LEAVES. 43 



(4.) Eqxdtunt ; 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. 



(5.) Obvolute ; when one margin of a leaf incloses the margin 

 of a leaf opposite, the remaining margin of each being outward, 

 as in the Sage. 



(6.) Plaited ; folded like a fan, as in the Vine. 



(7.) Involute ; when the margins of the leaves roll inward, as 

 in the Violet. 



(8.) Revolute ; where the margins are rolled outward, as in 

 the Willow. 



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

 in the Cherry. 



(10.) Circinate ; where it is rolled from the apex downward, 

 as in the Sundew (Drosera). 



Section 6. — Leaves. 



78. Leaves are organs arising at regular intervals along the 

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

 spaces of the stem between the leaves are called internodes. 

 They are expansions of the parenchymous portion of the bark, 

 with the spiral vessels and woody fiber of the medullary sheath 

 traversing them. The spiral vessels and woody fiber connect the 

 leaf with the center of the stem, and these vessels are expanded 

 on the upper surface of the leaf. The cellular tissue, liber, and 

 lactiferous vessels connect it with the bark, and are expanded on 

 the under surface of the leaf. A leaf consists gen- 

 erally of a petiole and lamina. The petiole, which 

 is the support of the lamina (Fig. 38, a), consists of 

 cellular and vascular tissue, and woody fiber. The 

 vascular tissue and woody fiber are formed into 

 bundles, the spiral vessels occupying the center of 

 the bundle, and the w^oody fiber forming a sheath 

 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 observed on the cut 

 surface are these bundles. The lamina (Fig. 38, b) of the leaf 

 consists of the expansion of the petiole, the materials of its com- 

 position being of course the same, but differently arranged. When 



Equitant? Obvolute? Plaited? Involute? Eevolute? Convolute? 

 Circinate? — 78. What are leaves ? How connected with the center of the 

 Btem ? How with the bark ? Of what does it consist? How is the fibro- 

 vascular system arranged ? Of what does the lamina consist ? 



