282 MODIFIED CIRCUMNUTATION. CHAP. VI 



pound leaves, forwards, that is, towards the apex of the 

 leaf, or backwards, that is, towards its base ; or, again, 

 they may rotate on their own axes without moving 

 either upwards or downwards. But in almost every 

 case the plane of the blade is so placed as to stand 

 nearly or quite vertically at night. Therefore the apex, 

 or the base, or either lateral edge, may be directed 

 towards the zenith. Moreover, the upper surface of 

 each leaf, and more especially of each leaflet, is often 

 brought into close contact with that of the opposite 

 one ; and this is sometimes effected by singularly 

 complicated movements. This fact suggests that the 

 upper surface requires more protection than the lower 

 one. For instance, the terminal leaflet in Trifolium, 

 after turning up at night so as to stand vertically, 

 often continues to bend over until the upper surface is 

 directed downwards whilst the lower surface is fully 

 exposed to the sky ; and an arched roof is thus 

 formed over the two lateral leaflets, which have their 

 upper surfaces pressed closely together. Here we have 

 the unusual case of one of the leaflets not standing 

 vertically, or almost vertically, at night. 



Considering that leaves in assuming their nycti- 

 tropic positions often move through an angle of 

 90 ; that the movement is rapid in the evening ; 

 that in some cases, as we shall see in the next 

 chapter, it is extraordinarily complicated; that with 

 certain seedlings, old enough to bear true leaves, 

 the cotyledons move vertically upwards at night, 

 whilst at the same time the leaflets move ver- 

 tically downwards ; and that in the same genus 

 the leaves or cotyledons of some species move 

 upwards, whilst those of other species move down- 

 wards ; from these and other such facts, it is hardly 

 possible to doubt that plants must derive some 



