60 PART I. THE MORPHOLOGY OF PLANTS. [ 12. 



face to face on the midrib as on a hinge (e.g. the Bean) ; it is plicate when the 

 leaf is folded in numerous longitudinal or oblique pleats (e.g. the Beech) ; it is 

 crumpled, when the foldings are in all directions (e.g. the petals of the Poppy) ; 

 it is involute, when the lateral halves are rolled inwards towards the mid-rib 

 on the ventral surface (e.g. the Violet) ; it is revolute, when they are rolled 

 inwards towards the mid-rib on the dorsal surface (e.g. the Dock) ; it is convolute 

 when the whole leaf is rolled up from one lateral margin to the other, so as to 

 form a single roll (e.g. Canna) ; or, finally, it is circinate, when the leaf is rolled 

 longitudinally on itself from the apex downwards (e.g. Ferns). 



The form of vernation of the individual leaf is determined by 

 the relative activity of growth in its different parts. For instance, 

 the conduplicate, involute, convolute, and circinate forms are all 

 due to the fact that the leaves which present them are hyponastic, 

 that is, that the dorsal surface grows more rapidly at first than 

 the ventral ; and this may be either in the transverse plane, when 

 the infolding is lateral or transverse (conduplicate, involute, con- 

 volute) ; or in the longitudinal plane, when the infolding is longi- 

 tudinal (circinate). Revolute vernation is, on the contrary, due 

 to the fact that the leaf is transversely epinastic, that is, that the 

 ventral surface grows more rapidly at first in the transverse plane 

 than does the dorsal. The expansion of the leaf, in either case, is 

 due to a reversal of the activity of growth ; that is, the surface 

 which grew the less actively in the early stage of development, 

 grows the more actively in the later stage (see also page 742, 

 Epinasty and Hyponastij). 



The prefoliation (or preparation) is said to be valvate when adjacent leaves in 

 the bud merely touch by their margins ; when some are overlapped by others it 

 is imbricate ; an intermediate form is that in which one margin of each leaf 

 is directed obliquely inwards, and the other obliquely outward overlapping the 

 inner margin of the next leaf, and is termed contorted or twitted (e.g. petals of 

 the Periwinkle). 



Valvate prefoliation is only possible in the case of whorled leaves, whereas 

 imbricate prefoliation is characteristic of spirally arranged leaves. A common 

 form of imbricate prefoliation or prefloration is the quincuncial, which occurs in 

 the many dicotyledonous flowers which have a g calyx ; the five imbricate sepals 

 are so arranged that two are wholly internal, two wholly external, and one partly 

 internal and partly external, connecting the outer two with the inner two (see 

 p. 496, Phyllotaxy of tne Flower). Where the phyllotaxy is distichous (^), 

 the vernation of the leaves is generally conduplicate, and the margins of each 

 older leaf overlap those of the next younger leaf, giving rise to a form of 

 imbricate prefoliation distinguished as equitant (e.g. Iris) ; in some cases the 

 overlapping is by one margin only, in which case the prefoliation is said to be 

 semi-cquitant. 



