78 ORIGIN OF CORMOPHYTA [ch. 



The Evolution of the Pteropsida and Palaeopliyllales. 



The next group which we may consider is the Palaeophyllales, 

 still entirely enigmatical. We may take these with the Pteropsida, 

 because some Psygmophyllums so closely resemble some species 

 of Archaeopteris that dispute has arisen as to the genus in which 

 they should be included^. In Psygtnophyllum (Fig. 27, p. 55) the 

 leaf appears to have arisen by the flattening oiit of a large branch, 

 in a wedge-shaped manner. The morphological unit here appears 

 to be an axis bearing alternated metamorphosed branches, each 

 branch being completely metamorphosed into a large cuneiform 

 leaf, with the radiating nervation so common among primitive 

 leaves. As a later development each leaf tends to become lobed 

 or divided longitudinally. 



Further the leaves are decurrent and thus the stem is to some 

 extent pericaulomic. 



Be5^ond this our knowledge of Psyg)nophyllum does not at 

 present extend, and, in our ignorance of its fructification, it is 

 for the moment maintained as the type of a distinct race. 



Nearly all, if not all, known Pteropsida from Palaeozoic rocks 

 are markedly pericaulomic and we agree with Potonie that in 

 this group the stem is largely built up of leaf bases (cf. Medullosa, 

 Calamopitys, and many other genera). Exact homologues of 

 such stems are common among the Red Algae, e.g. Polysiphonia. 

 It is clear also that this type of structure was common among 

 members of the Psilophyton flora, for it is well seen in Barrandeina 

 (Fig. 18, p. 36) and Ptilophyton (Fig. 12, p. 31) which appear to 

 be Thallophytes. 



The stem, in cases where large leaves are being evolved, would 

 naturally require some accession to its mechanical strength, and 

 this advantage would be gained by a pericaulome, a truly algal 

 feature. 



The differentiation of megaphylly from large branch structures 

 appears to have taken place in several different directions. There 



^ This is the case with Archaeopteris obtusa, (Dawson), inchided in that 

 genus by American Palaeobotanists (see Prosser (1894), Pi. II, j). 49) but 

 transferred more recently by the present writer to Psygmophyllum (Arber 

 (1912), p. 398) on the ground that in tliis type we appear to be deahng with 

 leaves spirally arranged and not a pinnate type of frond. 



