\- RUDIMENTS OF BOTA.NY. 



LEAVES. 



37. Leaves are those expansions which issue laterally from the stem and 

 branches of plants. They take their origin from the bark, and are always 

 to be observed, whether perfect or rudimentary, immediately below the leaf- 

 buds. 



38. Those leaves situated near the root are often larger, and of a differ- 

 ent shape from those higher up the stem ; the former are termed radical, the 

 latter cauline. 



39. A leaf consists of a petiole, a lamina or limb, and a pair of stipules : 

 but sometimes only one of these three parts can be observed. 



40. The petiole is the channel through which the vessels of the leaf are 

 connected with those of the stem ; it is formed of one or more bundles of 

 spiral vessels and woody fibre, enclosed in a cellular integument. 



41. The lamina of a leaf is an expansion of the parenchyma of the 

 petiole, and is transversed by veins which are ramifications or extensions 

 of the bundles of vascular tissue of the petiole, or when there is no petiole, 

 of the stem. 



42. These veins either branch in various directions among the parenchy- 

 ma, anastamozing and forming a kind of net-work, or they run parallel to 

 each other, being connected by single transverse unbranched veins ; the for- 

 mer structure being characteristic of Exogenous, and the latter, of Endoge- 

 nous plants. To this the Conifers and Cycadea form perhaps the only ex- 

 ceptions ; these having the stems of the Exogenous, but the same arrange- 

 ment of the veins as in the Endogenous ones. 



43. The principal vein of the leaf is a continuation of the petiole, run- 

 ning in a direct line from the base to the apex of the lamina, and is called 

 the midrib. 



44. The lamina is variously divided and formed ; it is usually thin and 

 membranous, with a distinct upper and under surface, but sometimes be- 

 comes succulent, when the surfaces cannot be distinguished. 



45. A leaf is either simple or compound ; simple when its lamina is undi- 

 vided, or when, if separated into several divisions, these segments are not 

 articulated with the petiole ; compound when the lamina is articulated with 

 the petiole. 



46. The modes in which leaves are divided are distinguished by particu- 

 lar names, as pinnate, pinnatifid, bipinnate, bipinnatiJLd, &c. &c. These 

 terms apply to the mode of division, and are equally applicable to simple 

 and compound leaves. 



47. Stipules are those small foliaceous organs sometimes situated on each 

 side at the base of the petiole. They never occur in the Endogenae, nor 

 in any Exogenous plants that have sheathing petioles, and are rarely found 

 in genera with opposite leaves. They are sometimes transformed into leaves ; 

 they sometimes have leaf-buds in their axils; and sometimes also they are 

 changed into spines. 



48. Leaves are originally continuous with the stem, but afterwards, from 

 a cause which is still unknown, an articulation more or less complete takes 

 place and the fall of the leaf ensues. 



49. The mode in which leaves are arranged within their bud is called 

 vernation or gemmation. This varies much in different groups of plants. 



