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 interfoliar 

 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 

 , . r, stipels, as in scarlet runner (fig. 20). 



xiu. 20.— Leaf of Knot-Grass. L . ' , . ,. \.° ?L 



I. lamina ; p. petiole ; g. sheathing As in the lamina, we can distinguish 

 stipules. 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 on 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 (Orohaiiche) 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), laminse 

 (lilac), or stipules (beech). The sticky substance bud-glue or bias- 

 tocolla, found in horse-chestnut and many other cases, is ex- 

 creted by special glandular hairs known as colleters. 



Structure of the Leaf. — It 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 mcsophyll, traversed by the 



