EXOGENOUS, ENDOGENOUS. AND ACROGENOUS LEAVES. 87 



hibit divisions on their margin. Their venation is generally parallel, 

 and their margin entire (figs. 135, 194). Exceptions to this rule 

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

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

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

 afterwards become split into various lobes. Endogenous leaves 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 hi Bananas), and do not fell off by an artic- 

 ulation. When there is only a slight divergence of then- veins, they 

 may be looked upon more as enlarged and flattened petioles than as 

 true lamina?. This remark is illustrated by the leaves of Typha and 

 Iris. In some Endogens, as in Sagittaria sagittifolia, the submerged 

 and floating leaves are narrow, like petioles, while those growing erect 

 above the water expand and assume an arrow-like shape (fig. 165). 



167. AcrogenoM or AcolTledonous I-eare*. In AcTOgens, the leaves 



vary much ; being entire or divided, petiolated or sessile, often feather- 

 veined, occasionally with radiating venation, the extremities of the 

 veins being forked. The fibre- vascular bundles of the leaves resemble 

 those of the stem both in structure and arrangement 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. 



168. 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, ramal: 

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

 minated seminal (semen, a seed), or cotyledons (wre/AuS**, a name given 

 to a plant); and those which succeed are primordial (primus, first, 

 and ordo, rank). 



169. The arrangement of the leaves on the axis and its appendages 

 is called phyllotaxis (0t/xxo, a leaf, and ri;, order). In their 

 arrangement, leaves follow a definite order. It has been stated 

 already (^[ 67) that there are regular nodes or points on the stem 

 (fig. 195 H) at which leaves appear, and that the part of the stem 

 between the nodes is the internode or menthol (fig. 195 m). Each 

 node is capable of giving origin to a leaf. Occasionally several nodes 

 are approximated so as to form as it were one, and then several leaves 

 may be produced at the same height on the stem. When two leaves 

 are thus produced, one on each side of the stem or axis, and at the 

 same level, they are called opposite (fig. 196); when more than two 



