COENOPTERIDACEAE 



[CH. 



derive their vascular supply from it, and their stele appears to be essentially 

 similar to that of the roots of modern Ferns. The stele of the axis consisted 

 of a solid xylem-core with endarch spiral protoxylem in one or more groups 

 (Fig. 312). The wider tracheides of the metaxylem are mostly pitted with a 

 transition to scalariform structure. There is no pith in any of these Ferns, 

 nor is the xylem differentiated into external and internal zones. The xylem 

 was surrounded by a broad band of phloem in which the sieve-tubes are 

 sometimes particularly well preserved (Fig. 312). In many cases it is not 

 clear whether an endodermis was present: nor is this surprising if it had 

 been of that ill-specialised type seen in Heljumthostachys (see Vol. I, Fig. 173). 

 But Dr Bancroft has recognised 

 it in the stem of B. cylindrica, 

 and in the young petiole there can 

 be little doubt of its existence 

 (Fig. 3 1 3). The whole stelar struc- 

 ture suggests a primitive state. 

 The character of the stele is the 

 feature in which the three genera 

 chiefly agree. From the protostele 

 of the stem of Botryopteris hirsuta 

 and ramosa the monodesmic leaf- 

 traces passed off with a phyllo- 

 taxis of two-fifths. The form of the 

 trace was approximately oval at 

 its departure, but it soon took a 

 very characteristic form. As it 

 leaves the stele the single proto- 

 xylem points inwards; as the trace 

 passes into the petiole three promi- 

 nent points appear upon the adaxial face of it, and to the apex of each of 

 these a protoxylem-group passes. The degree of projection of these points 

 varies in the species, while their orientation relative to the axis also varies, 

 doubtless in accordance with the habit, as is seen in modern Ferns 



(Fig. 311,/./.). 



In B. cylindrica from the Lower Coal Measures, with its isolated leaves 

 and long internodes, both stem and leaf have been carefully examined, so 

 that it is now probably the best known shoot of the family (Fig. 312, ^,^). 

 There are two forms of the stem recognised as (a) and (/S). In the latter 

 the stele is simpler and the cortex more bulky with superficial hairs, giving 

 the impression either of a rhizome or of an aquatic stem. In the former the 

 vascular tissue is better developed, and it gives important features for com- 

 parison. In the first place the axis dichotomises, giving rise to branches 



Fig. 313. Botryopteris hirsuta. Transverse section 

 of vascular bundle of young petiole, showing the 

 xylem in course of differentiation, px, lignified 

 protoxylem ; .r, tliin-walled xylem not yet lignified ; 

 /'/;, phloem-zone. The dark layer may be the endo- 

 dermis. (x 150.) S. Coll. 564 (R.S.). (After Scott.) 



