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 
VlG. 2i,i.—Sela£;inella inceqiialifolia ; longitudinal section through the right side of the axis of a spike .9, the base 
of the leaf the ligule n, and the sporangium j/ ,• point where the cauline and foliar fibro-vascular bundles unite; 
/ air-condueting intercellular spaces; ^'series of cells traversing the spaces. 
thickening bands, and of delicate parenchymatous cells, which is invested by a layer 
of clear, shortly prismatic ceils 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) ^'^^ Bnichmann (Jenaische Zeitschrift, 1874) are of opinion 
that the apical portion of the fibro-vascular body is really a cauline bundle. De Bary, however (Joe. 
cit.), does not accept this view.] 
I i 2 ■ 
483 
