132 THE MORPHOLOGY OF PTERIDOPHYTES 



an oblique or horizontal creeping rhizome; and on this basis, 

 too, Christensenia along with Danaea is to be regarded as 

 relatively advanced. 



The stem grows by means of a bulky type of meristem, not 

 referable to a single initial celP and is characterized by the 

 absence of sclerenchyma. Mucilage canals and tannin cells 

 are abundant throughout and give the tissues a very sappy 

 texture. The vascular anatomy of the stem is the most 

 complex of all living pteridophytes and is surpassed in 

 complexity only by fossil members of the group, such as 

 Psaronius. A transverse section of the stem of Angiopteris 

 (Fig. 19O) reveals a number of concentric rings of meristeles 

 which, in a dissection (Fig. 19S), are seen to be part of a 

 series of complex and irregular meshworks lying one within 

 the other, yet interconnected by 'reparatory strands'. The 

 whole system may be described as a highly dissected poly- 

 cyclic dictyostele, but can best be visuahzed as a series of 

 inverted cones of lace stacked inside each other. Although 

 each meristele in the sporeUng is surrounded by an endo- 

 dermis, in the adult state the endodermis is completely 

 lacking. 



The earliest protoxylem elements to Hgnify are 'annular- 

 reticulate', i.e. adjacent rings of lignin are interconnected by 

 a network of strands, whereas later ones are reticulate. The 

 metaxylem elements are scalariform and, in Angiopteris, the 

 orientation of the elongated bordered pits is sometimes 

 longitudinal, instead of transverse. This pecuhar arrange- 

 ment has been called 'ob-scalariform'^^ and occurs else- 

 where in the Ophioglossaceae and a few leptosporangiate 

 ferns {Dennstaedtia and Blechnum). 



Each leaf, in a mature plant, receives a number of traces 

 which arise from the outermost system of meristeles (the cut 

 ends of the leaf traces are represented in black in Fig. 19S), 

 but the root traces may arise from the innermost regions of 

 the stele, threading their way through successive cones on 

 their way to the cortex (cross-hatched in Fig. 19O). In those 



