STRUCTURE OF CONCENTRIC BUNDLES. 



345 



forms in Trichomanes, and of the approximate T which it forms in Schizaea pectinata ; 

 in Gleichenia dichotoma and polypodioides they lie isolated and often separated 

 from the tracheides by parenchymatous, cells along the edges of the V- shaped xylem. 



The xylem is surrounded in all cases, in annular bundles both within and without, 

 by a many-layered complexus of tissue, which is to be regarded as the phloem, Fig. 1 6 1 . 

 One or a few laj-ers of parenchymatous cells, containing starch, and similar to those 

 of the xylem, border immediately on the latter. Outside the parenchymatous layer 

 comes an annular zone, which contains the sieve-tubes, though in the smaller bundles, 

 as already mentioned at p. i8i, the latter are certainly not always to be clearly dis- 

 tinguished. When distinctly developed they form a usually single, but in some 

 places double annular ring, and are in contact with each other by means of those 

 walls which are radial with reference to the centre of the bundle. This zone is then 

 followed all round on the outside by a likewise annular zone of elongated fibre-like 

 elements with narrow lumina, characterised by thick, brilliant, and soft walls. These 

 are called by Dippel bast-fibres, and by Russow protophloem, because they appear as 

 the primitive elements of the phloem, and here also it remains doubtful whether they 

 are to be reckoned among the sieve-tubes, or regarded as special organs. They are 

 partly in immediate contact with the indubitable sieve-tubes, and even frequently in- 

 serted in the same circle, while in other cases they are separated from them by small 

 parenchymatous cells. Finally, a one or few-layered sheath of parenchymatous cells 

 containing starch, which are often tolerably wide, and always differ from those outside 

 the bundle in their form and their (smaller) size, completely surrounds the zone of 

 sieve-tubes and fibres, and, apart from a few exceptional cases to be mentioned below, 

 this is in its turn enclosed by a single-layered endodermis, which limits the bundle 

 sharply on the outside. This consists of prismatic, usually inconspicuous cells greatly 

 flattened from without inwards, with a moderately thick, usually brownish membrane, 

 which soon becomes cuticularised, and which in the radial walls is easily torn across, 

 so that in sections the whole endodermal sheath is often split, and difficult to recog- 

 nise. In especially favourable cases (e. g. species of Polypodium) every cell of the 

 endodermis stands exactly in front of a cell of the parenchymatous layer adjoining it 

 on the inside, so that the common origin of the two from one layer ef mother-cells is 

 recognised at once. Even where the latter is not the case, the origin of the two is 

 the same, at least aniong the true Filices and Marsiliacese ^. 



Among the plants belonging to this series the Marattiacese and Selaginellae are 

 destitute of an endodermis ^. The bundles of the former appear simply inserted in 

 the parenchyma, and this applies both to the petiole and to the stem. So at least 

 I found it in young stems of Angiopteris, and I can only suppose that the figure 

 cited by Russow from De Vriese and Harting's Monogr. des Marattiacdes (Taf. VII. 

 Fig. 3, 4), according to which the case' would be different in the stem of Angiopteris, 

 represents the section of a root passing through the stem, for in the root the 

 endodermis is always present ". In the Selaginellae the phloem is surrounded by a 

 dense layer of small-celled parenchyma. 



' Russow, /. <r. p. 19.1. 



' [Compare Treub, Recherches sur les Organes de la Veg. du Selaginella Martensii, Leide, 

 1877.] ' Compare Sachs, Textbook, 2nd Eng. ed. p. 420, supplementary remarks. 



