344 



PRIMARV ARRANGEMENT OF TISSUES. 



compression and distortion they are only to be detected in the mature stem with 

 great difficulty, and often there are only traces of them. 



The xylem is either composed of tracheides ■ only, without any non-equivalent 

 elements interposed between them, or there are groups and rows of parenchymatous 

 cells containing small starch grains, intermixed with them^ The two conditions are 

 distributed according to species and perhaps genera, not according to the form of 

 the bundle. The first, for example, occurs in Marsilia and Pilularia, where the xylem 

 is an uninterrupted, one to three-layered ring of tracheides ; also in the axial bundles 

 of the Selaginellas, and in many flat, round, and angular bundles of Polypodiacese, 

 e.g. in the stems of Polypodium vulgare (Fig. i6o), P. Lingua, Davallia pyxidata; 

 the petioles of Asplenium auritum, Scolopendrium vulgare, and many others 

 (cf. Russow, I.e.). In the bundles of the Marattiacese also, no parenchymatous cells, 

 or extremely few of them, occur among the tracheides. 



The other case occurs, for example, in the relatively thick cylindrical xylem of 



the rhizomes of Trichomanes radi- 

 cans, Gleichenia, and Lygodium, in 

 the annular bundle in the rhizomes 

 of Microlepia and Dennstsedtia, in the 

 round or flat bundles of the stems 

 of Pteris aquilina (Fig. i6i), Poly- 

 podium fraxinifolium, Platycerium 

 alcicorne, Alsophila microphylla, Cy- 

 athea Imrayana, and arborea ; in the 

 bundles of the leaf-stalks of Tricho- 

 manes, Aspidium Filix mas, moUe, 

 Lygodium and many others (corap. 

 Russow, /. c). The axial bundle in 

 the stem of the Schizseas is also 

 placed in this category by Russow, 

 and no doubt rightly, for although a 

 many-rowed uninterrupted ring of 

 tracheides appears to surround a 

 thick axial pith-cylinder of paren- 

 chyma, yet it is not separated from 

 the latter in the manner to be de- 

 scribed below, which is characteristic 

 of all other bundles of Ferns belonging 

 here, but the two are in immediate 

 contact, so that the strand in ques- 

 tion must naturally be regarded as a 

 xylem with a coherent axial cylinder of parenchyma. In the leaf-stalk of Tricho- 

 manes pinnatum and elegans ', and in that of species of Aneimia, Gleichenia and 

 Schizaea, very thick-walled, lignified, sclerenchymatous fibrous cells are added to the 

 tracheae ; there is a thick bundle of them in each corner of the V which the xylem 



FIG. i6i.— Pteris aquilina. A quarter of the cross-section tlirougfli 

 a large vascular bundle of tile stem; cf. Fig. 143, p. 255. .S spiral 

 tracheide, £■■—£■ wide scalariform vessels (cf. p. 162), j/ sieye-tubes, b the 

 protophloem of Russow, s^^ endodermis, fi the parenchyma surrounding 

 it, containing starch-grains ; A' thickened portions of the wall of the 

 vessels between the rows of scalariform pits. Between ft and s£, and 

 in the xylem, especially round S, are tliin-walled parenchymatous 

 cells, containing starch. From Saclis' Textbook, 



' Russow's 'companion cells' (Geleitzellen). 

 ' Mettenius, Die Hymenophyllaceen, p. 421. 



