230 DIPTEROID FERNS [CH. 



exile (Halle), or 128 for D. rngosuvi (H. H. Thomas). It was then a point 

 of peculiar interest to find in Cheiropleiiria that the typical number is 128 

 (Thomas); this is a fact so exceptional among living Leptosporangiate 

 Ferns that it points a definite connection with the Mesozoic Dipteroids. The 

 swing to a lateral dehiscence seems to have occurred in Mesozoic time, and 

 it is characteristic of all modern Dipteroids, while the organisation of the 

 stomium becomes more precise in the more advanced of them. The sporangial 

 stalk underwent changes from the short massive stalk of Gleichenia (a), and 

 that almost equally complex in Mato7iia (/), to the four-rowed stalk oiDipteris 

 and Clieiropleuria {k\ and onwards to the three-rowed in Platycerimn and 

 CJiristopteris (/), and a simpler structure still is seen in the Polypodioid 

 derivatives (see Vol. I, Fig. 243, to which the letters refer). This reduction 

 is not directly proportional to the spore-output, though it follows roughly 

 the diminishing number of spores in each sporangium. It appears to be more 

 closely related to the proportion of size of the sporangial head to the length 

 of stalk. The change from a four-rowed to a three-rowed stalk is linked with the 

 change of the initial segmentation of the sporangium: it has been discussed 

 at length in Chapter xxxiil (Vol. il, p. 303). Among the Dipteroids the 

 transition lies between Dipteris or Clieiropleuria on the one hand, and Platy- 

 cerium and Christopteris on the other: and a parallel may be drawn with a 

 like change on passing from the Cyatheoids to the Dryopteroids. The spores 

 themselves are without perispore throughout the series (Hannig, I.e. p. 339). 

 The comparison of the facts drawn from the living Dipteroid Ferns as 

 described above, when it is combined with the rapidly growing body of 

 facts relating to the Mesozoic Dipteroids, will leave little doubt that in 

 them all we see a natural phyletic sequence, leading from primitive 

 Simplices to advanced Mixtae. The series in fact pursued a morphological 

 course of its own, and preserved its identity from the Triassic Period to the 

 present day. It is this which gives a peculiar value to its study: for in 

 this series are illustrated a large body of progressive steps, both in form and 

 structure of the vegetative organs, and also in the characters of the sori and 

 sporangia. Not only do these follow along lines parallel in the main among 

 themselves, but they are also parallel with similar advances in other phyla 

 of Ferns, with which these have no near relationship. This remark applies 

 to the general morphology of the shoot : to the progression from hairs to 

 broad scales: to the growing disintegration of the vascular tracts: to the 

 elaboration of the sorus from the simple uniseriate type: to the indications 

 of change from the longitudinal to the transverse dehiscence: to the 

 diminution of size and spore-output of the sporangium, and to the change 

 from its simultaneous to its mixed origin. All of these progressions stand in 

 this very natural series of Ferns upon their own footing, and a close 

 observer would deduce them from observations on the Dipteroids them- 



