APICAL MERISTEMS 



651 



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 



D 



FIG. 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 meristeni 

 at margin of the wing. B=a. similar section of Todea barbara, with like structure. 

 C similar section of Scolopendrium vulgare, showing regular segmentation of a marginal 

 cell (m) by alternating cleavages. D = similar section of Trie ho manes radicans, showing 

 marginal cell (m) with transverse segmentation. X2i6. 



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, 



