FOLIAGE AND SCALE LEAVES. 



55 



strong sun, may be served, as in most of the Australian myrtles, 

 Arc. A very interesting example is the compass-plant (Silphium 

 laciniatum) of Xorth America, in which the vertical leaves are 

 directed with their edges north and south. All these cases are 

 only apparent exceptions, for careful examination shows that the 

 vertical position is the result of a twist at the base of the leaf. 

 A very curious case is presented by the leaves of the iris and its 

 allies. The internodes of the stem are here extremely short, so 

 that the two- ranked leaves are closely crowded together. They 

 would overlap one another by expanding horizontally, and the 

 densely packed sheathing bases put a twisting into the vertical 

 position out of the question. No attempt is therefore made 

 to expose the upper surface to the light, but the leaf doubles 

 inwards, so that the under side is so exposed. This folding 

 together causes the bases of the older leaves to overlap or stride 

 over those of the younger ones, on which account the term 

 equitant has been applied. But this is not all. In the free part 

 of the leaf more or less union has taken place between the two 

 halves, the edges of which can be easily recognized by their 

 whitish membranous appearance. It will also be easily seen 

 that in this free part there is present, in addition to the 

 obviously doubled portion, an outer thinner region. This is a 

 vertical outgrowth from the under side of the leaf, by which 

 a large amount of leaf-surface is gained. 



The lamina is usually bilaterally symmetrical, but radial 

 symmetry is exhibited in some cases, as in the tubular leaves of 

 onion and the cylindrical leaves of rushes. In these cases, as in 

 iris and many other monocotyledons, it 

 may perhaps be best to regard the 

 leaves not as laminas, but as examples 

 of undifferentiated leaf-structures, i.e., 

 showing no distinction of parts (cf. 

 p. 52). Xot only are there some in- 

 stances where the symmetry is greater 

 than usual, but also others where it is 

 less. The common begonias of green- 

 houses are the best example of such 1111- 

 symmetrical leaves. Here the base of 

 the lamina bulges out on either side 

 into a lobe, one of which is much larger 

 than the other. Xo plane will divide 

 such a leaf into two exactly correspond- 

 ing halves. The same peculiarity is 

 exhibited, to a less extent, by the elm fig. 



(% 13). 



Considerable importance is attached to the venation of leaves, 



13.— Oblique Leaf of Elm, 

 with serrate margin. 



