2o6 DIPTEROID FERNS [ch. 



past the stalk, while the lateral stomium is distinctly below the equator of 

 the sporangial head. The number of annular cells is large, 26 to 36 have 

 been counted (Fig. 717). The cell-cleavage of the young sporangium is, as in 

 Dipteris, by two rows of segments, giving the four-rowed stalk, a feature that 

 is shared with many Cyatheoids (see Vol. II, pp. 291, 302). The spore-output 

 has been shown b)- H. H. Thomas to be t}'pically 128: actual counts were 

 124, 123, 108. This is an important link with the fossil Dipteroids, coinciding 

 as it does with certain archaic features in the sporangium (Vol. II, p. 321). 



There is probably no living Fern which is so clearly a synthetic type as 

 Cheiropleiiria. Its characters present a singular combination of the primitive 

 and the advanced. The former include the protostelic axis, with a branching 

 susceptible of interpretation in terms of dichotomy: the presence of simple 



Fig. 716. Transverse section of part of a fertile 

 blade of Chciropleitria. ( x 50 ) It shows the dip- 

 lodesmic state, the storage tracheides of the re- 

 ceptacle being nearer to the lower surface than 

 the strands of the venation, which lie right and left 

 of it. 



hairs and the absence of dermal scales: the peculiarly primitive leaf-trace: 

 the prevalent dichotomy of the main veins of the blade : the absence of any 

 indusium protecting the superficial coenosori : the large size, two-rowed 

 segmentation, and oblique annulus of the sporangium, and the relatively 

 high spore-output. These features collectively confirm the Matonioid-Dipte- 

 roid affinity of Cheiropleiiria, while the vascular structure strongly suggests 

 the further link with the living Gleichenias, and a still further reference to 

 certain Mesozoic fossils on the ground of the relatively high spore-output. 

 Moreover, all of these Ferns have superficial sori, a feature which has been 

 constant for them at least from Mesozoic times. 



Characters of advance appear in the webbed leaf-blade, with {q\m and 

 irregular branchings, or none at all: the highly reticulate venation; and the 

 mixed character of the Acrostichoid coenosorus, of which the extended 



