FOLIAGE AND SCALE LEAVES. 53 
continuous with a sheath that closely surrounds the stem (fig. 10). 
It can, however, easily be removed, since it is not a complete 
tube, there being a longitudinal split traversing it on the side 
opposite to the lamina. In a sedge the sheath is a complete 
tube. ‘This structure is especially common among monocotyledons, 
though by no means confined to them. The sheath is often pre- 
sent in a very rudimentary way, as, @g., in 
the leaves of the common groundsel, which, 
by the way, illustrate the difficulty of distin- 
guishing between a winged petiole and a nar- 
rowed region of the lamina. Here we, per- 
haps, have to do with the latter condition. 
Again, many leaves possess stipules, which are 
membranous or leaf-like outgrowths situated 
on either side of the insertion of the leaf. 
Examine once more a young shoot of the 
garden geranium, and note a green expansion 
or stipule on either side of the point men- 
tioned. In an older shoot they will be 
withered and brown (cf, fig. 11). The dif- 
ferent regions enumerated, petiole, lamina, 
sheath, and stipules, may now be considered 
in greater detail. 
Little need be added to what has already 
been said about the Prriote. Like the leaf 
as a whole, it is generally bilaterally sym- y,. ... part of a Grass 
metrical, and its upper surface is very fre- Leaf. gv. sheath; f. 
quently grooved, its lower surface being atthe "#42; 9! lsule. 
same time ridged (fig. 29). In some leaves, especially those 
which possess the power of movement, the petiole presents a 
swelling at its base, the pulvinus or motile 
organ (fig. 29). This may be seen in 
the scarlet runner, sensitive plant, and 
the common garden acacia (Robinia pseud- 
acacia). The peculiar tremulous move- 
ment of the leaves of the aspen is due to 
the fact that the leaf-stalks are vertically 
flattened, thus presenting a considerable 
surface to the wind. Examp les are FIG. 11.—Base of Willow Leaf, 
known, but not among British plants, ~ showing stipules, s, s, and 
where climbing is effected by means of axillary bud, d. 
the leaf-stalks, which are capable of twisting round a support. 
Just as the stem may become flattened and assume the functions 
of the leaf, so also may the petiole make up for the small size 
or absence of the lamina. Such flattened phyllodes are espe- 
cially characteristic of the Australian eucalypti and acacias. These 
