l6o BOTANY OF THE LIVING PLANT 



nditions of light intensity and period of stimulation 



urvatures have been obtained. 



tain extenl the law of " quantity of stimulus " applies to photo- 



ponses. The result of briei exposure to a strong unilateral light 



ma) be the same ... thai of a longer exposure to a weaker light. The length 



potme th.it ifl required to produce a subsequent curvature {i.e. the 



presentation turn-) thus depends on the intensity of light that is used. Similar 



considerationa actually apply to geotropism. 



eotropic curvatures, so in those of phototropism there is 

 evidence that hormone re-distribution is involved. For the most 

 part this i- provided by experiments on the coleoptile, where there 

 is a similar localisation of sensitiveness and curvature as is encountered 

 in that organ with regard to gravity. If the tip is covered by an 

 opaque hood, and thus screened from light, the organ becomes much 

 less sensitive to lateral illumination, although the zone where curva- 

 ture would normally occur is exposed to the light. It thus appears 

 that the one-sided light acts more on the apex than directly on the 

 actual growing cells. Experiments suggest that there is in the later- 

 ally-illuminated tip a re-distribution of auxin, more accumulating 

 on the shaded side. This inequality is transmitted down to the 

 growth zone and is manifested there in a positive curvature, since the 

 hormone is growth-promoting (in this connection see Fig. 93). There 

 may, however, be in addition a direct, growth-retarding effect of 

 the light on the growing cells. Similar considerations apply to the 

 phototropic responses of stem structures in general. 



3. Other Tropisms. 



In addition to gravity and light, certain other factors exert directive 

 influences on the growth of plant organs. Roots are frequently sensitive to the 

 distribution of moisture in the rooting medium, and curve in their growth 

 towards the region of greater moisture-content, exhibiting the property known 

 rotropism. This particular tropism has not been very intensively in- 

 ted, and there is some doubt as to the degree to which it is developed 

 by rot ' eneral. The blocking of field drains by the invading roots of 



neighbouring trees is probably an example of hydrotropism. Aerotropism, as 

 a result of which an organ is sensitive to the distribution of gases, especially 

 . has been detected in roots, which tend to grow towards a higher 

 1 of oxygen, as is perhaps seen in " pot-bound " root-systems. 

 Pollen tiilK-s, which grow away from a source of oxygen, behave in the 

 Opposite fashion (] ;a). Chemotropism, or sensitiveness to the dis- 



tribution of chemical substances in the vicinity, is especially important 

 111 fu n ter XXI Y.), since as a result their hyphae grow towards 



d material : in the higher plants it is exhibited by roots. Hapto- 



