STRUCTURE.] ANATOMY OF THE LEAF. 253 



It is obvious that the hypothesis of the spiral arrangement 

 of the foliaceous organs of plants is a mathematical question 

 having but little relation to Botany. Those who wish to 

 investigate it, will find much more concerning it in Steinheil's 

 Observations upon the Theory of Phyllotaxis, in Ann. Sc. 

 n. s. IV. 100. 142, Bravais, sur la disposition des feuilles 

 curviseriees, ib. VII. 42. and VIII. 161., Link, Elementa 

 Botanica, ed. 2. II. 448., Dutrochet, Memoires, I. 238. 



In their normal state the edges of leaves are distinct, both 

 from each other and from the stem. But, in some cases, 

 adhesions of various kinds occur, and give them a new 

 character. Thus, in Cardui, and many other thistle-like 

 plants, the elongated bases of the leaves adhere to the stem, 

 and become what is called decurrent. The elevated lines upon 

 the stem, thus formed, are called by Link and Klotzsch 

 sterigmata : vera, when traversed by a cord of vessels ; spuria, 

 when mere elevated cellular plates. In Bupleurum perfoli- 

 atum the lobes of the base of the leaf not only cohere with 

 the stem, but projecting beyond it, grow together, so as to 

 resemble a leaf through which the stem has pierced : this is 

 called being perfoliate. Frequently two opposite leaves grow 

 together at the base, as in Caprifolium perfoliatum ; to this 

 modification the latter term is often also applied, but that of 

 connate is what more properly belongs to it. 



The usual anatomical structure of the leaf is this : From 

 the medullary sheath, or in Endogens, the woody bundles, 

 diverges a bundle of woody tissue, accompanied by spiral 

 vessels : this passes through the bark, and proceeds, at an 

 angle more or less acute, to a determinate distance from the 

 stem, branching off at intervals, and, by numerous ramifica- 

 tions, forming a kind of network. At the point of the stem 

 whence the bundle of fibro-vascular tissue issues, the 

 cellular tissue of the bark, mesophyll or cortical integument, 

 also diverges, accompanying the fibro-vascular tissue, expand- 

 ing with its ramifications, and filling up their interstices. 

 The tissue that proceeds from the medullary sheath, after 

 having passed from the origin of the leaf to its extremity, 



