152 THE LEAVES. 



258. Considered relatively to each other, leaves are valvate in 

 vernation when corresponding ones touch each other by their 

 edges only, without overlapping : they are imbricated when the 

 outer successively overlap the inner, by their edges at least, in 

 which case the order of overlapping exhibits the phyllotaxis, or or- 

 der of succession and position. In these cases the leaves are plane 

 or convex, at least not much bent or rolled. When leaves with 

 their margins involute are applied together in a circle without over- 

 lapping, the vernation is induplicate. When in conduplicate leaves 

 the outer successively embrace or sit astride of those next within, 

 the vernation is equitant, as the leaves of the Iris at their base : or, 

 when each receives in its fold the half of a corresponding leaf 

 folded in the same manner, the vernation is half-equitant or ob- 

 volute. These terms equally apply to leaves in their full-grown 

 condition, whenever they are then folded or placed so as to overlie 

 or embrace one another. They likewise apply to the parts in the 

 flower-bud, under the name of aestivation or prsefloration. 



SECT. II. THEIR STRUCTURE AND CONFORMATION. 



259. Anatomy of the Leaf, The complete leaf consists of the 

 BLADE (Lamina or Limb), with its PETIOLE or Leafstalk, and 

 at its base a pair of STIPULES. Of these the latter are frequently 

 absent altogether, or else they fall away as the leaf expands : the 

 petiole is very often wanting, when the leaf is sessile, or has its 

 blade resting immediately on the stem that bears it. Sometimes, 

 moreover, there is no proper blade or expanded portion, but the 

 whole organ is cylindrical or stalk-like. It is the general charac- 

 teristic of the leaf, however, that it is an expanded body. Indeed, 

 it may be viewed as a contrivance for increasing the green surface 

 of a plant, so as to expose to the light and air the greatest practi- 

 cable amount of parenchyma containing the green matter of vege- 

 tation (chlorophyll, 87), upon which the light exerts its peculiar ac- 

 tion. In a general, mechanical way, it may be said leaves are defi- 

 nite protrusions of the green layer of the bark, expanded horizon- 

 tally into a thin lamina, and stiffened by tough, woody fibres (con- 

 nected both with the liber, or inner bark, and the wood), which form 

 its framework, ribs, or veins. Like the stem, therefore, the leaf 

 is made up of two distinct parts, the cellular and the woody. The 

 cellular portion is the green pulp or parenchyma : the woody, is 



