54 LEAVES. 



4. Equitant ; when the leaves are folded around each other 

 with the midrib, of one corresponding to the margin of the 

 one contiguous to it as in the Iris, d. 



5. Ohvolvie; when one margin of a leaf encloses the mar- 

 gin of a leaf opposite, a:id the remaining margin of each be- 

 ing outward, as in the sage, e. 



6. Plaited; folded like a fan, as in the vine, f, 



7. Involute ; when the margins of the leaves rolled inwards 

 as in the violet, g. 



8. Revolute ; where the margins are rolled outwards, as in 

 the willow, h. 



9. Convolute ; where one leaf is rolled within another, as 

 in the cherry, i. 



10. Cercinnate ; where it is rolled from the apex down- 

 wards as in the sundew, (Urosera) k. 



Section 6. — Leaves, 



61. Leaves are organs arising at regular intervals along the 

 main axis or branches, having their origin at a node. They 

 are expansions of the parenchymous portion of the bark, 

 with the spiral vessels and woody fibre of the medullary sheath 

 traversing it. The leaves are connected to the center of the 

 stem by the woody fibre, spiral vessels on the one hand, and 

 with the bark by the parenchymous portion on the other. A 

 leaf consists generally of a petiole and lamina. The petiole, 

 which is the support of the lamina, (fig. 33, a) 

 33 consists of cellular and vascular tissue, and 

 woody fibre. The vascular tissue and woody 

 fibre are formed into bundles, the spiral vessels 

 .-^ occupying the center of the bundle, and the 

 woody fibre, forming a s-heath around them. 

 The bundles are imbedded in the cellular tissue, 

 as may easily be seen by observing a transverse 

 section of the petiole of a leaf. The dots ob- 

 served on the cut surface are these bundles. — 

 The lamina (fig. 33, b) of the leaf consists of the expansion 

 of the petiole, the materials of its composition being of course, 

 the same, but differently arranged. When the bundles of 

 vascular tissue enter the lamina, they divide and proceed in 

 various directions in difi*erent plants, but always in the same 

 manner in the same species, forming the veins (fig. 33, c) of 

 the leaves. The continuation of the petiole forms the middle 

 and larcrest vein of the leaf, called the midrib, (fi<^. 33 d.) 

 Those veins arising from the midrib are called primary, the 



