CHARACTERISTTC FORMS OF LEAVES AND SHOOTS. I91 



of the leaf is sometimes itself extended in the directions of the median plane, so 

 that the plane of extension coincides with an axial longitudinal section of the stem, 

 as in the genera Ixia, Iris, &c. But sometimes the leaves are not flat, but conical 

 or polyhedral ; conical with almost circular transverse section in Characeae, Pilularia, 

 &c., polyhedral in some species of INIcsembryanthemum and Aloe. 



The outline of leaves is either simple or segmented ; the former is the 

 case when no definitely separated parts can be distinguished in the leaf; a leaf 

 is 'segmented' when it consists of pieces of various shapes, which are more or 

 less separated from one another. Leaves which are not fiat are usually simple, as 

 are also those which are flat but small, their length and breadth being inconsider- 

 able relatively to the stem, and not exceeding a few millimetres or centimetres in 

 absolute measurement. Larger leaves are usually distinctly segmented, and in ge- 

 neral the degree of segmentation increases with the increase of size ; the small 

 simple leaves of Mosses, for instance, may be contrasted with the large segmented 

 leaves of Ferns, the small simple leaves of Lycopodiaceae and Coniferse with the 

 large compound leaves of Cycads, the small simple leaves of Linacese with the large 

 much-divided leaves of the nearly-allied Geraniaceae, &c. The segmentation of leaves 

 usually consists in the separation of a basal portion which generally remains narrow, 

 cylindrical, or prismatic, while an upper portion is flatly extended ; the former being 

 called the leaf-stalk or petiole, the latter the blade or lamina. Or the lower portion 

 of the leaf has the form of a sheath, and forms a lamella enclosing the stem and 

 younger leaves like a hollow cylinder. If the upper part is flatly expanded the leaf 

 then consists of a sheath or vagina and a blade ; it sometimes also happens that a 

 stalk intervenes between the shcath-like basal portion and the lamina, as in Palms and 

 some Aroidese and Umbellifera^. Segmentation into sheath, petiole, and blade may 

 be distinguished as longitudinal from lateral segmentation, which consists of actual 

 branching, as in i)innate, deeply lobed, or compound leaves, or of a rudimentary 

 branching, as in indented, toothed, and sinuate leaves. Leaves are termed divided 

 or compound in which the individual lateral pieces of the lamina are completely 

 separated at their base ; while those forms are termed lobed in which the lateral 

 branches are only more or less projecting portions w'hich unite at their base. If 

 the individual branches of a branched leaf are sharply separated, each branch forms 

 independently, so to speak, a leaf, and is hence distinguished as a Leaflet. The 

 division, Hke the formation of lobes, may be repeated. If the branches are obviously 

 arranged in two rows the leaf is said to be pinnate ; pinnately- divided if it is a 

 compound leaf; pinnately-lobed, pinnatisect, or pinnatifid if the divisions are in- 

 complete ; dentate, serrate, or crenate if the lateral projections are very small 

 relatively to the lamina. If, on the contrary, the branches or lobes of the lamina 

 are densely crowded at the end of the petiole, and radiate from it on all sides, the 

 leaf is said to be digitate, palmately-lobed, &c. It is termed peltate when the 

 lamina is fixed not by a portion of its margin, but by a point lying on its under 

 surface (as in Tropseolum, Nelumbium, &c.). These are only a few of the more 

 important forms; the beginner will find in every text-book a number of other 

 distinctions and terms employed which are of importance in the special description 

 of plants. 



As occasional appendages, which indicate a still further segmentation of 



