264 - Multicellular Plants 







Fig. 14-7. Positive chemotropism of pollen tubes, 

 which are growing toward a crushed stigma (central 

 dark area). 



auxin produces a curvature toward the source 

 of light; whereas in the root, the inhibiting 

 effect gives a bending in the opposite direc- 

 tion. 



Many other tropisms also depend upon the 



auxins. Chemotropisms, such as the growth 

 of pollen tubes through the style (Fig. 14-7), 

 or the growth of roots toward water (Fig. 

 14-8); and thigmotropisms, such as the twin- 

 ing of the tendrils of climbing plants around 

 solid supporting objects — all appear to be 

 typical growth movements, although these 

 reactions have not been studied very inten- 

 sively. However, some plant tropisms — such 

 as the turning of leaves in following the 

 course of the sun — appear to be turgor move- 

 ments, involving receptor-conductor-effector 

 structures that are generally similar to those 

 of Mimosa. 



Characteristics of the Auxins, fn distinct 

 contrast to the hormones of higher animals 

 (Chap. 22), the auxins do not display a very 

 high degree of specificity. Under proper con- 

 ditions, in fact, very similar effects are ob- 

 served when one auxin is substituted for an- 

 other. Each single compound, on the other 

 hand, displays a surprising versatility of 

 action when applied in different concentra- 

 tions and to different parts of the same plant. 

 In the stem, for example, the same concen- 

 tration of an auxin may stimulate growth in 

 the cells directly behind the main terminal 

 bud, but inhibit the development of the 



Fig. 14-8. Positive hydrotropism. These roots entered cracks in the drain pipes 

 and stopped them up. (Courtesy of Missouri Botanical Garden.) 



