64 THE FLOWERING PLANT. 
stem from the leaf insertion. Adhesions in this direction form 
transitions to sheathing or ochreate stipules, which, as in dock, 
sorrel, and knot-grass (fig. 28), or 
bistort, form a membranous sheath 
surrounding the stem for some dis- 
tance above the leaf insertion. The 
stipules of opposite leaves may unite 
on either side into an ¢nterfoliar 
stipule. Tendrils rarely and spines 
more frequently (garden acacia) re- 
sult from modifications of this part 
of the leaf. The leaflets of com- 
pound leaves may possess single 
Fa. 28.—Leaf of Knot-Grass. stupels, meen scarlet ian (tig. 29). 
l. lamina; p. petiole; g. sheathing As in the lamina, we can distinguish 
nee. different textures and varieties of 
surface. Membranous stipules often fall off early, and are then 
said to be deciduous, as in beech and lilac. 
Scale leaves are much simpler in character than foliage leaves, 
and in fact may be regarded as reduced representatives of these. 
They alone occur in subterranean stems, and in this case appear 
to represent leaf-sheaths. In this situation they may be insigni- 
ficant scales, like those in the axils of which potato-eyes are 
developed, or else, as in bulbs, they may be considerably thickened 
in connection with the storage of reserve materials. Scale-leaves 
are also found on underground stems, and in some forms which 
are parasitic or saprophytic they entirely supplant the ordi- 
nary leaves. The broom rape (Orobanche) and yellow bird’s-nest 
(Monotropa) are examples of this. Most frequently, however, 
overground scale leaves are found as bud scales, either for storage, 
as in bulbils, or for protection, as in the buds of trees (p. 46). 
In the latter case they often grade insensibly into the ordinary 
leaves of the bud, when their leaf nature becomes evident. They 
may correspond to leaf-sheaths (fir and horse-chestnut), lamine 
(lilac), or stipules (beech). The sticky substance bud-glue or blas- 
tocolla, found in horse-chestnut and many other cases, is ex- 
creted by special glandular hairs known as colleters. 
Structure of the Leaf.—lIt is not necessary to give any details 
regarding the structure of the leaf-stalk, since this is constructed 
almost precisely like a young stem, the chief difference being 
that the bilateral symmetry usually perceptible externally extends 
also to the interior. 
The lamina of an ordinary foliage leaf (fig. 7, G) consists of 
upper and lower layers of epidermis, between which is the green 
pulpy ground-tissue, here called mesophyll, traversed by the 
