, BOTANY OF THE LIVING PLANT 



wch circumstance* a type oi movement of plant organs is exhibited 



In adjusting the <.nmt.it.un of its organs with regard to directional 

 stimuli the plant exhibits Tropic responses, or Tropisms. The re- 

 spouse to the force of gravity is termed Geotropism, that to the direc- 

 j light, Phototropism. These phenomena will now be considered 

 in more detail. 



i. Geotropism. 

 That the upward growth of the main stem and the downward 

 growth of the mam root are due to the influence of gravity might 

 dedui ed from the consideration that gravity is the only directional 

 influence of general operation that could be involved. Confirmation 

 that inference is provided by experiments with Knight's wheel 

 (sec p. 155). The root, growing towards the source of the gravity 

 tmulus, is said to be positively geotropic, while the main shoot, 

 rowing upwards and away from the centre of the earth, is negatively 

 otropic : organs such as lateral shoots and roots and many leaves 

 which place themselves in oblique or transverse positions are plagio- 

 tropic. The assumption of a strictly transverse position with 

 gard to the direction of gravity is sometimes distinguished by the 

 1 term diageotropism. Stolons, runners and some rhizomes 

 and leaves provide examples of this. An organ may exhibit different 

 types of geotropism in the course of its development. Thus in seed- 

 lings the upper part of the plumule, or of the hypocotyl in epigean 

 types, is frequently bent over and points downwards, becoming 

 upright after emerging from the soil (Chapter I.). This is, at least in 

 part, due to changed geotropic response. An advantage to the seed- 

 ling following from this is that the terminal bud is less liable to be 

 damaged by being dragged through the soil than by being pushed. 

 The upper part of the peduncle of the Poppy is also positively geotropic 

 before flowering and points downwards, but during flowering and fruit- 

 production the upright posture is gradually assumed. Some flowering 

 become positively geotropic as the fruits ripen. This is the case 

 in the Monkey-nut {Arachis), where the fruits are as a result pushed 

 d <>•.>. awards into the soil. 



If the organs of a plant are displaced from their normal orientation, 

 in which a state of equilibrium exists with regard to gravity, then 

 •ropic curvatures may be exhibited. Thus if a seedling is fixed 

 horizontally or obliquely in a moist atmosphere, in the course of a 

 few hours curvature- will develop and will gradually bring the apical 

 parts of the root and stem back into the normal orientation (Fig. 94). 



