190 THE EVOLUTION OF PLANTS 



Can we trace anything of the history of the 

 Lepidodendrese after Palaeozoic times were over? 

 One or two rather doubtful Sigillarias appear 

 in Triassic rocks; in the Upper Triassic of Sweden 

 a large heterosporous cone has been found which 

 might well have belonged to one of the Lepido- 

 dendrese. A curious plant, Pleuromeia, from the 

 Triassic rocks of Germany and France, has been 

 regarded as a possible link between the Palaeo- 

 zoic family and Isoetes. The upright stem, a 

 yard or more in height, is marked with leaf-scars, 

 not unlike those of a Sigillaria, and terminates 

 below in four short arms, like the four main 

 "roots" of a tree-Lycopod, in a much stunted 

 condition. These four arms are studded with 

 the bases of rootlets. It is evident that we may 

 regard this strange structure as intermediate 

 between the four-rooted base of a Lepidoden- 

 droid stem, and the deeply lobed, root-bearing 

 stem of an Isoetes. The idea is certainly tempting, 

 though there are difficulties in carrying out the 

 comparison in detail. Large cones are found with 

 the Pleuromeia stems, and no doubt belonged to 

 them. They were heterosporous, for the mega- 

 spores have been found. But there is this great 

 difficulty: according to the statements of investi- 

 gators, the sporangium was seated on the lower 

 surface of the fertile leaf, a position quite con- 

 trary to what we find in Lycopods. It is impos- 



