36 THE MORPHOLOGY OF PTERIDOPHYTES 



rather irregular spiral. Whatever the appendages of Psilo- 

 phyton should be called, it is reasonable to call these leaves, 

 for they were up to 5 mm long, were dorsiventrally flattened 

 and were provided with stomata. 



Compared with Rhynia, Asteroxylon Mackiei was much 

 more complex in its stem anatomy (Fig. 5B). In the centre 

 was a fluted rod of tracheids which, in transverse section, 

 had a stellate outhne. Some morphologists apply the term 

 *actinostele' to such a structure. It was, nevertheless, a solid 

 protostele, fundamentally, and its xylem consisted solely of 

 tracheids, either with spiral or with annular thickenings. 

 The smallest elements (protoxylem?) were near, but not 

 quite at, the extremities of the ridges, with the result that the 

 stele is described as mesarch. Surrounding the xylem, was a 

 zone of thin-walled elongated phloem cells. The cortex was 

 composed of three distinct layers, the middle one of which 

 was trabecular (i.e. it consisted of a wide space, crossed by 

 numerous radial plates of tissue), while the innermost and 

 the outermost were of compact parenchyma. Within any 

 transverse section through a leafy axis are to be seen 

 numerous small vascular bundles which, although called 

 *leaf traces', nevertheless stopped short without entering the 

 leaves. (These are omitted from Fig. 5B, for the sake of 

 clarity.) If traced inwards and downwards, they are seen to 

 have had their origin in one or other of the protoxylems. 



No reproductive organs have been found actually in 

 organic connection with the leafy shoots of Asteroxylon 

 Mackiei, but, occurring along with them, were some 

 sporangial branches which are believed to represent the 

 fertile regions. These branches (Fig. 5C) were without leaves 

 and terminated in small-pear shaped sporangia about i mm 

 long (Fig. 5D). These contained spores in tetrads which were 

 shed by means of an apical dehiscence mechanism. Whether 

 these fertile branches were borne laterally or whether they 

 were the apical regions of the main axis is not known. 



The appearance of Asteroxylon elberfeldense is known 



