PHYSIOLOGY 



291 



towards the parent axis ; while if the growth of the upper side is the 

 stronger, the leaf is depressed. 



Movements of this nature are especially noticeable iu flower-leaves, and bring 

 about the opening and closing of the flower. A rise of temperature causes the 

 flowers of the Tulip and Crocus, and also those of Adonis, Ornithogalum, and 

 Colchicum, to open, while sudden cooling causes them to close. Tulips and 

 Crocuses, if brought, while still closed, into a warm room, open in a very short 

 time ; with a difference of temperature of 15-20 C., in from two to four minutes. 

 Crocuses respond to an alteration in temperature of C. ; Tulips to a variation of 

 2-3 C. In warm' sunshine the spring or summer flowers are open for the visits of 

 insects, but on a lowering of temperature the sexual organs are covered up and 

 protected. 



The flower-heads of Taraxacum, Leontodon, and other Composites, also the 

 flowers of Nyuiphaca, Cacti, etc., 

 open when illuminated, and close 

 when kept in darkness (Fig. 235). 



Variation of light produces also 

 unequal growth in foliage-leaves, 

 particularly in those of the Chenopo- 

 diaceae, Caryophyllaceae, and Balsa- 

 minaceae, and cause them to assume 

 so-called SLEEP POSITIONS. 



In many instances the move- 

 ments of the floral leaves are pro- 

 duced by variations of light as well 

 as of temperature ; for example, 

 some of the flowers mentioned open 

 in the light and close in the dark, al- 

 though the temperature remains con- 

 stant. In the case of opposing external influences, the resulting direction of the 

 movement of the flower-leaves is determined by the influence which is predominant. 

 The dependence of these movements upon different, and often opposing, influences, 

 together with the continuance of movements induced by previously operative 

 influences, led to their being for long misunderstood, especially as the movement 

 of opening is from internal causes followed by a closing movement even if a high 

 temperature is maintained ( 95 ). 



These movements, occasioned by variations in the illumination 

 and temperature, must not be confused with those of heliotropism 

 and thermotropism ; in both of which the movement induced in an 

 organ is dependent upon its relative position with respect to the 

 source of the light or heat, and not upon the varying intensity of 

 the stimulus. The movements last discussed are photonastic or 

 thermonastic, i.e., they result from the variation in illumination or 

 temperature, and not from the direction of the light or heat rays. 



FIG. 236. Flower-head of Leontodon hastilis, closed 

 when kept in darkness, open when illuminated. 

 (From DETMER'S PhysM. Pract.) 



3. Movements due to Changes of Turgor (Variation Movements) 

 The various movements hitherto considered are, to a large extent, 



