PSILOPHYTOPSIDA 29 



thought to represent a peat bog which became infiltrated 

 with siUca. In this way the plant remains became preserved, 

 some of them with great perfection. The chief plants to have 

 been described from these deposits are Rhynia major, Rhynia 

 Gwynne-Vaughani, Horneophyton Lignieri and Asteroxylon 

 Mackiei. Of these, the first three lacked leaves as well as 

 roots and are now grouped together in the Rhyniaceae along 

 with Cooksonia^^ and Yarravia^^ from Upper Silurian/Lower 

 Devonian rocks of Great Britain and Austraha respectively. 

 The general appearance of Rhynia major is illustrated in 

 Fig. 4E. It had a horizontal rhizome which branched in 

 a dichotomous manner and bore groups of unicellular 

 rhizoids at intervals. The tips of some rhizomes turned up- 

 wards and grew into aerial stems as much as 50 cm high and 

 up to 6 mm in diameter. These also branched dichotomously 

 and some of them terminated in pear-shaped sporangia up 

 to 12 mm long. The aerial parts were smooth and covered 

 with a cuticle in which stomata were sparingly present, their 

 presence indicating that the stems were green and photo- 

 synthetic. In transverse section (Fig. 4F) the stems are seen 

 to have had a cortex differentiated into two regions, often 

 separated by a narrow zone of cells with dark contents. 

 Whereas the outer cortex was of densely packed cells, the 

 inner cortex had abundant inter-cellular spaces with direct 

 access to the stomata; for this reason the inner cortex is 

 presumed to have been the main photosynthetic region. The 

 sporangium (Fig. 4H) had a massive wall, about five 

 cells thick, apparently without any specialized dehiscence 

 mechanism, and within it were large numbers of spores 

 about 65 ju. in diameter. The fact that these spores were 

 arranged in tetrads is taken to prove that they were formed 

 by meiosis and that the plant bearing them represented the 

 sporophyte generation. What the gametophyte might have 

 looked like no one knows, though the discovery of living 

 gametophytes of Psilotum containing vascular tissue has led 

 to a suggestions^ that some of the bits of Rhynia, identified 



