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L66. Tito spiral vessels of the leaf of Exogenous plajits derive\heir origin fromtlio 

 medullary sheath ; those of Endogenous plants from the bundles »fva cular tissue. 



167. The cellular integument of the petiole is a continuation of that of the 

 bark. 



108. When the petiole is leafy and the lamina is abortive, it is called phylloiium. 



169. When the petiole becomes dilated and hollowed out at its upper end, the 

 lamina being articulated with and closing up its orifice, it is called a.Sa>itcher or 

 otcidium. -• 



170. Sometimes the petiole has no lamina, or is elongated beyond the lamina, 

 and retains its usual cylindrical or taper figure, but becomes very long, and twists 

 spirally ; such a petiole is called a tendril, (Vrille, i'V.) 



171. The lamina of a leaf is an expansion of the parenchyma of the petiole, 

 and is traversed 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. 



172. The veins either branch in various directions among the parenchyma, anas- 

 tomosing and forming a kind of net-work, or they run parallel to each other, being 

 connected by single transverse unbranched veins. 



173. The former is characteristic of Exogenous, the latter of Endogenous plants. 



174. The principal vein of a leaf is a continuation of the petiole, and runs in a 

 direct line from the base to the apex of the lamina ; this vein is called the midrib. 



17~». Conifer<B and Cycadea, tribes the stem of which has an Exogenous struc- 

 ture, have the same arrangement of their veins as Endogensa. 



176. There are two strata of veins, the one belonging to the upper, and the other 

 to the under surface. 



177. The upper stratum conveys the juices from the stem into the lamina, for 

 the purpose of being aerated and elaborated ; the under returns them into the bark. 



178. The lamina is variously divided and formed ; it is usually thin and mem- 

 branous, with a distinct upper and under surface ; but sometimes becomes succu- 

 lent, when the surfaces are often not distinguishable. 



• ' 17!). The upper surface is presented to the sky, the lower to thn oarlh ; this posi- 

 tion is rarely departed from in nature, and cannot be altered artificially except by 

 violence. 



180. A leaf is simple when its lamina is undivided, or when, if it is separated 

 into several divisions, those divisions are not articulated with the petiole ; Ex. 



ILime Tree, Palm. 



181. A leaf is compound when the lamina is articulated with the petiole ; Ex. 

 Orange, Mimosa. 



182. The modes in which leaves are divided are distinguished by particular 

 names, such as pinnated, pinnatifid, bipinnated, bipinnatifid , and very many 

 others. These terms apply to the mode of division, and arc equally applicable to 

 simple and compound leaves. 



183. Stipule are attached to each side of the base of the petiole. Tbey have, 

 if foliaceous, veins, the anatomical structure of which is the same as of the veins 

 of the leaves. 



184. Stipula: are sometimes transformed into leaves ; they sometimes have buds 

 in their axilla' ; and may be, therefore, considered rudimentary leaves. 



185. Whatever arises from the base of a petiole, or of a leaf if sessile, occupy- 

 ing the same place, and attached to each side, is considered a stipula. 



186. The stipula; must not be confounded with celhdar marginal appendages of 

 the petiole, as in Apocynere. 



187. Stipula?, the margins of which cohere in such a way that they form a mem- 

 branous tube sheathing the stem, are called ochrccc ; Ex. Rhubarb. 



18H. All leaves are originally continuous with the stem ; as they grow, an inter- 

 ruption of their tissue at their junction takes place, by which a more or less com- 

 plete articulation is formed sooner or later. 



L89. As soon as the articulation between a leaf and stem is completed, the Fall 

 of the Leaf takes place. The cause of this articulation is unknown. 



190. All leaves ultimately fall off; evergreen leaves later than others. 



191. The mode in which leaves are arranged within their bud is called vernation, 

 or gemmation. 



192. Leaves have, under particular circumstances, the power of producing leaf- 

 buds from their margin (154) ; Ex. Bryophyllum, Malaxis paludosa, and proliferous 

 Ferns. 



