APICAL MERISTEMS 651 
4 
irregular and intermediate conditions have been found between the 
Marattiaceous type with four prismatic initials and that of the Lepto- 
sporangiate Ferns, where there is only one (Fig. 351). The axis has 
usually a single conical initial, but it is doubtful whether this is 
always so, and irregularities certainly occur (Fig. 352). The leaf of the 
Osmundaceae is alone among Ferns in showing the more complicated 
three-sided initial in place of the two-sided common for the Lepto- 
sporangiates, while the wings of the leaf have a segmentation of the 
Marattiaceous rather than of the Leptosporangiate type (Fig. 353). In 
Fic. 353- 
Sections transversely through young wings of the leaves of various Ferns. A=Angi- 
opteris evecta, showing that there is no single marginal cell, but a small-celled meristem 
at margin of the wing. =a similar section of Todea barbara, with like structure. 
C=similar section of Scolopendrium vulgare, showing regular segmentation of a marginal 
cell (2) by alternating cleavages. D=similar section of Trichomanes radicans, showing 
marginal cell (7) with transverse segmentation. 216. 
point of fact, all the meristems of the Osmundaceae show nearer relation 
to those of the Marattiaceae than do any other Leptosporangiate Ferns 
that have been examined. 
These facts are in obvious accord with the segmentation of the sporangia 
{see above, p. 637): it appears, therefore, that in stem, leaf, root, and 
sporangium those types of Ferns which are held as more primitive 
commonly show a more bulky initial construction, while the true Lepto- 
sporangiates, which are held as derivatives, show in all their parts a less 
bulky type of segmentation. It has been seen above that the spore- 
‘output per sporangium follows a similar sequence, and indeed the numbers 
might be taken as a rough indication of the complexity of organisation 
for the plant at large. The conclusion follows that in the Filicales 
there has been a progressive simplification of the meristic plan: in the 
more ancient forms the meristems are not referable to a single initial, 
