FOLIAGE AND SCALE LEAVES. 55 
strong sun, may be served, as in most of the Australian myrtles, 
&e. A very interesting example is the compass-plant (Si/phium 
laciniatum) of North 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 
eqguitant 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 lamine, but as examples 
of undifferentiated leaf-structures, 7.e., 
showing no distinction of parts (¢/. 
p- 52). Not 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 un- 
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. No plane will divide 
such a leaf into two exactly correspond- 
ing halves. The same peculiarity is 
exhibited, to a less extent, by the elm Fic. 13.—Oblique Leaf of Elm, 
(fig. I my: with serrate margin. 
Considerable importance is attached to the venation of leaves, 
