Dr. G. Ogilvie on the Forms and Structure of Fern-stems. 407 



arched fibres in the interior^ characteristic of the Endogenous 

 stem, and the annual layers of descending woody tissue on the 

 exterior (so conspicuous a feature in that of Exogens)_, are alike 

 wanting ; for the fasciculi, immediately on entering the stem, 

 branch out and anastomose with those derived from former pe- 

 tioles, forming a very beautiful reticulated cylinder near the 

 exterior, analogous evidently to the " fibrous layer '* of the Mo- 

 nocotyledonous stem, but presenting, instead of the dense fibrous 

 interlacement of the latter, a network of thick cords of scalari- 

 form tissue, ensheathed in a layer of brownish pleurenchyma, and 

 separated by large open meshes, through which the cellular 

 tissues of the central and cortical regions are freely continuous 

 with each other (PI. X. fig. 11). The pile of superposed trun- 

 cated cones, diagrammatically representing the Exogenous stem 

 — already cut down all to one length, and more opened at the 

 top to adapt them to the Endogenous type — must now be re- 

 duced to a series of simple rings placed one upon another. 



On a longitudinal section along the axis, a fern-stem presents 

 a uniform expanse of cellular tissue, marked only by an inter- 

 rupted line near each margin, indicating the position of the 

 fibro-vascular bundles of the reticulated cylinder, the interrupted 

 spaces answering to the meshes of the network (fig. 10). On a 

 horizontal section we have a corresponding interrupted circle 

 formed by the cut extremities of the fibro-vascular anastomosing 

 cords (fig. 12). This circle divides the section into a large central 

 and a smaller cortical region, both composed of cellular tissue, as 

 in the Monocotyledonous stem, but without the descending fibres 

 which form so conspicuous a feature in the interior of the latter. 

 In a certain point of view, these regions correspond to the pith 

 and cellular bark of a young Dicotyledonous stem, in which the 

 cellular tissue has not yet been displaced by the fibro-vascular 

 element ; and the interspaces of the reticulations may be held 

 to represent the medullary rays. Indeed, in all the fibrous 

 tissues of Dicotyledonous plants a tendency may be observed to 

 such an interlacement about the medullary rays : it is quite ob- 

 vious to the naked eye in the bast-fibres of the Lime and many 

 other trees; it has been noticed in the medullary sheath of 

 Coniferse, and it may be seen with great distinctness under the 

 microscope, in tangental sections of mahogany and other hard 

 woods. 



The stems of Tree-ferns are said to be generally hollow, but 

 the character is not an essential one. Fistulous stems are well 

 known to occur both in Endogenous and Exogenous plants*, and 



* In Endogens, the Cocoa-nut and other Palms and most grasses, in 

 Exogens, many UmbelUferae, Compositse, and Labiatse furnish instances of 

 this. 



