PSILOPHYTOPSIDA 37 



with more certainty and it lends support to the supposed 

 reconstruction of the Scottish species. A portion of the plant 

 is illustrated in Fig. 5E. The lower portions of the aerial axes 

 were clothed with leaves like those of ^. Mackiei, then came 

 a transition region with spine-Hke outgrowths like those of 

 Psilophytoji, while the distal regions were quite smooth. 

 Young developing branches were circinately coiled and the 

 tips of the ultimate branchlets were frequently recurved, 

 some of them bearing terminal sporangia. An interesting 

 feature of its internal anatomy was the presence of a central 

 pith region in the xylem of the larger axes — constituting a 

 medullated protostele. 



It is impossible to overestimate the importance of the 

 Psilophytopsida to botanical thought. Their discovery not 

 only caused many botanists to abandon the classical theory 

 that there are three fundamental categories of plant organs 

 (stems, leaves and roots), but also led some of them to 

 develop new and far-reaching theories of land plant evolu- 

 tion. Thus, the simple Rhynia was adopted as the ideal 

 starting point for the 'telome theory' of Zimmermann—, 

 while Psilophyton and Asteroxylon were taken by others to 

 illustrate the 'enation theory' of the evolution of leaves. 

 These various theories will be discussed in greater detail in 

 the final chapter; in the meantime, one should bear in mind 

 the remarks of Leclercq^^ that these simple plants were by 

 no means the earhest land plants, that more complex plants 

 preceded them in the fossil record and that several other 

 types of land plant existed alongside them in Upper Silurian/ 

 Lower Devonian times. These will be described in succeed- 

 ing chapters, along with the groups to which they are 

 beheved to be related. 



