34<5 PRIMARy ARRANGEMENT OF TISSUES. 



In petioles, wlien the xylem is concave or notched, strands of cells 3-4 rows thick, 

 which are distinguished from the rest of the parenchyma by their very wide lumen, 

 are to be found in its depressions and furrows, sometimes just in front of the primitive 

 tracheides ; ' in longitudinal sections they are conspicuous from the fact that their 

 walls are irregularly bent in and out, and are connected with those of the neighbour- 

 ing cells in such a manner that large cavities or intercellular spaces arise; in old 

 bundles it is usually found that the walls have become brown.' Russow calls them 

 cavity-parenchyma (Liickenparenchym). The tracheides bordering on them usually 

 have very irregularly developed spiral bands. Examples : species of Asplenium, 

 Cyathea microlepis (Dippel), /. c, Aspl. Ulix femina, Cyatheaceae ^, e. g. Cyaihea 

 medullaris. The wide cells in Osmunda regalis to be mentioned below may also 

 belong here. 



From its general distribution among the Fern group in the widest sense, one 

 may term the structure of the bundle just described the Fern-type. At the same 

 time different degrees of deviation from the type occur within this group. Those de- 

 scribed in the Marattiacese and Selaginellae are trifling. The Lycopodiaceae, of which 

 we shall treat in the next section, are closely connected with the Selaginellas in the 

 structure of the bundle, as well as in other points. In this respect the Equiseta are 

 most widely different from the Fern-type ; their strictly collateral bundles, which 

 most closely resemble those of Monocotyledons, were described above (p. 329). 

 Besides these, the Ophioglosseae, and in part at least the Osmundacese, have colla- 

 teral bundles. The two parts have normal orientation in the round or flat bundles, 

 the xylem is similar to that of Ferns, with some narrow spiral tracheides (primitive 

 elements) at its inner edge; its main bulk consists of large prismatic tracheides, 

 which in Ophioglossum (pedunculosum and vulgatum) show narrowly scalariform 

 reticulate thickening without pit-borders, while in Botrychium they have very thick 

 reticulate fibres, with elliptical bordered pits in the meshes of the reticulations. 

 Parenchyma is present in the xylem of the annular bundle in the stem of Botrychium 

 rutaefolium, in the form of radial bands resembling medullary rays ; I could not 

 find any in the examples of B. Lunaria, which I investigated. The phloem appears 

 very similar to that of the typical form ; the wide elements presumed to be sieve- 

 tubes still require more exact investigation (p. 180). The bundles of the petiole, and 

 the small bundles of the stem of Ophioglossum, which in cross-section are arranged 

 in a circle, are not bounded externally by any kind of distinct sheath. The bundle 

 of the stem of Botrychium Lunaria and rutsefolium, which is annular in cross-section, 

 is encircled on the outside by an endodermis, the cells of which do not differ from those 

 of the surrounding parenchyma, except in the exquisite undulating longitudinal bands 

 in the middle of their radial side-walls. 



In Osmunda (compare p. 280) the bundles of the stem are collateral. The xylem, 

 where it enters the circle of bundles, is horseshoe-like in cross-section, and during its 

 descending course becomes narrowed to a wedge-like form ; internally it borders 

 directly on the parenchyma of the pith ; it has the same structure as in the typical 

 Ferns, and has hardly any parenchyma inserted among the scalariform tracheides. 

 The groups of xylem are separated from one another in the whole longitudinal 



* Russow, /. c. 



