XX XIV] 



ANATOMY 



315 



D, Lobbiana or qiiinquefurcata. Lastly, in the large-leaved D. conjiigata 

 the bifurcate lamina is broadly webbed, and the very numerous sori, which 

 are distributed over the wide expanse, may be circular or oval, and not always 

 distinct from one another. They may vary much both in size and shape, and 

 their individuality is often lost, so that nearly the whole of the lower surface 

 of the frond appears as though densely covered with a mass of sporangia 

 (Fig. 573). 



There seems to be only one probable way of reading these facts phyletically. 

 Comparison points to Gleichenia and Matonia as primitive types of leaf to 

 which that of Dipteris is related by D. Lobbiana. But from this simple 

 narrow-leaved type, with its single row of sori on either side of the midrib, 

 the broader-leaved types of Dipteris have broken away as the leaf-area 

 enlarged, and the sori have spread over the extended surface Tcompare Vol. I, 



Fig. 573. Dipteris conjugata Reinw. Portion of leaf showing its extended surface, the 

 webbing between the pinnae, the venation, and the numerous sori spread over the 

 surface. (Natural size.) (After Maxwell.) 



Fig. 222, p. 227). The loss of their individuality suggests one way in 

 which the result may have been brought about, viz. by fission. This process, 

 so clearly seen in the species of this very natural genus, has probably occurred 

 also in other types of Ferns. It is suggested by Christensenia among the 

 Marattiaceae, and by Hypoderris among the Woodsieae, but much more 

 obviously among certain other advanced Leptosporangiates. It will have to 

 be reckoned with in any general conception of the phylogeny of Ferns. 



Anatomy 

 The adult rhizome of Dipteris is traversed by a simple solenostele the 

 general character of which resembles that of Metaxya (Vol. I, Fig. 149, 2). 

 In D. conjugata the leaf-traces that arise from it come off each as a single 

 ribbon-like strand, and the leaf-gap soon closes. The margins of this petiolar 

 strand curve inwards to form the usual horse-shoe, which is uninterrupted 



