24 COENOPTERIDACEAE [CH. 



again by three or four layers of cells with more resistant walls, forming a 

 sheath. The internal xylem forms a narrow median band traversing each 

 arm, and consisting of tracheides of small bore, presumably including 

 protoxylem. At the centre of the star these widen into a small mass of 

 thin-walled parenchyma embedding some internal tracheides. At the peri- 

 phery of the arms a single protoxylem-group passes into each leaf-trace. 

 The trace at its origin is small (i mm. diam.) in proportion to the whole 

 stele (5"5 mm.), as is also the petiole (6*5 mm.) in proportion to the whole 

 stem (5 cm,). As it passes outwards the trace widens, and the single 

 protoxylem branches into two which pass to the poles of the Clepsydroid 

 trace, so that as it enters the petiole it measures 2 mm. diam. as against the 

 6"5 mm. diam. of the relatively small petiole. From the margins of the 

 meristele the supply passes out to the two rows of pinnae. Thus it would 

 appear that the type of Asteroclilaena offers near analogies with that of 

 Ankyropteris of which it may be held as a variant. Perhaps the comparison 

 would lie better with Clepsydropsis australis, the stem of which has recently 

 been described by Mrs Osborn {Brit. Ass. Report, 191 5). It appears to have 

 been an upright, fairly large Tree-Fern of the Osmundaceous type. 



Fig. 324. Asteroptcris iiovcboi-acensis. Incomplete transverse section of stem, 

 showing part of the star-shaped stele and several leaf-traces i-vii. The 

 fracture at the middle of the stele is accidental. ( x nearly 3.) (From 

 Dr Bertrand. After Scott.) 



If A sterochlaena were the only example of a stellate stele in these plants, 

 its stele might be held to be of a late derivative type, since the genus is 

 recorded only from the Permian. But a stellate stele has been described 

 by Dawson from the Portage Beds of New York State of Devonian age, in 

 Asteroptcris noveboracensis, which is the oldest of the known Zygopterideae. 

 The diameter of the stem is here also considerable (2"5 cm.), and it bears 

 numerous though less crowded leaves. The transverse section shows a 

 xylem-star with about 12 arms, each with a single peripheral loop. Each 

 arm gives off a single trace. The leaves are in verticils which are super- 

 posed (Fig. 324). The x}'lem is simpler than in the previous examples, 

 being homogeneous, without either pith or any distinctive internal system 

 of tracheides. The frond is unknown, but the trace is at first Clepsydroid, 

 passing later into a more complex form. 



If. C. Stete CoHw 



