Dr. G. Ogilvic on the Forms and Structure of Fern-stems, 405 



of Lycopodium the successive stages of annual growth, and the 

 separation of the shoots as independent plants by the gradual 

 decay of the older portion of the stem, are more clearly brought 

 out than even in the stoloniferous roots of Ferns. In Isoetes we 

 have a peculiarly modified form of the simple caudex. 



Internal Structure of Fern-stems. 



The next point to be noticed is the internal structure of the 

 vegetative axis of the Fern, principally as it afifects the disposi- 

 tion of the fibro-vascular bundles. Woody fibre and vessels 

 (which, except in a few degraded species, are a universal charac- 

 teristic of phanerogamic plants) occur, it is well known, only in 

 the higher forms of cryptogamic vegetation — the Ferns and 

 allied orders, — and even there but sparingly and with some 

 remarkable peculiarities both in their minute structure and their 

 general disposition. The most observable structural characters are 

 the dark colour of the woody or hard tissue, and the large size, 

 angular section, and scalariform markings on the ducts. In the 

 disposition, too, of the fibro-vascular tissue, the stems of Ferns 

 present some remarkable peculiarities, quite as distinct as those 

 characteristic of the better-known divisions of the so-called En- 

 dogenous and Exogenous stems. These differences do not so 

 much affect the original constitution of the stem as the mode in 

 which the successive annual increments are applied to the pri- 

 mary axis. In vascular shoots of the first year, whether Dico- 

 tyledonous, Monocotyledonous, or Cryptogamic, there is com- 

 paratively little difference. In each we have a single circle of 

 fibro-vascular bundles imbedded in the general cellular tissue of 

 the stem ; but when the new bundles of subsequent years come 

 to be added, we find the characteristic differences clearly brought 

 out. Thus the peculiarities of the Exogenous stem depend on 

 the addition to the exterior of the wood and interior of the 

 liber of former years, of annual layers, forming in section con- 

 centric circles of ligneous and cortical tissue. Hence the accre- 

 tions to the wood of a Dicotyledonous stem have been repre- 

 sented by a number of cones of continually increasing dimen- 

 sions placed one over the other, and slightly truncated at the 

 top. In horizontal sections the wood of such a stem presents a 

 series of concentric rings, which represent the bases of the cones, 

 interrupted by radiating bands of muriform tissue, which are the 

 outward continuations of the cellular interspaces between the 

 bundles of the original circle, and represent the small residue of 

 the cellular element of the stem left after the abundant develop- 

 ment of fibro-vascular tissue characteristic of the Dicotyledonous 

 organization. In a tangental section, these same bands are 

 seen cut across, and may be observed to occupy the meshes left 



