COMPARATIVE ANATOMY 339 



anomalies, finds its nearest parallel in certain fossils belonging to the 

 Lycopodiales. 



It may thus be concluded from comparative examination of all the 

 leading types of the Lycopodiales that the vascular structure of the mature 

 shoot is referable in origin in all cases to the non-medullated monostele. 

 This is actually seen existent in the stems of Selaginella spmulosa, though 

 in its lower portion the protoxylem is central ; but it is also shown more 

 amply developed, and with the protoxylem in the accustomed position at 

 the periphery in the upper region of that plant, as well as in certain 

 stems of Lepidodendron. Comparative study of the Lycopodiales shows 

 that all the variants of vascular structure known in them may be referred 

 in origin to this simple type. In Lycopodium the modification has been 

 by intrusion of the phloem more or less deeply into the xylem-core, till 

 this may at last be divided into distinct plates, or riddled like a sponge. 

 In Selaginella there is amplification in various ways, the most obvious 

 being by the adoption of a solenostelic structure, or more commonly by 

 segregation of the enlarging stele to form a varying number of meri- 

 steles. Among the dendroid fossils, where the demands on the conducting 

 system were large in consequence of the large size of the plants, the 

 extended stele became first medullated, as seen in most stems of Lepido- 

 dendron : and then in the later Sigillarias the . residual ring of xylem 

 became broken up into more or less distinct strands. In these types 

 additional vascular tissue was supplied by the potentially unlimited 

 developments from an external cambium. Finally, in Isoetes a complicated 

 structure, partly primary, partly secondary, is found, which would be 

 hardly intelligible except when studied in the light of the dendroid fossils ; 

 but even this, in common with the rest, is referable in origin to the 

 non-medullated monostelic type, together with the results of secondary 

 thickening. The bearing which this constant reference to a primitive 

 monostele has upon a strobiloid theory is plain : as is also the fact that 

 throughout the Lycopodiales the foliar traces are inserted peripherally, and 

 with only slight local disturbance upon the periphery of the cauline 

 xylem-core : for this indicates structurally that the leaf is in them all the 

 minor, while the axis is the dominant feature of the shoot. 



Scott and Hill rightly point out that the view of the central cylinder 

 as cauline applies only to the adult stem of Isoetes : in embryonic stages 

 the construction of the vascular system is from the union of definite 

 leaf-traces : this is the case also in the embryonic stages of certain other 

 Lycopods. The question of the relation of these facts to a theory of 

 the strobilus will be taken up in connection with the embryology of the 

 Lycopods, which forms the subject of the next chapter. 



