STRUCTURE OF CONCENTRIC BUNDLES. 



343 



where they lie in the corners themselves, and are continued into the leaf-bundles, 

 which join on here (comp. p. 282) ; and in the band-shaped or symmetrically many- 

 ra3'ed bundles of the leaf-stalk of Ferns ^ In bundles which are elliptical in cross- 

 section, e. g. in the rhizome of Pteris aquilina, their position corresponds approxi- 

 mately to the foci of the ellipse. Besides these primitive groups occupying the ends 

 and corners, others may be present in the same xylem, e. g. in the band-shaped, 

 symmetrically curved bundles from leaf- stalks, represented by Russow ; a median 

 group occurs in Gleichenia vulcanica, Aneimia Phyllitidis, Marsilia Drummondii ; 

 in Asplenium Filix-femina there is a median group and an intermediate one in each 

 side between this and those at the margin ; in Balantium Culcita there are two inter- 

 mediate groups, and so on. In roundish or circular and in annular xylems several 

 primitive groups are scattered about the cross-section ; five for example in the annular 



Fig. 160. — Polypodiuni vulgare; rhizome; cross-section through a weak vascular bundle (225). s phloem region, 

 sieve-tube structure not clear ; sp narrow spiral tracheides of the xylem, the wider elements which constitute the 

 majority are scalariform tracheides ; 7e endodermis apparently derived by tangential division from the same layer of 

 mother-cells with the parenchymatous layer adjoining it on the inside. Outside 11 is parenchyma. The pitting on 

 its cell-walls is essentially the same everywhere ; in the figure it has only been indicated at certain points. Those of 

 its cells which border on u are thicker-walled on the inside than elsewhere. 



bundles of the stem of Marsilia Drummondii ; six, according to Russow's description, 

 in the vascular cylinder of Trichomanes radicans, three near the middle in the round 

 axial bundle in the small stem of Selaginella spinulosa. In very small bundles there 

 is often only a single primitive group present, which is more or less eccentrically 

 placed, e. g. small bundles in the rhizome of Pteris aquilina, Angiopteris (Mettenius, 

 I.e. 517). In the large flat bundles in the stem of Cyatheacese the primitive groups 

 have only recently been discovered by Trecul. Here they occur in the form of 

 narrow reticulated tracheides at the edges bordering the leaf-gap, enclosed among 

 the scalariform tracheides, or in a narrow notch between them ; and from here they 

 proceed, or send branches into the bundles of the leaf-stalk. In consequence of early 



' Cf. Russow, /. c. Taf. X; especially abundant details in Trecul, /. c, 



