CHAP. VII. SLEEP OF LEAVES. 349 



recommenced, and became rapid late in the evening, when 

 the leaflet was beginning to go to sleep. The awaking or 

 sinking movement had already commenced by 6.45 A.M. on both 

 mornings. 



Trifolium (Tribe 3). The nyctitropic movements of 11 

 species were observed, and were found to be closely similar. If 

 we select a leaf of T. repens having an upright petiole, and with 

 the three leaflets expanded horizontally, the two lateral leaflets 

 will be seen in the evening to twist and approach each other, 

 until their upper surfaces come into contact. At the same time 

 they bend downwards in a plane at right angles to that of their 

 former position, until their midribs form an angle of about 45 

 with the upper part of the petiole. This peculiar change of 

 position requires a considerable amount of torsion in the pul- 

 vinus. The terminal leaflet merely rises up without any twist- 

 Fig. 141. 



A. B. 



Trifolium repens: A, leaf during the day ; B, leaf asleep at night. 



ing, and tends over until it rests on and forms a roof over the 

 edges of the now vertical and united lateral leaflets. Thus the 

 terminal leaflet always passes through an angle of at least 90, 

 generally of 130 or 140, and not rarely as was often observed 

 with T. subterraneum of 180. In this latter case the terminal 

 leaflet stands at night horizontally (as in Fig. 141), with its 

 lower surface fully exposed to the zenith. Besides the difference 

 in the angles, at which the terminal leaflets stand at night in 

 the individuals of the same species, the degree to which tha 

 lateral leaflets approach each other often likewise differs. 



We have seen that the cotyledons of some species and not of 

 others rise up vertically at night. The first true leaf is generally 

 unifoliate and orbicular ; it always rises, and either stands verti- 

 cally at night or more commonly bends a little over so as to expose 

 the lower surface obliquely to the zenith, in the same manner 

 as does the terminal leaflet of the mature leaf. But it does not 

 twist itself like the corresponding first simple leaf of Melilotus. 



