THE PTERIDOPHYTA : FILICALES, THE FERNS 



527 



Todea, for in the latter the sporangia are superficial on the lower surfaces of 

 unmodified pinnules. The absence of separate sori and the difference of 

 sporangia! position in the two genera emphasizes the primitive character 

 of the family. 



The leaf of Osmiinda grows by means of a three-sided apical cell, which 

 is exactly like that of the stem (Fig. 525). This is in contrast with the two- 

 sided apical cell which is common to the leaves of most of the Lepto- 

 sporangiatae, and it is in accord with the large size and robust structure of 

 the leaf. The lamina does not grow from a single row of marginal initial 

 cells, as in the Leptosporangiatae, but has a massive, marginal, meristem 

 tissue with segmentation of a complex type, which may be derived from a 

 double row of initial cells. 



The similarity of the apical region in the leaf and the stem lends special 



APICAL CELL OF 

 LEAF 

 jRUDIMENT« 



APICAL CELL OF 

 LEAF 



APICAL CELL 

 OF STEM 



Fig. 525. — Osmunda regalis. A, Apex of stem showing origin of leaf rudiment. B, 

 Osmunda cinnamoniea. Apex of young leaf, before pinna formation, showing apical 

 cell and stipular expansions of leaf base. {After Boicer.) 



interest to the observations of Lang, that in starved plants of Osmunda very 

 much reduced leaves may be formed, which appear as short cylindrical 

 growths without a lamina. On these abnormal growths buds may appear 

 at various levels, or the apices may be directly transformed into buds which 

 develop normal leaves in regular succession. The actual transformation of a 

 leaf into a shoot is a very unusual phenomenon, and it has been suggested 

 by Wardlaw that there has been here, as in Dryopteris, a shifting of a branch 

 apex, laterally, on to a leaf rudiment. In the latter Fern a bud normally 

 appears high up on the abaxial side of the petiole, and in the Osmunda leaves 

 it may have reached the leaf apex and apparently replaced it. 



Development of the Sporangium 



When the sporangium first becomes visible as a small protuberance on 

 the fertile pinnule there is a conspicuous initial cell, but several cells take 



