170 THE LEAVES. 



tosa ; which is distinguished from a really simple leaf by the joint 

 at the junction of the partial with the general petiole. 



290. The palmate or digitate form is produced when a leaf of 

 the palmately veined sort becomes compound ; in which case the 

 leaflets are necessarily all attached to the apex of the common 

 petiole, as in the Horsechestnut and Buckeye (Fig. 211). Such 

 leaves of three, five, or any definite number of leaflets are termed 

 palmately (or digitately) trifoliolate, five-foliolate, &c. A leaf of 

 two leaflets, which rarely occurs, is unijugate (one-paired) or bi- 

 nate. By this nomenclature, the distinction between pinnately and 

 palmately compound leaves is readily kept up. 



291. The stalk of a leaflet is called a partial petiole (petiolula) ; 

 and the leaflet thus supported is petiolulate. 



292. The partial petioles may bear a set of leaflets instead of a 

 single one, when the leaf becomes doubly or twice compound. 

 Thus a pinnate leaf again compounded in the same way becomes 

 bipinnate, or if still a third time divided it is tripinnate, &c. In 

 these cases the main divisions or branches of the common petiole 

 are called pinna. So a trifoliolate leaf twice compound becomes 

 biternate ; or thrice, triternate, &c. When the primary division 

 is digitate, the secondary division is often pinnate, thus combining 

 the two modes in the same leaf. A leaf irregularly or indeter- 

 minately several times compounded, in whatever mode, is decom- 

 pound. 



293. The blade of a leaf is almost always symmetrical, that is, 

 the portions on each side of the midrib or axis are similar ; but oc- 

 casionally one side is more developed than the other, when the leaf 

 is oblique, as is strikingly the case in the species of Begonia (Fig. 

 210), now common in gardens. 



294. The blade is also commonly horizontal, presenting one 

 surface to the sky, and the other to the earth ; in which case the 

 two surfaces differ in structure (262) as well as in appearance, 

 each being fitted for its peculiar offices : if artificially reversed, 

 they spontaneously resume their natural position, or soon perish if 

 prevented from doing so. But in erect and vertical leaves, the 

 two surfaces are equally exposed to the light, and are similar in 

 structure and appearance. In such erect leaves as those of Iris, it 

 is what corresponds to the lower surface of ordinary leaves that is 

 presented to the air ; for the leaf is folded together lengthwise and 

 consolidated while in the nascent state, so that the true upper sur- 



