GENERAL 



101 



view is correct, the blocking of the stomata is more likely to be a serious matter by 

 preventing the assimilation of carbon dioxide during the early morning hours. Both 

 the avoidance of dew-formation and of cooling may be of importance, and possibly 

 the sleep-movements may have still other biological advantages. 



In many cases organs are brought into positions which enable them to utilize the 

 light best or to avoid it when intense, and this may take place by the aid of photo- 

 nastic reactions, coupled with tropic movements. Although photonastic reactions 

 may often be feeble, there is no reason for restricting the term nyctinastic to pro- 

 nounced sleep-movements. 



SECTION 22. Instances of Photonastic and Diurnal Movements. 



Pronounced daily movements are performed by the foliage and floral 

 leaves of many plants, and these movements are in some cases induced by 



FIG. 28. Leafy shoot of Desmodiunt gyrans. A in the day position, B in the night position (after Darwin). 



the daily changes of illumination, but in other cases by alterations of 

 temperature. Frequently the power of photonastic response is accompanied 

 by a feeble or pronounced capacity for thermonastic reaction. In such 

 cases decreases of temperature or illumination usually induce similar 

 curvatures ; and, since the movements produced by increases are also alike, 

 both factors co-operate at morning and evening in producing the awakening 

 or sleep-movements as the case may be. 



The photonastic sleep-movements are the result either of growth or of 

 variation curvatures, the latter of which are commonly performed wherever 

 motile pulvini are present. These movements are shown by the leaves of 



