160 BRITISH FLOWERING PLANTS 



Trifolitjm (Clover) 



The seed-pods are protected in several ways. In 

 T. suhterraneum by the rigid, persistent flowers ; in 

 T. fragiferum by the swollen calyx ; in T. globosum 

 by large brushes of hairs which completely envelop 

 them. Those of T. badium are flattened. In this 

 genus also leaf movements may be observed ; for in- 

 stance, in the common White Clover [T. repens). 

 If we take a leaf with an upright stalk and three 

 horizontal leaflets, we find at night that the lateral 

 leaflets twist and approach one another until their 

 upper surfaces come in contact, bending down at the 

 same time at right angles to their original position, 

 until the midribs form an angle of about 45° with 

 the petiole. The terminal leaflet, on the contrary, rises 

 up and turns over so as to form a sort of roof over 

 the other two leaflets, with its lower surface turned 

 upwards. In T. pratense the leaflets rise at night, 

 thus presenting a smaller radiating surface, and 

 Darwin found that leaves which were prevented from 

 doing so sufi"ered more from frost. The autonomous 

 movements of the leaflets of T. pratense are very 

 considerable ; the terminal leaflet may accomplish a 

 movement of 30°- 120° in the course of one and 

 a half to four hours. " If such a plant, previously 

 exposed to daily variation of light, be illuminated in the 

 evening and thenceforth kept continuously in the light, 

 no further closing movement corresponding to the daily 

 period is to be observed even on the following evening, 

 apparently because it is obscured by the autonomous 

 movements, which go on with an amplitude of as 

 much as 100°, and with a rhythm of about two hours. 

 These autonomous movements continued also unchanged 

 while the plant was kept constantly illuminated for 

 two days longer." ^ 



The stipules ofier many points of interest. For 



' Sachs, Physiology of Plants. 



