DIV. ii 



PHYSIOLOGY 



357 



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 changes of 

 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 two to 

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

 the Tulip to 2-3 C. 



2. PHOTONASTY. In a similar fashion other flowers (Nymphaea, 

 Cacti) and also the flower heads of Compositae (Fig. 289) open on 

 illumination and close on darkening. The night-flowering plants 

 such as Silene noctiftora and 

 Victoria regia behave in an 

 opposite manner. 



The significance of these move- 

 ments must lie in only exposing the 

 sexual organs when insect- visits may 

 be expected ; at other times they are 

 protected against injury by rough 

 weather, especially by rain. These 

 plants are adapted to pollination by 

 moths. 



3 NYCTINASTY _ Manv ^ IG ' ^* Flower-head of Leontodon hastilis, closed 

 , . i M ' when kept in darkness, open when illuminated. 



foliage leaves exhibit nyctl- (From DETMER'S Physiol. Pract.) 



nastic movements which are 



usually influenced more by light than by temperature. In some 

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

 Compositae) these movements are entirely growth-movements as in 

 the floral leaves ; in the Leguminosae, Oxalideae, and other plants 

 provided with pulvini, variation-movements are found. The former 

 are naturally of short duration and cease when the leaves are full- 

 grown. The latter, however, continue for a long period. In the 

 movements of variation an increase 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 

 characterised by a 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 out- 

 wards. In the day-position the surfaces stand horizontally or at right 

 angles to the incident light (p. 351) (Fig. 290).. 



That these phenomena are not due to phototropism is shown by the day-position 

 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 significance of the vertical position assumed by foliage leaves at night is 

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



