146 ORGANOGRAPHY. 



bent or rolled. Of this we shall describe four varieties: — 1st. 

 When involute leaves are aj^plied together in a circle -without 

 overlapping, they are said to be induplicate {fig. 280); 2nd, If the 

 leaves are conduplicate, and the outer successively embrace and 

 sit astride of those next within them as if on a saddle. As 

 in Privet, and the leaves of the Iris at the base (^$r. 281), they 

 are equitant ; or 3rd, If the half of one conduplicate leaf re- 

 ceives in its fold the half of another folded in the same manner, 

 as in the Sage (fig. 282), the vernation is half-equitant or obvo- 

 lute ; and 4th, when a convolute leaf encloses another which is 

 rolled up in a like manner, as in the Apricot, the vernation is 

 supervolute {fig. 283). 



The terms thus used in describing the different modes of ver- 

 nation are also applied in like manner to the component parts 

 of the jflower-bud, under the collective name of (estivation or 

 prcefioration. We shall have therefore to refer to them again, 

 together Avith some others, not found in the leaf-bud, when 

 speaking of the flower-bud. 



3. THE INTERNAL STRUCTURE OF LEAVES. 



In describing the structure of leaves we shall divide them into 

 tv^o\imCiS,naxnt\y , aerial axiA submerged-, by the former, we under- 

 stand those that arc produced and live in the air; by tlie latter, 

 those that are formed and dwell wholly immersed in water. 



1. Aerial Leaves. — In the lowest plants which possess leaves 

 as already noticed (see p. 132), these consist simply of a cellular 

 plate formed by an expansion or growing outwards of the paren- 

 chyma of the circumference of the stem; while in the majority 

 of the higher plants they contain in addition to this parenchyma, 

 a framework or skeleton formed of wood-cells, and vessels of 

 different kinds, which are in direct connexion with the fibro- 

 vascular system of the stem. We distinguish therefore, in such 

 leaves, as in the stem — both a cellular and a flbro-vascular 

 system — the former constituting the soft ])arts or the paren- 

 chyma of the leaf; the latter the hard parts, Avhich by their 

 ramification form what are called veins, ribs, or nerves. 



The whole of the leaf is clothed by the epidermis, wliich is 

 commonly furnished Avith stomata in the manner already de- 

 scribed. The stomata arc, however, almost confined to that 

 portion of the epidermis which corresponds to the paren- 

 chyma of the leaf. The e])idermis is also furnished with 

 various appendages, as Hairs, Glands, &c. These, togctlier with 

 the ei)idcrmis, have ))een already fully described under their re- 

 spective heads: we have now tliereforc only to allude to the 

 parenchyma situated between the epidermis of tlie upper and 

 lower surfaces of the leaf, and the fibro- vascular system consti- 

 tuting its veins. 



a. Fibro-vasvular Si/stem. — This is in direct connexion with that 



