THEIR STRUCTURK AND CONFORMATION. 145 



When leaves with their margins involute are applied together in a 

 circle without overlapi^ing, the vernation is itiduplicate. Wlien, in 

 conduplicate leaves, the outer successively embrace or sit astride of 

 those next Avithin, the vernation is equitant, as the leaves of the Iris 

 at their base (Fig. 296) ; or when each receives in its fold the half 

 of a corresponding leaf folded in the same manner, the vernation is 

 half-equitant or ohvolute. These terms equally apply to leaves in 

 their full-grown condition, whenever they are then so situated as to 

 overlie or embrace one another. They likcAvise apply to the parts 

 in the flower-bud, under the name of aestivation or j)r£efloration. 

 Chap. IX. Sect. V. 



Sect. II. Their Structure and Conformation. 



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

 Blade {Lamina or Limb, Fig. 229, h), with its Petiole or Leaf- 

 stalk, p, and at its base a pair of Stipules, st. Of these the latter 

 are frequently absent altogethei*, and in many cases where they 

 originally exist they fall away as the leaf expands. The petiole is 

 very often wanting ; when the leaf is sessile, or has its blade rest- 

 ing immediately on the stem that bears it (as in Fig. 210, 211). 

 Sometimes, moreover, there is no proper blade, but the whole organ 

 is cylindrical or stalk-like. It is the general characteristic 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 practicable amount of paren- 

 chyma containing the gi-een matter of vegetation {chlorojjhyll, 92), 

 upon which the light exerts its peculiar action. Leaves as foliage, 

 accordingly, are what we ai'e now principally to consider 



260. 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 Avoody, is the 

 skeleton or framework which ramifies among and strengthens the 

 former. The woody or fibrous portion fulfils the same purposes in 

 the leaf as in the stem, not only giving firnmess and support to the 



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