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. e.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 (ef. fig. n). 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 Petiole. Like the leaf 

 as a whole, it is generally bilaterally sym- 

 metrical, and its upper surface is very fre- 

 quently grooved, its lower surface being at the 

 same time ridged (tig. 29). In some leaves, especially those 

 which possess the power of movement, the petiole presents a 

 swelling at its base. the put ri)t us 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. Examples are 

 known, but not among British plants, 

 where climbing is effected by means of 

 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 



Fig. 10.— Part of a Grass 

 Leaf. gv. sheath; f. 

 blade ; gl. ligule. 



FIG 



Base of Willow Leaf, 

 showing stipules, s, s, and 

 axillary bud, b. 



