l60 THE MORPHOLOGY OF PTERIDOPHYTES 



Sporangium, dehiscence being lateral, although there is no 

 special stomium of thin-walled cells (Figs. 24H and 24I). 

 The spore output is sixty-four. 



Dipteridaceae 



This family is represented at the present day by some 

 eight species of the single genus Dipteris, restricted to the 

 Indo-Malayan region, but in Triassic times there were at 

 least three genera, Clathropteris, Dictyophyllum and Camp- 

 topteris. Again, the architecture of the leaf is quite character- 

 istic, and there can be little doubt as to the correct taxonomic 

 placing of these fossil forms. After an initial dichotomy, the 

 frond shows successive unequal dichotomies in an anadromic 

 direction (i.e. towards the median plane). This pattern is 

 represented in present-day species, in the venation of the 

 two halves of the frond. However, while the primary veins 

 are dichotomous, the smaller ones form a reticulum of a 

 highly advanced type, with blind-ending veinlets, as in the 

 leaves of many flowering plants. 



The fronds arise at distant intervals along a creeping hairy 

 rhizome, whose vascular structure is a simple solenostele. 

 While some species have only a single leaf trace, others have 

 two entering the base of the stipe. 



The sorus is superficial, completely without an indusium, 

 and the sporangia are interspersed with glandular hairs. In 

 Dipteris Lobbiana the sporangia arise simultaneously, but in 

 D. conjugata they are mixed. Thus, the single genus cuts 

 right across the division of the ferns into Simplices, 

 Gradatae and Mixtae. 



The sporangia have relatively thin stalks (only four cells 

 thick) the annulus is obUque (Fig. 24Q-S), and dehiscence 

 is lateral. The spore output is sixty-four. 



Cyatheaceae 



This is the family to which most of the tree-ferns of the world 

 belong. Indeed, at one time, all tree-ferns were placed in it, 



