Chapter IX 

 EMBRYOGENESIS IN EUSPORANGIATE FERNS 



GENERAL INTRODUCTION 



THE living ferns constitute a large and coherent class descended from 

 early Palaeozoic ancestors. Two subdivisions, the Eusporangiatae 

 and the Leptosporangiatae, are recognised, the former being the more 

 ancient and primitive, the latter the more modern and derivative 

 (Bower, 1923 etc.). There is, however, no absolute separation of the 

 two subdivisions: a gradual emergence of the leptosporangiate from 

 the eusporangiate condition can be traced along several phyletic lines. 

 Bower has argued that the ferns can be arranged in a phylogenetic 

 scheme on the basis of twelve criteria of comparison, including external 

 morphology and symmetry, the initial constitution of the plant body as 

 indicated by segmentation, the nature of the vascular system, the 

 inception and character of the sporangium, the morphology of the 

 prothallus and the embryology of the sporophyte. If these criteria are 

 applied to the whole class of living ferns, a rough division can be made 

 into those, at one end of the series, in which the general plan of con- 

 struction is on a robust or massive basis (the Eusporangiatae) and those 

 at the other end, in which the general plan of construction is on a less 

 massive but structurally more exact basis (the Leptosporangiatae) 

 (Goebel, 1880). Between these extremes various intermediate conditions 

 have been found. Campbell (1890) concluded that the massive euspor- 

 angiate ferns are primitive and the more delicate leptosporangiate ferns 

 derivative. This conclusion is supported by the fossil evidence. 



In Bower's comparative treatment, the young sporophytes of 

 eusporangiate and leptosporangiate ferns afford materials for com- 

 parison. The two main features in the embryogeny which he regards 

 as possibly having a phylogenetic bearing are (i) the relative complexity 

 of the initial segmentation, and (ii) the presence or absence of a 

 suspensor. 



In the older classifications, the ferns constituted one of the six 

 related classes of the Pteridophyta, or Vascular Cryptogams. But in 

 more recent classifications {see Chapter XVI), their affinity is with the 

 Gymnospermae and Angiospermae. As these problems of taxonomy 

 have by no means been solved, it will be of interest to consider if the 

 facts of embryology throw any light on the various relationships that 

 have been suggested. 



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