34 THE MORPHOLOGY OF PTERIDOPHYTES 



supply; so none of the descriptions seems to be really 

 appropriate. Since stomata were present in the cuticle 

 covering the stem, it is presumed that the principal site of 

 photosynthesis was in the cortex of the stem itself. However, 

 only mummified specimens have been found, with the result 

 that little is known about the internal anatomy of the stem, 

 except that the xylem tracheids had annular or scalariform 

 thickenings. 



During their growth, the aerial axes were circinately 

 coiled in a manner similar to that seen in the young fronds 

 of a modern fern — a method of growth which, no doubt, 

 gives some protection to the dehcate stem apex, from 

 mechanical damage and from desiccation. Some of the 

 ultimate branches bifurcated and each fork terminated in a 

 sporangium up to 6 mm long and 2 mm wide (Fig. 5H), 

 within which were numerous spores in tetrads. 



Two species of Aster oxylon are known, A. Mackiei^^, 

 which occurred along with Rhynia and Horneophyton in the 

 Rhynie chert, and A. elberfeldense^^ from Middle Devonian 

 rocks near Elberfeld, in Germany. While the German species 

 is known to have attained a height of about i m, the 

 Scottish species is believed to have been somewhat smaller, 

 but one can only guess at its height, for only portions of the 

 whole plant have been found. A. Mackiei had dichotomous 

 rhizomes whose internal structure was so hke that of Rhynia 

 that the two were, at first, confused. However, they were re- 

 markable in being completely without rhizoidal hairs. 

 Instead, small lateral branches of the rhizome grew down- 

 wards into the underlying peat, branching dichotomously as 

 they went, and it is assumed that they acted as the absorbing 

 organs of the plant (Fig. 5A). 



The erect aerial axes were about i cm across at the base 

 and they branched monopodially, dichotomies being res- 

 tricted mainly to the lateral branches. Except right at the 

 base, and in the presumed reproductive regions of the shoot, 

 all the aerial axes were clothed with leaves arranged in a 



