On Pallavicinia decipiens , Mitten . 39 
What has been said will show that it is impossible to draw 
any conclusions as to the presence or absence of adventitious 
branches (in the true sense of the word) on the rhizome, but 
structures of this character occur occasionally on the ventral 
surface of the ‘ frond,’ and commonly beneath a mid-rib near 
the spot where it forks. I have never seen these adventitious 
shoots develop, but their rudiments may not seldom be seen, 
especially when examining sections in which archegonia occur 
(cf. PI. VI, Figs. 11-13). As however in the majority of the 
species of this genus, ventral shoots occur, it is a matter of 
some interest to meet them also in this aberrant type, 
although they never, under normal circumstances, pass out 
of a rudimentary state. 
Structure of the Rhizome . — Transverse sections of the rhi- 
zome show a clearly marked strand of fibrous cells, whose 
walls become much thickened and exhibit a number of 
shallow pits on their walls, at first view resembling ordinary 
striation. The individual cells are very much elongated, and 
possess long pointed ends ; when examined in sections near 
the apex, it is seen that the thickening commences in the 
cells in the middle of the strand, and extends from thence to 
those at the periphery. Immediately surrounding the strand 
are the large parenchymatous cells which constitute the chief 
portion of the rhizome, and which are bounded externally by 
an epidermis-like layer, the cells of which frequently grow out 
to form root-hairs (PI. VI, Fig. 8). 
The apex of an underground shoot is occupied by a single 
cell of peculiar form. The actual determination of its shape 
is beset with numerous difficulties, but after comparing 
sections cut in different directions one is led to the conclusion 
that it resembles a prism whose cross-section is that of an 
isosceles triangle, whilst the outer (free) and inner surfaces are 
parallel and they lie in planes at right angles to the long axis 
of the prism. This is a most unusual form for an apical cell, 
but the evidence is conclusive as to its occurrence in this 
plant. Transverse sections of the apex in good preparations 
always show a triangular cell occupying the centre, with 
