THEIR FORM, DIVISION, ETC. 171 



face is concealed in the interior, except near the base, where they 

 override each other in the equitant manner (258). 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 of the petiole ; as is strikingly seen in a large number of 

 New Holland trees of the Myrtle Family, now common in green- 

 houses. 



295. Leaves assume extraordinary appearances when they be- 

 come succulent, as in the different species of Mesembryanthemum 

 (Ice-plant), &c., and no less so when, on the contrary, producing 

 little or no green parenchyma, they become scale-like, as in Beech- 

 drops, Monotropa, and other parasitic plants ; where they do not 

 perform the ordinary office of leaves. Not unlike these are the 

 altered or degenerate leaves that form the in- 

 teguments of scaly buds (146). The primary 



leaves on every shoot of the Pine are merely 

 thin and dry scales ; from the axils of which 

 the ordinary foliage is developed in fascicles 

 of needle-shaped leaves (253). 



296. Leaves which grow under water are 

 often nearly or quite destitute of parenchyma ; 

 as in Ranunculus Purshii (Fig. 207), and Ra- 

 nunculus aquatilis, Bidens Beckii, Myriophyl- 

 lum, &c. A very remarkable instance of the 

 kind occurs in Ouvirandra fenestralis, a South 

 African aquatic plant, with nerved leaves, 

 which exhibit a complete framework or skel- 

 eton, while the parenchyma is entirely want- 

 ing. In the Barberry some of the summer 

 leaves harden as they grow into compound or 

 branching spines (Fig. 222). 



297. When the blade of the leaf is want- 

 ing, its office is sometimes performed by the 

 petiole, or by the stipules. 



298. The Petiole, or Leafstalk, is usually either round, or half- 

 cylindrical and channelled on the upper side. But in the Aspen, 

 it is strongly flattened at right angles with the blade, so that the 

 slightest breath of air puts the leaves in motion. It is not unfre- 



FIG. 222. A summer shoot of the Barberry, showing a lower leaf in the normal state; the 

 next partially, those still higher completely, transformed into spinea. 



