stem becomes longer 

 than the lighted side and actually pushes the tip of the stem toward 

 the light. 



These hormones may be extracted from plants and* dissolved in wool 

 tat, called lanolin. If a little of this lanolin containing the extracted hor- 

 mones is placed on one side of a root or stem tip, the expected curva- 

 tures are obtained. Similar curvatures may be obtained by placing any 

 of several closely related chemical compounds in the lanolin instead of 

 the naturally occurring hormones (Fig. 74). 



Fig. 74. Curvatures of stems and petioles resulting from the effects of chemical 

 compounds similar to naturally occurring hormones: A, untreated plants; B, treated 

 plants. The compounds in low concentration (O.OI mg. in 100 mg. of lanolin) 

 were placed on the right side of the stems and on the upper surface of the two 

 petioles that have curved downward. Photos from P. W. Zimmerman and A. E. 

 Hitchcock. 



Leaf mosaics. The formation of leaf mosaics (Fig. 75) as a result of 

 differences in elongation, bending, and twisting of petioles and stems is 

 a consequence partly of unequal illumination of petioles and stems and 

 partly of unequal illumination of the blades of the leaves. Bending and 

 twisting of petioles are the results of differences in cell enlargement in 

 opposite sides of the petiole. This cell enlargement is dependent in part 

 upon hormones that are formed in young leaf blades exposed to light 

 and pass down the veins to the petiole and stem. If the right and left 

 halves of a blade are unequallv illuminated, the amounts of hormones 

 that are formed and get to the right and left halves of the petiole will be 

 unequal. Consequently, there will be a difference in the amount of cell 

 enlargement on the two sides of the petiole and a curvature will result. 



