3i8 DIPTERIDACEAE [CH. 



similarity of form, and of position of the annulus (Chap. XXIV, Fig. 489,/, ni). 

 But there the induration is complete, and the dehiscence is median and distal. 

 In D. Lobbiana the dehiscence is obliquely lateral, and as the figures {a, b) 

 show, it may be either right or left of the median line. The stomium is not 

 well defined : the stalk is short, and shows two regular rows of cells as seen 

 from the peripheral side. The "central" or basal side is presented in Fig. 

 576, d, but the stomial side of the annulus is hidden: again the stalk 

 appears as two rows of cells. It follows that the stalk is actually composed 

 of four rows of cells, a point definitely proved by transverse sections. This 

 is a slightly simpler structure than that of Gleichenia lineata (see Vol. I, Fig. 

 243, b). But it exactly corresponds to that oi Metaxya and of the Cyatheaceae. 

 It will subsequently be seen also to tally with that of Cheiropleuria. Exami- 

 nation of the early development of the sporangium proves that the sporangial 

 primordium undergoes a two-sided cleavage, with two rows of segments, as 

 in Metaxya and Alsophila,e.\.c. The spore-output as recorded by H. H. Thomas 

 {Camb. Phil. Soc. 1922, p. 109) is 64 for D. conjiigata, while that appears also 

 to be the typical number for D. Lobbiana, though the full number is not 

 always developed. 



The germination of the spores of Dipteris has not been observed, but the 

 structure of the prothallus is represented in Fig. 577, from a specimen 

 ascribed by Prof l^3ir\gX.oD.Ho7'sfieldii{=conJugata), 

 collected by him on the Malay Peninsula. From 

 this it appears to resemble that of Gleichenia pecti- 

 nata (Campbell, Mosses and Ferns, 3rd Edn. p. 366, 

 Fig, 208), in its foliaceous lateral lobes, while its 

 massive structure is reminiscent of relatively early 

 types of Ferns. It has been seen how young plants 

 of D. Lobbiana show early bifurcation of their Fig. =;;^. prothallus of Z^.>/..z> 

 narrow leaves, at first with open venation (Fig, 569): coujugata seen from below: 



1^.7-, . j,,i , 1 , 1, 1 from a specimen collected by 



but m //. 6-t^;/;//§vr/« the broader characterand closely Dr Lang on Mount Onhir, 



reticulate venation make their appearance in the Malay Peninsula, 



earliest leaves, though underlying these features is the clearly marked 

 bifurcation of the main veins (Fig. 570). 



Comparisons 

 The preceding description has dealt only with the living representatives 

 of the Dipteridaceae, and they all belong to the genus Dipteris. Nevertheless 

 they provide interesting material for comparison with other Ferns, both 

 more primitive and more advanced. Moreover, within the genus itself it is 

 possible to see indications of advance which will be found to facilitate the 

 wider comparisons. But still more important as a means to assigning their 

 phyletic position is the fact that many Mesozoic fossils have with a high 



