MONOCOTYLEDONS. 



547 



some others, a long and comparatively slender stalk developes between the sheath 

 and the lamina. When the leaf-stalk is absent, and the lamina sharply marked 

 off from the sheath, a Ligule is not unfrequently present at the point where the 

 two meet, as in Grasses and Allium (Fig. 394). 



The lamina is generally entire and of a very simple form, commonly 

 long and narrow (ligulate), rarely roundish and disc-shaped {e. g. Hydrocharis), 

 or cordate or sagittate (as in Sagittaria and some Aroidese). Branching of the 

 lamina is a rather rare exception among IMonocotyledons ; and then takes the 

 form either of lobes from a broad common base or less often of deep divisions, 

 as in some Aroidese {e.g. Amorphophallus, Fig. 133, p. 162, Atherurus, and 

 Sauromatum). The division of the com- 

 pound and pinnate leaves of Palms is 

 not due to a branching occurring at an 

 early stage, but to a splitting which takes 

 place on unfolding, and is caused by the 

 drying up of certain strips of tissue with- 

 in the lamina, which is at first sharply 

 folded up. The formation of the tendrils 

 of Smilax appears, on the other hand, to 

 depend on actual branching of the leaf- 

 stalk. 



The Venation of the foliage-leaves 

 differs from that of most Dicotyledons, 

 in the weaker veins not generally pro- 

 jecting on the under side of the leaf, but 

 running through the mesophyll ; in the 

 smaller leaves there is even no projecting 

 mid-rib. The mid-rib is, on the other 

 hand, strongly developed in the large 

 stalked leaves of the Spadiciflorae and 

 Scitamineae, and is permeated by a num- 

 ber of fibro-vascular bundles. When the 

 leaf is ligulate and its insertion broad, the 

 fibro-vascular bundles run nearly parallel 

 to one another; in broader leaves with- 

 out a conspicuous mid-rib they describe 

 curves from the mid -rib to the margins 



(as in Convallaria). But when a strong mid-rib occurs in a broad lamina, as 

 in Musa &c., the fibro-vascular bundles which run through it give off laterally 

 smaller thin bundles, running parallel to one another in large numbers to the margin 

 of the leaf. These parallel transverse nerves are sometimes united into a 

 lattice-like network by short straight anastomosings (as in Alisma, Costiis, and 

 Ouvirandra, the mesophyll being absent within the meshes of the latter). It 

 is only rarely (as in some Aroidese), that projecting lateral veins are given off 

 from the mid-rib, a finer reticulated venation springing from them. 



The Floiver of jMonocotyledons usually consists of five alternating whorls each 



N n 2 



/ 



Fig. 394.— a leaf of AUnim Cepa divided length- 

 wise ; z the thickened base of the sheatli, which persists 

 as a bulb-scale after the upper part of the leaf has died 

 down, s the membranous part of the sheath, / the hollow 

 lamina, h hcUow of the leaf, i' inner side of the lamina, 

 X ligule. 



