320 B ower . — The comp a ra live examina tion of the 
midal apical cell ; but that this rule is not without exceptions 
among dorsiventral forms, such as Pteris aquilina , Azolla , and 
Salvinia , in which a two-sided cell is present. 
On various grounds it had seemed to me not improbable 
that the dorsiventral axes of the Hymenophyllaceae might also 
prove to be exceptions, and have the simpler construction of 
the apex : these have, as far as I am aware, never hitherto 
been thoroughly examined on this point 1 : it is true that 
Prantl 2 has made some observations on Trichomaiies specio- 
sum and T. pyxidiferum> but he admits that they were insuffi- 
cient owing to lack of suitable material. Having had access 
to the fine collection of living specimens in the Royal 
Gardens, Kew, observations have been made on several of 
the more robust species of Hymenophyllaceae, in which the 
difficulties of manipulation are less than in the minute forms, 
and the security of the observations accordingly greater. The 
result is constantly to find the apex occupied by a three-sided 
pyramidal initial cell. 
In Trichomanes radicans the rhizome is rather bulky, and 
its apex flat (Fig. 18): upon this rhizome the leaves arise 
alternately on opposite sides. Seen in surface view (Fig. 1 9) the 
apex is occupied by a three-sided cell, of which one side is 
directed downwards, and one edge upwards. The segments, 
which are cut off from the successive sides, are relatively 
bulky, and from the appearance of the whole meristem it would 
appear that the growth of the segments is much more rapid 
than that of the apical cell itself. Only a small proportion of 
the segments take part in the formation of the leaves, and I 
have been unable to recognise any numerical relation between 
the segments and the leaves formed. The latter arise from 
that part of the upper or dorsal segments which adjoins the 
ventral or lower segments (Fig. 19 b). In the axil of each 
leaf arises an axillary bud, which is constantly present, and 
may therefore be styled a ‘ stable bud ’ in the sense of 
Strasburger 3 . 
1 No species of Hymenophyllaceae were examined by Klein. 
2 Hymenophyllaceae, p. 26 ; Plate I, Figs. 6, 7, 9. 3 Azolla, p. 22. 
