STRUCTURE OF RADIAL BUNDLES. 351 



former consists of a few small vessels lying on one side of the cylindrical bundle. In 

 the roots of L. Selago and inundatum a strongly-curved, diarch vascular plate, sickle- 

 shaped in cross-section, lies, according to Russow's description, inside the cylindrical 

 phloem, the sieve-lubes being situated between its limbs. 



=• Sect. 108. In the great majority of roots the axial bundle which traverses them 

 is of very regular radial structure, which in its principal characters is uniform in all 

 cases '. 



The approximately cylindrical bundle is surrounded by an endodermis, which is 

 either permanently undulated, or is only so at first, becoming sclerotic in the mature 

 condition. According to its origin the endodermis is not to be assigned to the 

 bundle, but forms the innermost (limiting) layer of the surrounding cortex. The 

 xylem is according to the particular case diarch or polyarch, and its starting-points, 

 corresponding to what are afterwards its peripheral corners, all lie at equal distances 

 from one another : in diarch bundles at diametrically opposite points of the circular 

 cross-section; in all other cases removed from each other by the fraction of the 

 periphery determined by their number (|-, \, &c.). From the starting-points vascular 

 plates develope in a radial direction, and in centripetal order of development ; and these 

 either meet in the centre or do not reach it, but remain separated by a parenchyma- 

 tous or sclerenchymatous mass, which permanently occupies the middle of the bundle. 

 The same number of phloem-groups alternate with the vascular plates, to which they 

 thus correspond in number and arrangement. 



The xylem and phloem-rays are separated from each other by delicate paren- 

 chymatous cells, and in fact two layers of the latter may as a rule be distinguished 

 between each xylem and the next phloem group ; more rarely only a single layer is 

 present, or there are more than two. On the- outside an uninterrupted zone of 

 parenchyma, which usually forms a single layer, more rarely two layers, and rarely 

 several, constitutes the limit of the whole bundle towards the endodermis. In the 

 case of the Ferns, Nageli and Leitgeb have called this limiting layer the Pericamhium, 

 a name which'it may here bear generally, even where, as in Equisetum, its origin is 

 different from that in the cases for which it was first introduced. In Monocotyledons 

 however cases are not rare, where the outermost vessels border directly on the endo- 

 dermis, and the Pericambium is thus interrupted at every xylem-plate, and only 

 surrounds the phloem-rays. 



Van Tieghem calls the whole of the cells, which are interposed between the groups 

 of xylem and phloem, and thus unite them into a dense cylinder, conjunctive tissue 

 (tissu conjonctif). The latter forms, as has been said, the usually two-layered 

 bands between the xylem-plates and phloem-groups, and is continued inwards between 

 the former in cases where they do not meet. Externally it borders on the pericam- 

 bium. The latter is called by Van Tieghem in the case of the Phanerogams the 

 rhizogenic layer, from the function which it performs in the origination of lateral 

 roots. 



' Nageli, Beitrage, I.e. p. 23.— P. van Tieghem, Recherches sur la symmetrie de structure dans 

 les Plantes vasculaires. I. La racine. Ann. Sci. Nat. 5 s^r. torn. XIII.— Nageli und Leitgeb, Ent- 

 stehung d. Wurzeln, Miinchen, 1867. — Nicolai, I.e. (compare p. 231).— See also Link, Icones 

 anatomicEe. — Schacht, Lehrbucli, p. 167, etc. 



