254 PART III. VEGETABLE PHYSIOLOGY. 



Some flowers have very regular hours of opening and closing ; 

 for example, according to Linne and De Candolle, the Purple 

 Morning-glory opens at 2 a. m. ; the White Water-lily at 7 a. m. ; 

 the Blue Passion-flower at 12 m. ; the common Evening Primrose 

 at 6 p. M., and the Night-blooming Cereus between 7 and 8 p. M. 

 Sometimes the movements appear to be dependent on variation 

 in the intensity of light ; at others they seem to be quite inde- 

 pendent of it, as in the case of the Goat's-beard (Tragopogon 

 pratensis), which opens in the morning and closes at or 

 before noon. 



The opening and closing movements of the floral organs are 

 accomplished, like those of ordinary leaves, by unequal growth, 

 or, sometimes, mainly by unequal turgescence of the upper and 

 under surfaces of each organ, or of its basal portion. 



So far as the utility of these floral movements are concerned, 

 they mostly have reference in some way to cross-fertilization. 

 The closing of a flower at night, or when the sky is darkened at 

 the approach of a storm, serves to prevent the wastage of its 

 nectar and pollen by dew and rain, and the closing at night, in 

 some cases at least, prevents the access of night-flying insects 

 that could not be serviceable to the flower in cross-fertilization, 

 while in the reverse case of flowers that open by night and close 

 by day, they are mostly adapted to cross-fertilization by night- 

 flying insects, and it is an obvious advantage to them to protect 

 themselves, by closing, from unserviceable day-flying insects. 



Besides these, there are other more conspicuous movements 

 observed in some plants, the use of which is not so well under- 

 stood. The Telegraph Plant (Desmodium gyrans), a native of 

 India, affords a conspicuous example. The plant has compound 

 leaves, with the leaflets in threes, two small lateral ones, and a 

 much larger terminal one. The lateral leaflets are in constant 

 motion, sometimes moving up and down, and sometimes circu- 

 larly. The motions are often rapid, particularly in bright sun- 

 shine, and they are frequently unequal and jerky. 



These movements, as well as those described as nyctitropic, 

 are also to be regarded as modifications of circumnutation. 



Irritability. Attention has already been called to various 

 phenomena under this head, such as the sensitiveness to contact 

 shown by the leaves of the Sensitive Plant, by those of Venus' 



