VERTICAL AND PERFOLIATE: LEAVES. 



165 



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 said to be 

 decompound. 



293. Leaves of Peculiar Conformation. The blade of a leaf is almost 



always symmetrical, that is, the portions on each side of the midrib 

 or axis are similar ; but occasionally one side is more developed than 

 the other, when the leaf is oblique, as is strikingly the case in the 

 species of Begonia (Fig. 246) of our conservatories. 



294. Vertical and Equitant Leaves. 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 Avell as in 

 appearance, each being fitted for its peculiar of- 

 fices : if artificially reversed, they spontaneously 

 resume their natural position, or soon perish if 

 prevented from doing so. But in erect and verti- 

 cal leaves, the two surfaces are equally exposed 

 to the light, and are similar in structure and ap- 

 pearance. In such erect and equitant leaves as 

 those of Iris (Fig. 291), it is really the lower sur- 

 face that is presented to the air ; for the leaf is 

 folded together lengthwise 

 (conduplicate), and consoli- 

 dated while in the nascent 

 state, so that the true upper 

 surface is concealed in the 

 interior, except near the 

 base, where they alternately 

 cover over each other in the 

 equitant manner (258, Fig. 

 292). True vertical leaves, 

 which present their edges instead of their surfaces to the earth and 

 sky, generally assume this position by a twisting of the base or 

 the petiole ; as is strikingly seen in the Callistemon and many other 

 Australian trees of the Myrtle family, some of which are now com- 

 mon in green-houses. 



295. Perfoliate Leaves. While in Iris the two halves of the 



FIG 291. Equitant erect leaves of Iris, with the rootstock. 



FIG £92. A section across these leaves at the base, showing their etputant character. 



