242 ■ PRIMAUr ARRANGEMENT OF TISSUES. 



leaf-trace, radially and tatigentially vertical, in the periphery of the axial cylinder. Each 

 passes through two internodes, and at the third node, above one of the leaf-bundles which 

 pass out here, splits into two short, strongly-diverging limbs, each of which attaches itself 

 to the nearest of the bundles coming down from the second node.— The mam mass of 

 the axial cylinder consists permanently of thin-walled prismatic cells. When elongation 

 is complete, the axial bundle of spiral vessels disappears, while round its circumference 

 larger thick, and usually reticulated, vessels appear, which are scattered and arranged in 

 irregular rings. Also the number of the supposed sieve-tubes at the periphery increases 

 with age, so that the original arrangement may become indistinct. 



Sect. io6. The vascular bundles in the stem and leaves of the Ferns ^ belonging 

 to the divisions Polypodiacese, Cyatheacese, Hymenophyllacese, GleicheniaceEe, 

 Schizaeaceffi^ and Marattiacese, to which the Selaginellae are to be added, are of 

 various size and form ; in cross-section they may be circular, elliptically trapezoidal, 

 band-shaped or plate-like ; the outline of the broad ones being even, wavy, folded 

 in a furrow-like manner, or with the edges bent in ; others are annular or tubular 

 (e. g. stem of Marsiliaceae, Microlepia, Dennstsedtia, &c., see p. 284), or forming 

 peculiar symmetrical figures resembling an X, V, U, O, &c. : the bundles of the 

 leaves may be similar to those of the stem to which they belong, or very different 

 from them. Comp. Figs. 128-141. Their structure is as uniform as it is distinct 

 from that of most other forms of bundle. Comp. Figs. 160, 16 r. 



The middle is occupied by the xylem, the form of which is either identical with 

 that of the entire bundle, or similar to it, or in various degrees different ; the former 

 for example is the case in the annular or band-shaped bundles, and also in the 

 approximately cylindrical ones of the stem ; while the latter condition occurs 

 especially in leaf-stalks, in such a manner that the symmetrical figures- mentioned 

 are peculiar to or especially marked in the xylem, the outline of the whole being 

 simpler. The former may even be severed into two symmetrical groups in one 

 bundle, as for example in the leaf-stalk of Aspidium molle, Polypodium phymatodes. 



The xylem consist's, as regards its main mass, of wide, long, prismatic to spindle- 

 shaped, scalariform tracheides with bordered pits (comp. p. 165), and only in rare 

 cases of scalariform vessels, with septa perforated in a scalariform manner (Pteris 

 aquilina, p. 162). Between, or more rarely on the outside of these, lie, at definite 

 points, some narrow spiral and narrow scalariform tracheides, which are the primitive 

 elements at the origin of the xylem; from these the development of the wide 

 tracheides starts, and advances centrifugally with reference to each point of departure, 

 though, it may be, centripetally with reference to the whole bundle. The position 

 and number of these primitive groups are different in the individual cases. In 

 bundles presenting an angular or unilaterally elongated cross-section, one such 

 group lies at or near each corner, or at each end of the greater diameter of the 

 section, as in the flat bundles in the stem of most Selaginellas (comp. Fig. 131, p. 282), 



' Von Mohl, Structiira filic. arborearum, I.e. — Link, Icones selectse, Heft. III. iind IV. — Met- 

 tenius, Angiopteris, /. c. — Karsten, Vegetationsorgane der Palmen, /. c. pp. 1 1 7, 1 30, &c. — Dippel, 

 Verhandl. der Naturforscher-Versammlung zu Giessen (compare p. 180), and Mikroskop, p. 198 ff.— 

 Trecul, Sur la position des Trachees dans les Fougeres, &c. ; Ann. Sci. Nat 5 s^r. torn. X. p. ,^44, 

 torn. XII. p. 219 fif. — Russow, Vergl. Untersuchungen — With reference to the form of the bundles, 

 compare also Presl and Reiohardt in the works cited at p. 298. 



''■ [See Prantl, Unters. i. Morph. d. Gefasskrypt, Heft. II. Leipzig, iSSi.]' 



