THE CLUB-MOSSES 189 



petuated or led on to any further advance. The 

 Seed-plants of our own time appear to have been 

 derived from other sources (see Chapters II-IV). 



It is an interesting question whether all Lepi- 

 dodendreae died out after the Palaeozoic period, 

 or whether some of them may have left descend- 

 ants. As we have seen, they can hardly have 

 given rise to the Selaginella line, for the latter 

 type itself goes back to Palaeozoic times, and 

 still shows no connection with the tree-Lycopods. 

 There is an evident affinity between the two 

 groups, but we cannot trace them to a common 

 origin. 



On the other hand, there are several points in 

 common between the Lepidodendreae and Isoetes. 

 The sporangium is attached by a long vase to 

 the fertile leaf in just the same way in Lepido- 

 dendron and in Isoetes, and this is the case in no 

 other groups of Lycopods. The presence of tracts 

 of sterile tissue in the sporangium, and, in cer- 

 tain cases, of an external coat enclosing it, may 

 also be reckoned as common to the fossil and 

 recent groups. Secondary thickening of the stem 

 occurs in both, and the root-structure is similar. 

 On the other hand, nothing could be more dif- 

 ferent than the habit tall trees on the other 

 hand, and dwarf-water plants with a flat disc for 

 a stem, on the other. Also, there is no cone in 

 Isoetes. 



