504 Plant Physiology 



Nastic curvatures, however, are not necessarily the 

 result of external stimuli, hence they may be either auto- 

 nomic or paratonic. 



In the development of leaves (section 181) there is 

 usually a growth response whereby the under or dorsal 

 surface grows faster, yielding an upward curvature (hypo- 

 nasty). As a result of this each leaf in turn becomes a 

 part of the bud. Later the growth on the upper or ventral 

 surface is more rapid and there is outward bending (epi- 

 nasty) during exfoliation. There may be a recurrence of 

 epinastic and hyponastic curvature under the influence 

 of various stimuli until maturity of the leaf. Growth 

 upon the upper surface called forth by light is a paratonic 

 nastic bending, or photepinasty. 



307. Nyctitropism. The old idea of floral clocks was 

 founded on the observation that flowers of diverse species 

 open and close with different light and temperature 

 relations. There are some flowers which remain closed 

 during the night, opening in the early morning with in- 

 creased temperature or sunshine. Others are less readily 

 stimulated and remain closed until the conditions are 

 further intensified. Again, some blossom when the heat 

 of the day begins to decline, while the night-blooming 

 Cereus and certain other flowers bloom at night. 



Movements of floral leaves have been shown to be typi- 

 cally nastic growth movements and they disappear as 

 soon as the power of growth is lost in these organs, unless 

 accompanied by special basal articulations which may 

 show turgor movements. 



Quite as characteristic are the sleep movements of leaves 

 in a number of families, especially Leguminosse and Mi- 



