268 



BOTANY 



leaf-stalks, the inner and outer sides of floral leaves) show an unequal 

 growth in response to even transitory and slight variations in tempera- 

 ture and in the intensity of light. Whenever, on account of such 

 variations, the growth of the under side of a leaf exceeds that of the 

 upper side, the whole leaf moves upwards and 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 in flower-leaves, 



Fig. 210. — Loplmspermum scandens climbing by means of its tendril-like petioles. 



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 Golchicum, 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 dif- 

 ference 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 stronger growth of one side occurs in this case either at the base 

 or upper part (Colchicum) of the perianth leaves. 



Fig. 211. — Composite flower of Leontodon hastilis, 

 closed when kept in darkness, open when illu- 

 minated. (From Detmer's Physiol. Frnct.) 



