FORMS OF COMPOUND LEAVES. 



79 



152. Compound leaves have been classified according to the nature 

 of the venation, and the development of parenchyma. In a feather- 

 veined leaf, if 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 articu- 

 lated to the petiole or midrib (fig. 177), the leaf is pinnate (pinna, a 

 wing or feather). If the midrib and primary veins are not covered 

 with parenchyma, 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 

 bipinnate (fig. 178). In this case the secondary veins form as it were 

 partial petioles. A farther sub-division, in which the tertiary veins 

 only are covered with parenchyma and have separate leaflets, gives tri- 

 pinnate or decompound, in which case, the tertiary veins form the partial 

 petioles; and a leaf divided still more is called supradecompound (fig. 

 179). 



153. When a pinnate leaf has one pair of leaflets, it is unijugate 



(unurn, one, and jugum, a yoke); when it has two pairs, it is bijugate; 

 many pairs, multijugate (fig. 175). When a pinnate leaf ends in a 

 pair of pinnae (fig. 177), it is equally or abruptly pinnate (pari-pinnate); 

 when there is a single terminal leaflet (fig. 175), the leaf is unequally 

 pinnate (impari-pinnate) ; when the leaflets or pinna? are placed alter- 

 nately on either side of the midrib, and not directly opposite to each 



Fig. 177. Pari-pinnate leaf with six pairs of pinnae (sexjugate). 



V\K. 178. Bipinnate leaf, with sessile foliola or leaflets. 



Fig. 179. Part of the supradecompound leaf of Laserpitium hirsutum. 



