FILICINEX. 



483 



and lignification of the wider tracheides proceed (Fig. 339). The layer of phloem which 

 invests the xylem is itself surrounded by two or three layers of parenchymatous cells, 

 which Russow compares to the phloem-sheath of the Ferns, but which must at any 

 rate be regarded as forming a bundle-sheath belonging to the ground-tissue, investing 

 the bundle within the above-mentioned air-cavity. A well-defined sheathing layer com- 

 posed of cells with folded lateral walls is not to be found in the stem or in the leaves. 

 In the latter, the fibro-vascular bundles are delicate and of simple composition : the 

 xylem consists of spiral and reticulated tracheides, and it is invested by a scanty phloem. 

 To this brief description must be added a few words with respect to Isoetes. The 

 short stem of the mature plant contains an axial woody body which can scarcely be 

 termed a bundle, consisting of short tracheides loosely united, with spiral or reticulated 



Fig. ■3,i,\.—SelagineUa inaqnalifoUa ; longitudinal section through the right side of the axis of a spike 5, the base 

 of the leaf*, the ligule n, and the sporangium j/ ; ^ point where the cauline and foliar fibro-vascular bundles unite; 

 / air-conducting intercellular spaces ; X series of cells traversing the spaces. 



thickening bands, and of delicate parenchymatous cells, which is invested by a layer 

 of clear, shortly prismatic cells with large but delicate pits on their walls : these Russow 

 considers to represent the phloem. The bundles appear to be built upon the collateral 

 type, the phloem forming a continuous ring external to the xylem. The fibro-vascular 

 bundles proceed, one into each of the very numerous leaves (Fig. 333) and into the 

 roots. The stem of losetes probably does not possess any cauline fibro-vascular bundle ^ 



^ [Hegelmaier (Bot. Zeitg. 1874) ^.nd Bruchmann (Jenaische Zeitschrift, 1874) are of opinion 

 that the apical portion of the fibro-vascular body is really a cauline bundle. De Bary, however {loc. 

 cit.), does not accept this view.] 



112 



