9f> 



THE LEAVES OF PLANTS. 



Fig. 155. 



the veins diverge in different planes, the leaf is orbiculai (round), as the leaf of the 

 common sheep-rot (Hydrocotyle vulgaris. Fig. 154). In succulent or fleshy leaves, such 

 as the leaf of the house-leek, sedum, several pinks, &c., the veins spread in different 

 planes, and the parenchyma is so much developed as to conceal the veins, which con- 

 sequently are neither prominent nor visible, as they are in the greater number of 

 leaves. 



The leaf, when complete, consists of two parts (Fig. 155), a, the petiole, or leaf- 

 stalk ; $, the lamina or blade. The petiole connects the 

 leaf with the branch or stem, and is composed of the unex- 

 panded bundle of fibres, covered by the epidermis ; the ra- 

 mification of the nerves constitutes the skeleton, and the 

 veins and veinlets, with the cellular tissue and epidermis, 

 constitute the entire leaf. When the petiole is not present, 

 the leaf is termed sessile. Sessile leaves often partially or 

 entirely surround the stem, and in this case they are termed 

 semi-amplexical or amphxical (half embracing, or quite sur- 

 rounding the stem). 



The most obvious division of leaves is into simple and com- 

 pound. In simple leaves the limb consists of one piece, either 

 quite entire, or variously indented, cleft or divided at the 

 margin. (See Fig. 155, an entire leaf; 154, a crenate; and 

 153, a toothed or incised leaf.) Compound leaves are com- 

 posed of one or more pieces, called leaflets, each of which is 

 jointed to the common petiole or rach, as it is termed when 

 the leaf is winged. (See Fig. 156, which represents a pinnate, 

 or winged leaf.) 



Simple Leaves. The shape or contour of the leaf is regu- 

 lated or modified by the angle of divergence of the lateral or 

 secondary veins, and by their length. When the divergent 

 veins are but slightly distant, and extend from the base to the 

 apex, inclosing only a narrow slip of parenchyma, the leaf is 



called linear. The leaves of grasses are familiar Fig. 156. a, the rach, 

 examples of this form. When the veins extend t Ve' compound leaf 6 ' 8 * 

 from end to end, and are rather more distant in the 



middle of the leaf, the lanceolate form is produced. In these 

 two forms the veins usually diverge at 

 the base ; but in the second, viz. the lan- 

 ceolate form, the relative length of the 

 secondary veins and their wideness of 

 angle produce a lanceolate leaf (Fig. 

 157). When the secondary or branching 

 veins are nearly of equal length, both at 

 the base and apex, the leaf is elliptico- 



Fit? 1 5S Fii? 1 59 



leaf with branching nerve*; la * ceolate ( F %- 158). The oblong leaf 

 Fipr. 158. Eiliptico-lanceolate leaf. Fig. 159. differs from the latter merely in being 

 Oblong leaf. rather broader at the base and tip (Fig. 



159). When the branching veins are nearly equal, the leaf, being obtuse at both ends, 

 is called a rounded leaf. 



