PTEROPSIDA 131 



may be as much as five times pinnately compound or, in 

 some species, only once pinnate, like a Cycad leaf, while 

 a few species have a simple broad lamina. Christensenia is 

 pecuhar in having a palmately compound frond, as the 

 specific name, C. aesculifolia, imphes. It is also pecuhar in 

 having reticulate venation, for all the other genera have 

 open dichotomous venation. All show circinate vernation, 

 i.e. the young frond is coiled Hke a crozier and gradually 

 uncoils as it grows. This is a feature which they share with 

 Leptosporangiate ferns but which is absent from the 

 Ophioglossales. With the exception of Danaea trichoman- 

 oides, all the Uving members of the group have very leathery 

 pinnules in whose ontogeny several rows of marginal initials 

 are active (instead of a single row of marginal initial cells, 

 as is more usual in leaves of other plants). In many 

 species there are swelhngs, or pulvini, at the base of the 

 pinnae and pinnules, which play a part in the geotrophic 

 responses of the leaf, and in all species there are thick fleshy 

 stipular flanges at the base of the petiole. Fig. 19V illustrates 

 the appearance of the growing point of Christensenia, show- 

 ing how the stipules (i) are joined by a commissure (2), 

 and how they are folded over the primordium. After the 

 frond has died and has been shed, the stipules and the leaf 

 base remain attached to the axis and contribute much to its 

 overall diameter. 



The young parts of Marattia and Angiopteris are covered 

 with short simple hairs, while those of Christensenia 2ind 

 Danaea bear peltate scales. Bower^ suggested that the nature 

 of the dermal appendages in ferns can be a useful indicator 

 of primitiveness or advancement, hairs being more primitive 

 than scales; on this basis, therefore, Christensenia is rela- 

 tively advanced and this conclusion is supported by its 

 possessing reticulate venation. A comparison of fossil and 

 recent members of the group suggests that there has been 

 progressive reduction in height, from the tree-hke Carbon- 

 iferous forms, through an intermediate stumpy erect axis, to 



