THE ORIGIN OF THE VASCULAR PLANTS. 283 



enormous majority of all existing Pteridophytes, the 

 sporangia are derived from a single epidermal cell, and in 

 all cases provided with a curious ring of thickened cells, 

 the annulus. In the Eusporangiatae the sporangia are much 

 more massive and multicellular from the first, and the 

 annulus is extremely rudimentary or usually entirely absent. 

 The latter group is unquestionably nearer to the seed 

 plants in the structure of the sporangium, and this has been 

 the cause of their being regarded as higher in the series 

 than the Leptosporangiatae which are connected with them 

 by almost insensible gradations ; and it is only quite recently 

 that it has been suggested that the Eusporangiatae, while 

 doubtless related on the one hand more nearly to the 

 flowering plants, are at the same time nearest the Bryophytes. 

 This hypothesis considers the Leptosporangiatae as a 

 comparatively modern group of plants, derived from the 

 Eusporangiatae, which are also the ancestors of a portion, 

 at least, of the seed plants. If this view is correct the 

 Eusporangiatae are an ancient generalised type connecting 

 the Bryophytes directly with the higher vascular plants, 

 and from them as a starting-point have diverged two, and 

 possibly more, distinct lines of development, one of which 

 has terminated in the most specialised of the Lepto- 

 sporangiatae, and another in the Angiosperms. According 

 to this theory the Hymenophyllaceae, instead of represent- 

 ing the most primitive group of vascular plants, are really a 

 somewhat specialised one, whose peculiarities are largely 

 the result of adaptation to a peculiar environment, all of 

 them being inhabitants of dark moist localities to which 

 their delicate structure is especially adapted. In their 

 essential characters they show most evidently a close 

 relationship with the other Leptosporangiatae and no 

 suggestion of structure comparable to that of the Bryo- 

 phytes. 



The more carefully the Eusporangiatae are examined, 

 the stronger becomes the conviction that they are really 

 primitive forms connected with the higher Bryophytes. 

 This is true both as regards the prothallium and the sporo- 

 phyte. The former reaches among these a degree of 



