AN EXTINCT PLANT OF DOUBTFUL AFFINITY. 429 



length. Occasionally long and narrow strobili are found 

 attached to the vegetative branches ; in external appearance 

 they resemble to some extent the corresponding structures 

 in calamitean plants, but a closer inspection at once reveals 

 a very distinct individuality for this type of strobilus. 



In Williamson and Scott's work three specific forms of 

 Sphenophyllum are dealt with. We may first of all give a 

 short description of the general type of structure char- 

 acteristic of the genus, and afterwards attempt a diagnosis 

 of the specific characters. 



Primary Structure of the Stem. — Traversing the young- 

 stem there is a single vascular cylinder or stele, consisting 

 of a triarch and centripetally developed axial strand of 

 xylem. A transverse section of such a stem shows in 

 the centre a triangular group of reticulate, scalariform, 

 and spiral tracheids, the latter having a smaller diameter 

 than the others, and constituting the three protoxylem 

 groups at the prominent angles of the solid vascular axis. 

 It is a fact of considerable interest, that we have in this 

 primary structure an arrangement and manner of develop- 

 ment of the tracheids which a student of Botany is always 

 taught to regard as characteristic of root rather than stem 

 structures. External to the xylem there is occasionally 

 preserved a thin-walled phloem tissue, and beyond this 

 may be recognised the pericycle or limit of the stele. 

 Passing beyond the central cylinder we have a thicker 

 walled cortex, of which the outermost layer or epidermis 

 has not been clearly preserved. 



Secondary Structure. — On examining a series of trans- 

 verse sections of stems in different stages of secondary 

 growth, we find that the triangular group of primary 

 tracheids becomes gradually surrounded by radially dis- 

 posed rows of large elements, forming in older stems a 

 considerable thickness of secondary wood, in which, as a 

 rule, there is a striking uniformity in the diameter of the 

 tracheae. Smaller xylem elements occasionally occur, but, 

 as in the majority of Coal-period plants, there are no definite 

 rings of growth. The development of secondary xylem, 

 beginning in the interfascicular region, that is, in the broad 



