LEAVES OF EXOaENS, ENDOGENS, AND ACEOGENS. 101 



this kind are seen in Acacias (fig. 204), as well as in Ranunculus 

 gramineus and K. Lingua. 



Leaves op Monocotyledons. — In Monocotyledons, the leaves 

 do not present an angular network of vessels, nor do they exhibit 

 divisions on their margin (figs. 150, 210). Exceptions to this rule 

 occur in some plants, as Tamus and Dioscorea, which have been called 

 Dictyogens by Lindley, on account of their somewhat netted venation ; 

 and in Palms, in which, although the leaves are entire at first, they 

 afterwards become split into various lobes. Leaves of Monocotyle- 

 dons are rarely stipulate, unless the ligule of grasses be considered as 

 being a stipule. Their leaves are often sheathing, continuous with 

 the stem (forming a spurious stem in Bananas), and .do not fall off by 

 an articulation. When there is only a slight divergence of their 

 veins, they may be looked upon more as enlarged and flattened petioles 

 than as true laminae. This remark is illustrated by the leaves of 

 Typha and Iris. In some Monocotyledons, as in Sagittaria sagitti- 

 folia, the submerged and floating leaves are narrow, like petioles, 

 while those growing erect above the water expand and assume an 

 arrow-like shape (fig. 180). 



Leaves of Acotylbdons. — In Acotyledons, such as Ferns and 

 their allies, the leaves vary much ; being entire or divided, stalked or 

 sessile, often feather-veined, occasionally with radiating venation, the 

 extremitieg of the veins being forked. The flbro-vascular bundles of 

 the leaves resemble those of the stem both in structure and arrange- 

 ment. In Thallogens, the leaves when present have no vascular 

 venation. In many of them, as Lichens, Fungi, and Algae, there are 

 no true leaves. 



Phyllotaxis, or the Arrangement of the Leaves on the Axis. 



Leaves occupy various positions on the stem and branches, and 

 have received different names according to their situation. Thus 

 leaves arising from the crown of the root, as in the Primrose, are 

 called radical; those on the stem are cauline; on the branches, raimal; 

 on flower-stalks, floral leaves. The first leaves developed are deno- 

 minated seminal (semen, a seed), or cotyledons (xoruXridiiv, a name given 

 to a plant or a seed-lobe) ; and those which succeed are primordial 

 (primus, first, and ordo, rank). 



The arrangement of the leaves on the axis and its appendages is 

 caljed phyllotaxis (<pfjXKov, a leaf, and rct^'S, order). In their arrange- 

 ment leaves follow a definite order. It has been stated already, p. 45, 

 that there are regular nodes or points on the stem (fig. 211 n) at 

 which leaves appear, and that the part of the stem between the nodes 

 is the internode (fig. 211 m). Each node is capable of giving origin 

 to a leaf. Occasionally several nodes are approximated so as to form 



