150 THE MORPHOLOGY OF PTERIDOPHYTES 



Antheridium — large, containing several hundred anthero- 



zoids ; wall cells more than four in number. 

 Archegonium — with a relatively long neck. 



In the more advanced ferns, the dermal appendages are 

 usually scales instead of hairs and, as the stem assumes an 

 erect position, the leaves tend to form a crown at the apex. 

 With increasing size, the stelar anatomy becomes more 

 complex, the leaf-gaps overlap, and a dictyostele results. 

 True vessels are known to occur in at least two genera. ^^ 

 The fronds become reduced in size and may have a simple 

 broad lamina with an entire margin and with anastomosing 

 veins, while the stipe receives a number of leaf traces. In 

 the most advanced ferns, the fronds are frequently 'jointed' 

 at the base, i.e. they are shed by means of an absciss layer, a 

 habit which may well be associated with Ufe outside the 

 tropics, in regions where seasonal changes in chmate may 

 be severe. Evolution of the sorus appears to have taken place 

 in stages, the first of which involved a regular gradate 

 sequence of development of the sporangia. The next resulted 

 in a mixed arrangement of old and young sporangia within 

 the sorus. Still more highly advanced is the condition des- 

 scribed as 'acrostichoid,' where the individuahty of the sorus 

 is lost and the sporangia form a 'felt' that covers the dorsal 

 surface of the lamina, irrespective of the position of vein 

 endings. 



The various stages in soral evolution are often held to be 

 the most important indicators of relative advancement and, 

 on this basis, many pteridologists subdivide the Filicales into 

 SimpHces, Gradatae and Mixtae. It is important to reahze, 

 of course, that these subdivisions represent levels of evolu- 

 tion and not taxonomic groups. However, it is debatable 

 whether one character should be weighted to this extent, for 

 it is almost universally agreed among taxonomists that the 

 maximum possible number of characters should be used in 

 the assessment of phylogenetic status. If all the primitive 



