IRRITABILITY. 51 



in the leaf-blade, but the impression is transmitted to the pul- 

 vinus at the base of the leaf or leaflet and produces movement. 

 The region which receives the stimulus is designated the per- 

 ceptive zone, and the one which causes the movement, the 

 motor zone. The two may coincide in position. 



42. Geotropism. The power by which a plant responds to 

 the influence of gravity is termed geotropism. The response 

 of an organ to this stimulus may occur in three ways, as follows, 

 (i) The organ may point its apex toward the centre of the earth, 

 the source of gravity, in which instance it is said to be pro- 

 geotropic. This action is generally manifested by primary 

 roots. (2) It may point its axis away from the source of gravity, 

 directly upward, when it is said to be apogeotropic. Erect 

 shoots are generally apogeotropic. In general organs of 

 radial structure exhibit one of these two forms of geotropism. 

 (3) The organ may place its axis in a horizontal position, at 

 right angles to the force of gravity, when it is said to be dia- 

 geotropic. This is characteristic of the larger number of bilateral 

 organs, such as leaves, although also shown by organs of radial 

 structure, such as branches of stems, secondary roots, etc. 



EXPERIMENT 59. 



PROGEOTROPISM. 



To a cork in the top of a bell-glass fasten a seedling of a 

 Bean with the radicle which is i 

 to 2 cm. in length in a horizontal FlG - 44- 



position. In a few hours the tip 

 is found to be pointing downward 

 more or less directly. (Fig. 44.) 



EXPERIMENT 60. 



GEOTROPIC REACTIONS OF SEEDLINGS. 



Place a layer of sawdust be- 

 tween two horizontal parallel rings Progeotropism of seedling. (Det- 



mer.) b, dish partly filled with 

 covered with wide-meshed gauze. water ; g, bell-glass ; s, seedling. 



