hl 
STRUCTURE OF THE PETIOLE. 179 
5. PETIOLE OR LEAF-STALK. 
The petiole or leaf-stalk is that part which connects the 
blade of the leaf with the stem or branch (figs. 273, p, and 274, p). 
It is frequently absent, and the leaf is then said to be sessile 
(fig. 286). It consists, as already described (page 143), of fibro- 
vascular tissue (jig. 372, fv), surrounded by parenchmya pce, and 
the whole covered by epidermis, which contains a variable number 
of stomata, and is frequently furnished with hairs and other epi- 
dermalappendages. The fibro-vascular tissue varies in its nature 
in the leaves of Dicotyledons, Monocotyledons, and Cormophytes, 
being in structure essentially the same in each case as that of the 
Fic. 374, 
Bie. 302. Fie. 373. 
Serer 
suaeres 
Fig. 372. Vertical section of a portion of the stem and the base of a leaf, 
showing the passage of the fibro-vascular tissue, fv, into the petiole. pe, pe. 
Parenchymatous tissue of the stem and petiole. c. Pulvinus. jf. Articu- 
lation between theleafand stem. 5. Leaf-bud in the axil of the petiole. m. 
Pith Fig. 373. A portion of a branch and leaf of the Sensitive Plant, 
showing pulvinus at the base of the petiole-——Fig. 374. A portion of 
the stem of aGrass with a leaf attached, 7. Blade. g. Sheathing petiole. 
lig. Ligule. 
three kinds of stem already fully described ; thus, in Dicoty- 
ledons, the fibro-vascular tissue (fig. 276), consists of spiral, 
pitted, annular, or some other vessels (see page 143), and sieve 
tubes, and wood and liber cells, that is, of the same elements 
essentially as the wood and liber. The distribution of this fibro- 
vascular tissue in the lamina forms the veins, which have been 
already described under the head of Venation (page 157). 
The petiole is either simple or undivided, as in all simple 
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