428 



PRIMARY ARRANGEMENT OF TISSUES. 



Blechnum occidentale, and Scolopendrium vulgare \ in such a form that one or more 

 layers bordering directly on the endodermis acquire brown walls, strongly thickened 

 chiefly on the inner side, so that these layers form either a sheath going uniformly 

 round the whole axial bundle, or one which is interrupted, or at least thinner, over 

 the corners of the xylem-plates. 



The sclerotic sheath in stems and petioles has, in the first series of forms, the 

 same position relative to the endodermis as in the roots. And indeed the sclerosis 

 very often at first affects only the inner walls bordering on the endodermis, and the 

 lateral walls of the layer of cells which is in direct contact with it ; the outer walls 

 of this layer, like those of the surrounding parenchyma, are not sclerotic, e. g. stem 

 and petiole of Polypodium I^ingua, and pustulatum, and the stem of Davallia elegans. 

 The converse condition in the thickening of the walls occurs rarely : petiole of 

 Blechnum brasiliense ; or the sclerotic thickening of the wall exists all round, though 



i it may be weakest on the outside : 



-^ Polypodium Phyllitidis'l In other cases • 



the endodermis is surrounded by an 

 uninterrupted sclerotic sheath, consist- 

 ing of one or more layers (rhizome of 

 Polybotrya IVIeyeriana and Hymeno- 

 phylleae), or by an interrupted sheath, 

 i. e. by one or several many-layered 

 strands of sclerenchyma adjacent to it. 

 This is the case, for example, in the 

 rhizome of Platycerium alcicorne, and 

 in very many petioles. In the very 

 frequent case where the bundles in the 

 latter have projecting corners and de- 

 pressed incurvations of their surface, a 

 definite relation between the latter and 

 the strands of sclerenchyma exists ; e. g. 

 they lie on the concave side of the 

 runnel-shaped bundles in the petiole of 

 Balantium Culcita and Cyathea medul- 

 laris, in the corners of the figure x 

 which the bundle shows as seen in 

 cross-section, in the petiole of Scolo- 

 pendrium vulgare, &c. (Comp. Rus- 

 sow, /. c. and the special pteridological descriptions.) 



In a second series of forms, the sclerenchyma accompanying the bundle is not 

 adjacent to the endodermis, but is separated from it by a zone of delicate parenchyma, 

 usually consisting of many layers of cells. This is the case in the stem of Todea 

 africana, but more especially in that of the Cyatheaceae. In most of the latter, e. g. Cy- 

 athea arborea, Imrayana (Fig. 189), Alsophila microphylla, and many others, the band- 



FIG. 189. — Cyathea Imrayana; cross-section through the hving 

 stem, natural size ; seen from above. X\.b, c, rf, foliar gaps. AH the 

 guue black bands and points are cross-sections of sclerenchyma ; 

 the paler ones those of vascular bundles. Within or adjacent to the 

 foliar gaps, especially d and *, are root-bundles on their way to the 

 periphery; /"furrow of the base of the leaf; a vascular bundle of 

 the main tube ; s outer, s' nmer plate of its sclerenchymatous sheath. 

 Inside s" is the pith, outside s the cortex, with theu- small bundles. 



Tieghem, /. c, p. 66, Taf. 5. 



/ ,- ., 4i ■ 



'^ Russow, /. c, p. 81. 



