356 BOTANY PART i 



leaflets twine around slender supports, while the parasitic shoots of Cuscuta (Fig. 

 221) are adapted for both twining and climbing. In many tropical plants 

 axillary shoots are transformed into tendril-like climbing hooks. Climbing parts 

 of the thallus occur in some Thallophyta (Florideae). 



More recent investigations have shown that haptotropism is more widespread 

 than was previously supposed. Etiolated seedlings are always haptotropic, and 

 this holds frequently for older shoots of green plants, especially of twining and 

 climbing plants. No use appears to attach to this power ( 112 ). The roots of 

 seedlings are only exceptionally irritable to contact. 



(b) Nastie Movements ( 113 ) 



In the tropistic and tactic movements of irritability, the direction 

 of the stimulus stands in direct relation to the direction of the move- 

 ment; the nastic movements, on the other hand, are either brought 

 about by diffuse stimuli with no definite direction or are not influenced 

 by the direction of the stimulus. The direction of the movement 

 always depends on the reacting organ and not on the environment, 

 the movements are not movements of orientation such as those we 

 have hitherto considered. 



Typical nastic movements of variation are shown by stomata ; the structural 

 relations of these determines the opening or closing of the pore by changes in the 

 curvature of the guard-cells brought about by variations in their turgescence. It is 

 frequently assumed that the closing on loss of water and the opening on illumina- 

 tion are purely mechanical results. Loss of water will have as its direct result a 

 diminution of the osmotic pressure, and illumination will increase the pressure by 

 increasing the production of assimilates. It cannot, however, be doubted that in 

 addition to purely physical influences true stimulus-movements also take place. 

 Thus light and some other factors also may act as stimuli directing the production 

 of osmotic substances by the protoplasm in particular directions. 



In other nastic movements, as in the case of the stomata, light and 

 heat, chemical substances, and sometimes also vibrations, may play the 

 part of stimuli. Often the movement of a particular organ results 

 from several of these stimuli in the same or in different ways. 



1. NYCTINASTIC MOVEMENTS ( m ) 



Many foliage leaves and floral leaves assume different positions by 

 day and by night. According as the change from the one position to 

 the other is brought about by variations in the intensity of light, in 

 the temperature, or in both factors at once, we distinguish between 

 photonasty, thermonasty, and nyctinasty. The movements are carried 

 out partly as growth-movements, partly as variation-movements. 



1. THERMONASTY. Growth - movements due to variations in 

 temperature are found especially in flowers, e.g. Crocus, Tulip, 

 Ornithogalum, Cokhicum, and Adonis. These flowers on a rise of 

 temperature exhibit a sudden and limited acceleration of the growth 



