656 PLANT MOVEMENTS 



numerous adventitious roots on the aerial stems of many climbing plants. 



Most of our knowledge of the mechanism of phototropic movements has 

 been derived from a study of the behavior of the coleoptile of the oat plant 

 (J vena sativa) when subjected to one sided illumination. Because of its re- 

 activity, its structural simplicity, its uniformity of behavior under similar con- 

 ditions and its general suitability for such work this structure has been widely 

 used in experimental studies of phototropism, geotropism, and some other tropic 

 movements (Chap. XXXII) 



It has been known since the time of Darwin that curvature of a coleoptile 

 will occur if only the extreme tip of the coleoptile is exposed to unilateral 

 illumination. The region in which the cell enlargement responsible for this 

 curvature occurs, however, is some distance below the tip (Chap. XXXII). 

 If the tip of the coleoptile is shaded by means of a tin foil cap and the entire 

 coleoptile illuminated unilaterally little or no curvature results. Likewise, 

 decapitated coleoptiles react feebly to one sided illumination, but if coleoptile 

 tips which have been subjected to one sided illumination are placed upon 

 unilluminated coleoptile stumps, marked phototropic curvature of the stump 

 results. Experiments like these indicate clearly that the tip of the coleoptile 

 profoundly influences the enlargement phase of growth in cells below the tip 

 of the coleoptile. 



The positive phototropic curvature of oat coleoptiles is caused principally 

 by greater elongation of the cells on the shaded side of the coleoptile than of 

 the cells on the illuminated side. Since cell elongation is known to be in- 

 fluenced by the quantity of auxin present (Chap. XXXII) it is logical to 

 seek an explanation of such phototropic reactions by studying the effect of 

 light upon the distribution of auxin in the coleoptiles. Extensive investiga- 

 tions have shown that, in oat coleoptiles at least, unilateral exposure to light 

 increases the quantity of the auxin reaching the shaded side of the coleoptile 

 from the tip and decreases the quantity on the illuminated side. The photo- 

 tropic curvature of oat coleoptiles and presumably of many other plant struc- 

 tures apparently results from the presence of unequal quantities of auxin on 

 the two sides of the coleoptile. Some of the evidence upon which these con- 

 clusions are based will be reviewed briefly. 



Went (1928) removed the tip of a coleoptile that had been exposed uni- 

 laterally to light of suitable intensity and placed it upon twu small blocks 

 of agar, separated from each other by a thin metal plate, in such a way that 

 the auxin from the shaded and illuminated sides diffused into different agar 

 blocks (Fig. 144). The blocks were then tested for auxin content by the 

 oat coleoptile technique. The resulting curvatures indicated that more auxin 

 diffused out of the shaded half of the coleoptile tip than out of the illuminated 



