THE LYCOPODIALES 



255 



polystelic. In still another modification, presented by S. laevigata 

 from Madagascar (Fig. 181), a true siphonostele is exemplified which 

 is complicated by the presence of medullary strands joining up with 

 the walls of the tube in the regions where branches are given off. 

 In the species under discussion the traces of the leaves illustrate 

 the condition typical for the Lycopsida and pass off from the 

 cylinder without causing any x 



gaps in its continuity. 



Isoetes has a protostelic stem 

 which is remarkable among ex- 

 isting Lycopsida in manifesting 

 well-marked secondary growth 

 (Fig. 182). The external prod- 

 uct of cambial activity is a 

 radially disposed storage paren- 

 chyma, while internally the di- 

 viding layer originates additions 

 to the fibrovascular tissues 

 which are most commonly inter- 

 preted as consisting of alternate 

 zones of xylem and phloem. 

 The situation here, however, is 

 disputed, and uncertainty per- 

 sists as a result of the indifferent 

 development of the tissues re- 

 sulting from the characteristic- 

 ally aquatic habit of the plant. The roots in Isoetes are distin- 

 guished, in common with the smaller radical organs of a number 

 of the lower Lycopsida, by the fact that they develop a single 

 mass of xylem in proximity to a single strand of phloem. The 

 leaf is not worthy of special note. 



In the Lepidodendraceae the stem manifests great diversity 

 of structure, as would naturally be expected in a group which 

 in Paleozoic times displayed numerous types with generally 

 marked secondary growth. The primary structures of the stem 

 were either protostelic or siphonostelic. In the former condition 

 a considerable amount of parenchymatous tissue was developed 



FIG. 182. Cambial activity in Isoetes 



