STRUCTURE OF CONCENTRIC BUNDLES. 347 



course of the bundles, by medullary rays 6-10 cell-layers in breadth. Round this ring 

 of separate groups of xylem runs a common annular phloem- region, which is similarly 

 constructed to that of the typical Fern-bundle : outside each group of xylem are first 

 some layers of small-celled parenchyma, then an almost uninterrupted zone of large 

 sieve-tubes running round the whole stem ; this zone is usually one layer thick outside 

 the xylem-groups, while in front of the medullary rays it is many-layered, and projects 

 into them like a wedge. The layer of sieve-tubes is next immediately bounded on the 

 outside by a layer of transversely elongated, partly thick- walled elements, which in their 

 turn are separated from the brown sclerotic tissue of the stem by a many-layered zone 

 of parenchyma. Outside the transversely elongated zone runs an endodermis, which 

 in the mature condition can be recognised by the brittleness of its radial walls. In 

 the petiole of Osmunda the runnel-shaped xylem is surrounded by a similarly-formed 

 zone resembling the phloem of typical Fern-bundles, which in the mature state is 

 bounded on the outside by a very indistinct endodermis ; this zone however, as also 

 stated by Dippel, only contains sieve-tubes in its broader convex half. On the con- 

 cave side it is parenchymatous, and distinguished in cross-section by 10-12 small 

 groups of conspicuously wide cells, which still need investigation \ In the stem of 

 Todea Africana and T. hymenophylloides the structure of the vascular bundle is 

 like that described for Osmunda, only the form of the xylem is in some degree dif- 

 ferent in consequence of the fusions of laterally adjoining bundles. In the lowest 

 part of the leaf-bundle, which has the same shape as in Osmunda, sieve-tubes are, in 

 T. Africana at least, present on the concave side as well. In the leaf-stalk of 

 T. Africana I found the endodermis scarcely recognisable, while in T. hymenophyl- 

 loides it is very clear. 



The axial strand, which the collateral bundles in the stem of Isoetes unite to 

 form, consists of a roundly angular mass of short and irregularly spindle-shaped, 

 reticulated and spiral tracheides, and of thin-walled parenchymatous cells irregularly 

 distributed between them, these elements together forming the xylem. The latter is 

 completely surrounded by a transparent mantle of shortly-prismatic or tabular cells, 

 with contents clear like water, and a strongly refractive membrane, which is provided 

 with broad and very delicate pits, but no clear sieve-pores. Russow is no doubt 

 right in considering this mantle as a peculiarly imperfect phloem of the axial strand, 

 especially as the equivalent parts of the leaf-bundles pass over into it directly. 

 With reference to its phenomena of growth it will have to be further spoken of in 

 Chap. XVIII. 



This may probably be the most fitting place to mention the axial bundle, which tra- 

 verses— Ipngitudinally the leafless stolons of Nephrolepis tuberosa, N. acuminata, and 

 N. exaltata K In the structure, form, and centripetal develojjment of its xylem this agrees 

 completely with the 5-6 rayed radial bundles in the roots of Ferns to be described below. 

 Here also, as in the latter, phloem-groups alternate with these rays, and appear to con- 

 tain relatively wide sieve-tubes, but I am doubtful whether the narrow primitive elements 

 . of the phloem do not also completely surround the rays of the xylem. At any rate the 

 whole inner part of the bundle is surrounded by about two layers of very narrow 

 elements, and the latter usually by two layers of wider parenchymatous cells, on which 



* Compare Djppel, Russow, Lc. ^ Trecul, /. c.^Riissow, I.e. p. 100. 



