92 



FORMS OF COMPOUND LEAVES. 



from the axis as a single piece, and its subsequent divisions in the 

 form of leaflets are all in one plane. The leaflets are either sessile 

 (fig. 192), or have stalks, caRed petiolules (fig. 191), according as the 

 vascular bundles of the veins spread out or divaricate at once, or remain 

 united for a certain length. 



Compound leaves have been classified according to the nature of 

 the venation, and the development of parenchyma. If we suppose that 

 in a simple feather-veined unicostate leaf, the divisions extend to 

 the midrib, and each of the primary veins spreads out or branches 

 so as to become covered with parenchyma, and thus form separate 

 leaflets, which are usually articulated to the petiole or midrib (fig. 193), 

 the leaf becomes compound and pinnate (pinna, a wing or feather). 

 If the midrib and primary veins are not covered with parenchyma. 



Fig. 193. 



rig. 196. 



while the secondary (or those coming off in a feather-like manner from 

 the primary veins) are, and separate leaflets are thus formed which 

 are usually articulated with the veins, the leaf is hipinnate (fig. 194). 

 In this case the secondary veins form as it were partial petioles. A 

 farther subdivision, in which the tertiary veins only are covered with 

 parenchyma and have separate leaflets, gives tripinnate or decompound, 

 in which case the tertiary veins form the partial petioles ; and a leaf 

 divided still more is called supradecon^ound (fig. 195). 



When a pinnate leaf has one pair of leaflets, it is unijugate (unum, 

 one, and j-ugum, a yoke) ; when it has two pairs, it is hijngate; many 



Fig. 193, Pari-pinnate leaf with six pairs of pinnae (sa^ugate). Fig. 194. Bipinnate leaf, 

 witli sessUe foliola or leaflets. Fig. 196. Part of the supradeeompound leaf of Laserpitium 

 hirsutmn. 



