320 BOTANY 



I'AIIT I 



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

 the movements are not movements of orientation such as those Ave 

 have hitherto considered. 



Typical nastic nioveinents of variation are sliowii by stoinata ; tlie 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. 



As in the case of the stomata, so also in other nastic movements, light 

 and heat (photonasty, thermonasty, nyctinasty), chemical substances 

 (chemonasty), and sometimes also vibrations (seismonasty) 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. 



(a) Nyetinastic Movements Q^^) 



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. 



Growth-movements due to variations in temperature are found 

 especially in flowers, e.g. Crocus, Tulip, Ornithoyaluiii, Colchicum, and 

 Adonis. These flowers on a rise of temperature exhibit a sudden and 

 limited acceleration of the growth of the inner side of their perianth- 

 leaves or petals. The flowers consequently open. On the other hand 

 they close on a fall in the temperature. 



The flowers of the Tulip and Crocus are especially sensitive to clianges ot 

 temperature. Closed flowers brought from the cold into a warm room open in a 

 short time ; with a difference of temperature of from 15°-20° they open in four to 

 five minutes. Sensitive flowers of the Crocus react to a difference of V C. ; those of 

 the Tulip to 2°-3° C. 



In a similar fashion other flowei's {Niimplmea, Cacti) and also the 

 flower heads of Compositae (Fig. 238) open on illumination and close 

 on darkening. The night-flowering plants such as Silene nodiflora and 

 Cereus grancliflunis behave in an opposite manner. 



The significance of these movements must lie in only exposing the sexual organs 

 wlicn insect-visits may be expected ; at other times they are protected against 

 injury by rough weather, especially by rain. 



