358 



BOTANY 



consequent favouring of transpiration. The fact that the stomata lie on the surface 

 protected in the sleep-position may be noted with regard to the furthering of 

 transpiration. 



Excessively high temperature or illumination causes the leaves to depart from 

 the usual day-position and to assume a different one ; this is either externally 



FIG. 290. Amicia zygomeris, showing diurnal and nocturnal position of leaves. 



similar to the night-position or is diametrically opposite to this. Thus the leaflets 

 of fiobinia are bent downwards at night, in diffused daylight they are spread out 

 flat, while in the hot mid-day sunlight they stand vertical. This so-called diurnal 

 sleep is only found in leaves with pulvini and is brought about in a different way 

 to the evening change of position ; there is no increase of turgescence but a condition 

 of flaccidity, which is unequal on the two sides of the pulvinus. 



PERIODIC MOVEMENTS ( 115 ) 



When a plant has carried out regular nyctinastic movements for a long period 

 under the influence of the alternation of day and night, the periodic movements con- 

 tinue for some days in constant light or constant darkness. In some plants it is 

 possible to bring about experimentally a shorter or longer period of change than the 

 usual one of twenty-four hours ; this new periodicity also shows an after effect. 



On the other hand it is established that, in certain flowers (Calendula] and 

 leaves (Phaseolus), there are also movements with a period of 24 hours, determined 

 not by the rhythm of light and darkness or their after effect. The possibility 

 that these movements are antonomous is excluded. Their cause is unknown 

 but there is much in favour of the view of STOPPEL that variations in the electrical 

 conductivity of the atmosphere are of importance in determining them. It is 

 true that there is no exact basis for this view. 



2. CHEMONASTY ( 116 ) 



Chemonasty bears the same relation to chemotropism as photonasty 

 does to phototropism. From whatever side a chemical stimulus (such as 

 the vapour of ether, chloroform, or ammonia) acts on a sensitive tendril 

 the same side of the latter always becomes concave ; this is the side 

 which is especially sensitive to haptotropic stimulation. 



