532 A TEXTBOOK OF THEORETICAL BOTANY 



characters, more closely resembling the Eusporangiatae than the Lepto- 

 sporangiatae. The first leaf does not, however, penetrate the prothallus, 

 but grows up round its side, which is a character of the latter group. 



Apogamous embryos have been observed in Osmunda, formed by a 

 meristem which originates on the under side of the central cushion of the 

 prothallus. Such embrj^os are formed without any union of gametes taking 

 place, but a fusion of vegetative cells may occur, giving a diploid sporophyte, 

 as in the normal case. 



Relationships 



We have remarked above that the Osmundaceae have a fossil history 

 going back to the Palaeozoic period. The fossil representatives were fully 

 investigated by Kidston and G\\rynne Vaughan, who showed that the 

 peculiar dictyoxylic stele of the living types was derived from an early 

 protostelic condition by the development of a parenchymatous pith. Early 

 traces of this change can be seen in the differentiation of the xylem mass into 

 an outer zone of conducting elements and a central core of short storage 

 tracheids, a condition shown in the Permian types Zolesskya and Thamnopteris. 

 A further stage in the change is shown by Osmundites kolhei from the upper 

 Jurassic, which has a " mixed pith " consisting of parenchyma with scattered 

 tracheids, obviously an indication of the progressive parenchymatization of 

 the central part of the stele. The Palaeozoic types have broad zones of wood 

 with no leaf gaps, but the Mesozoic Osmundites has developed gaps corre- 

 sponding to those in the living genera, associated with a much-narrowed 

 xylem zone. As Bower points out, the living Osmunda has preserved the 

 condition arrived at by the family in Jurassic times, while Todea corresponds 

 rather to the Cretaceous types of Osmundites. The change of stem structure 

 has been accompanied by an expansion in diameter and opening out of the 

 stele as a whole, which is apparently correlated with increasing size of the 

 plants. 



The Osmundaceae as a family probably stand closest in relationship to the 

 protostelic Botryopteridaceae. The main difference between the two families 

 so far as the vascular anatomy of the stem is concerned lies in the position 

 of the protoxylem, which is mesarch in Osmundaceae and endarch in 

 Botryopteridaceae. The older species of Botryopteris showed also some 

 degree of mesarchy in the leaf trace, a condition corresponding to that at the 

 base of the petiole in Thamnopteris. The probable relationship of the 

 sporangial structure in the two families has already been touched upon. 

 There is good reason for the suggestion that the two families Osmundaceae 

 and Botryopteridaceae are related and have probably been derived from a 

 common ancestry (see Volume III). 



We may perhaps usefully summarize very briefly the points in which 

 Osmunda shows a primitive condition : — 



1. Dichotomy of the main axis. 



2. The three-sided apical growth of the leaves and the thick marginal 



meristem of the lamina, together with the open venation. 



