132 



OSMUNDACEAE 



[CH. 



Cellular Construction 

 It is not often that a comparative argument can be effectively applied 

 from the facts of cellular construction: but it has already been shown in 

 Vol. I, Chapter Vi, that the Osmundaceae hold an exceptional position in 

 that feature. The Filicales are broadly distinguished as Leptosporangiate 

 or Eusporangiate, according as the sporangium arises from a single cell or 

 from a group of parent cells. It has been shown that a similar distinction 

 holds as a rule for all the apical meristems, the Leptosporangiates having 

 in each case a single initial, while the Eusporangiates have as a rule a plurality 

 of initials. But the Osmundaceae take an intermediate position between the 

 two types as regards their cellular construction. The apex of the stem of 

 Osmwida is, it is true, occupied by a three-sided conical initial, thus resembling 

 that of many Leptosporangiate Ferns, but it also corresponds to what is 



Fig. 4-22. A, apical region of an adult plant of Osviunda regalts^ seen from above, showing 

 the three-sided initial cells (shaded) of stem and leaf. ( x 33. ) B, apex of young leaf of 

 O. cinnamomea, before the first pinnae appear, showing the three-sided initial cell, and the 

 relation of the lateral stipular leaf-bases to its segmentation. ( x 35.) 



found in young stems of Marattia (Charles, Bot. Gas. li, p. 94), and in the 

 adult of the Ophioglossaceae (Fig. 422, A). As far as recorded, this is constant 

 in the Osmundaceae. The apex of the leaf, both in Osviunda and Todea, has 

 a three-sided pyramidal cell, which is a more complex organisation than the 

 two-sided type of all observed Leptosporangiate Ferns: this accords with 

 the relatively robust structure of the Osmundaceous leaf-stalk (Fig. 422, B). 

 A very distinctive feature is the wing of the leaf or pinna, which in 

 Leptosporangiate Ferns shows definite segmentation from a single row of 

 marginal cells (see Vol. I, Figs. 105, 107). But in Osmunda and Todea 

 barbara, as in the Marattiaceae, the massive wing has no single row of 

 marginal initials (Fig. 396, B, p. loi). Similarly with the roots, though small 

 roots may have the usual tetrahedral initial cell, large roots both of Osviunda 

 and Todea may exhibit various irregularities of segmentation ; or they may 

 settle down to a structure with four truncated prismatic initials, as is seen 



