SECONDARY THICKENING IN ISOEfES. 633 



are specially thickened and sclerotic. Accordingly, the endodermis is split longitudi- 

 nally by the growth in thickness, with the same phenomena as have been described 

 for the splitting of cortical fibrous rings, p. 543. The primary outer cortex follows 

 the growth in thickness, at least for a time, by means of growth by dilatation. It is 

 uncertain whether it may be thrown off later by an internal formation of periderm. 



Sect. 200. Among the Fern-like plants now living there are known, among 

 secondary changes of tissue-distribution, some indications of formation of periderm, 

 which have already been mentioned on p. 108. A secondary thickening, starting 

 from a cambial layer, which produces secondary wood and secondary cortex, is only 

 found in the Isoetese ^ The phenomena in question, which there appear, differ in 

 many points from those known in Phanerogams, but are to be connected with 

 these as a very simple member of the series, which they themselves form. The 

 single zones and parts related to the secondary thickening may without difficulty be 

 compared with those of the Phanerogams, and may be designated by the same 

 names without altering their meaning. Scruples, which have been felt against this, 

 are allayed, if, in drawing the comparison, a start be not made from the normal 

 Dicotyledons alone, but if the whole series of phenomena described in the above 

 paragraphs be kept in view. 



As is known from the descriptions '', the short simple stem of the Isoetese is 

 2- or 3-lobed, in exceptional cases 4-lobed, and the lobes are separated from one 

 another by longitudinal furrows, from which the roots arise. The middle of the 

 stem is traversed longitudinally by the axile bundle containing tracheides, mentioned 

 on pp. 280 and 347, which extends on the one hand to close below the meristematic 

 group of the flat apex of the stem, and there developes further in an acropetal manner 

 as a conical-cylindrical body, according as new leaves appear ; on the other, basiscopic 

 side it widens out into as many arms or wings as there are furrows in the stem. 

 Each wing is opposite one furrow of the stem : its generally convex lower margin 

 and its almost straight upper margin join into a single angle opposite the furrow. 

 The wings increase in breadth as new roots arise, and the vascular bundles of the 

 latter are inserted on them. The above data lead to the view put forward by 

 Hofmeister and Sachs, that the whole axile bundle arises simply from the sympodial 

 coalescence of the points of insertion, on the one hand of the bundles of the leaf- 

 trace, on the other of the bundles of the roots. 



The upper end of the tracheide-containing bundle, surmounted by the small 

 gi-oup of apical meristem, is surrounded laterally by radial rows of meristem directed 

 towards the surface, from which the thick parenchymatous primary cortex is derived : 

 the latter retains the arrangement in radial rows. As far as a point close below the 

 tracheide-bearing end, the bundle is surrounded by a layer of those tabular cells, 

 which in I. lacustris are transparent, with lustrous walls ; these have been above 

 mentioned, p. 347, as probable representatives of the sieve-tubes. This layer has, 

 like the bundle of tracheides, passed over into the permanent condition. While thi? 



' [On the occurrence of a cambial ring in Botrychium, cf. RussoW, Vergl. Unters. p. 119.] 

 ' Von Mohl, Ueber den Bau des Stammes von Isoetes lacustris. . Verm. Schr. p. 122. — Hof- 

 meister, Beitr. z. Kenntn. d. Gefasskryptog. I. Abhandl. d. Sachs. Ges. d. Wissensch. Bd, IV. — A. 

 Braun, I.e., compare p. 418. — Russow, Vergl. Unters. p. 139. — Hegelmaier, Botan. Zeitg. 1874, P- 

 481.— Compare also Sachs' Lehrb. p. 473 [2nd Eng. Ed. p. 483]. 



