SEOT. U 



PHYSIOLOGY 



321 



Many foliage leaves also exhibit nyctinastic movements which are 

 usually influenced more by light than by tempei-ature. In some cases 

 (e.g. in Chenopodiaceae, Caryophyllaceae, Balsamineae, and some 

 Compositae) these movements 

 are entirely growth-move- 

 ments as in the floral leaves ; 

 in the Leguminosae, Oxali- 

 deae, and other plants pro- 

 vided with pulvini, variation 

 movements are found. The 

 former are naturally of short 

 duration and cease Avhen the 

 leaves are full grown. The 

 latter, however, continue for 

 a long period. In the move- 

 ments of variation an increase 



Fig. 238. — Flower-head of Leontudon hastilis, closed 

 when kept in darkness, open when illuminated. 

 (From Detmeb's I'hynwl. Pract.) 



of turgor probably takes place 



in darkness in both halves of the pulvinus but more weakly or slowly 

 on the concave side. The night- or sleep-position is always characterized 

 by the vertical position of the laminae, the leaf-stalk or the pulvinus 

 curving either upwards or downwards ; the laminae themselves have 

 thus either their under or upper faces turned outwards. In the day- 



FiG. 239. — Aiiiiciu zygomeris, showing diurnal and nocturnal position of leaves. 



position the surfaces stand horizontally or at right angles to the 

 incident light (p. 306) (Fig. 239). 



Tliat tlie.se phenomena are not due to phototropisni is shown by the day-po.sition 

 being assumed whether the under or the upper side is more strongly lighted or 

 when the illumination is equal. The same holds for the effect of darkness. 



The signilicauce of tlie vertical position assumed by foliage leaves at night is 

 regarded by Stahl as consisting in the diminution of the formation of dew and the 

 consequent favouring of transpiration. 



Excessively high temperature or illumination causes the leaves to depart from 



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