244 



MORPHOLOGY. 



vessels of the leaves, form loops by their mutual combination, from which 

 these are given off. In the undeveloped internodes they rise up in ser- 

 pentine lines, forming, by their alternate approach and retreat, longer or 

 shorter, narrower or broader, meshes, from the edges of which the vascular 

 bundles of the leaves branch off. These latter not unfrequently run for 

 a time upwards in the pith, before they pass through the meshes and enter 

 into the leaves. H. Mohl has given us minute anatomical observations on 

 the Fern-stems, but we are unfortunately still wholly deficient in a 

 history of their development. 



The Equisetacece have all developed internodes and a simple ring of 

 vascular bundles. We are still without any very exact observations, or 

 special history, of their development. 



157 



II. Sexual Plants. 



A. RHIZOCARPEJS. 



These, again, have an extremely simple structure of stalk and stem, 

 together with a central vascular bundle, which contains only a few 

 weakly-developed vessels. 



B. GYMNOSPERM^E. 



The whole of this division is devoid of stalks, having only stems. The 

 Cycadacece have only undeveloped, and the Conifer ce and Loranthacece only 

 developed, internodes. All have a cambium-layer under the bark on the 

 outer side of the simple vascular bundle ring, and the stems are conse- 

 quently capable of being indefinitely thickened ; the vascular bundles are 

 indefinite. In the Coniferce we find a ring of vas- 

 cular bundles (the medullary sheath of older bota- 

 nists) surrounding a pith, which even in the first 

 year close into a woody cylinder (fig. 157. h, k). The 

 portions of wood corresponding to the vascular bun- 

 dles run quite perpendicularly, and only leave very 

 narrow crevices for the escape of the little bundles, 

 branching from the inner part of the vascular bun- 

 dles, which intersect the wood obliquely, and ascend 

 for a time into the bark before they pass into the 

 leaves (fig. 157. h, i). These points will be made more 

 plain by means of the accompanying diagram of a 

 section of a piece of the stem of the Fir. The 

 Loranthacece do not appear to differ in their ar- 

 rangement from the Conifers. 



InCycas revolutawe find also a simple ring of vas- 

 cular bundles, from the innermost part of which the 

 vascular bundles for the lateral parts run through 

 long meshes (which are formed by the alternate 

 retreat and closing together of the vascular bundles, 



d. a. <?. c 



157 Abies excelsa. A longitudinal section of the apex of a tree, of two years' growth 

 in the lower part, and only one year's growth above the lateral branch. a, The wood 

 divided into two annual portions (annual rings). b, The medullary sheath ; that is to 

 say, the oldest portion of the first annual ring, or original vascular bundles. Between 

 a and b, the pith, shaded with cross lines, c, Cambium-layer, d, Bark. /, Lateral 

 branch, e, A leaf, from the axil of which the lateral branch has originated, g g, Re- 



