CARBONIFEROUS LYCOPODS AND SPHENOPHYLLS. 117 



shaped to filiform, veins radiating from the base, dichotomously 

 divided. Fructification in the form of cones, terminal, often on 

 short lateral branches, formed of more or less modified leaves, 

 whose lower portions unite to form a saucer-like collar round the 

 axis, distal portion free, erect. Sporangia varying in number 

 according to the species, sessile or pedicellate; in the pedicellate 

 forms the pedicels spring from the saucer-like collar. 



Internal structure of axis. — Stem consisting of a solid axis, 

 composed of primary (centripetal) and secondary (centrifugal) 

 xylem, surrounded by a stout cortex. 



The young stem of Sphenophyllum contains a solid primary 

 triarch or hexarch central bundle in the form of a three-rayed 

 star, composed in great part of large pitted or scaliform tracheides, 

 though smaller scalariform tracheides occur towards the ends of 

 the arms. The original protoxylem elements situated at the 

 extreme limit of the arms of the star contain small spiral vessels. 



When the primary xylem is fully developed it assumes the 

 form of a triangle with concave sides and truncated angles. In 

 due course this is surrounded by a secondary exogenous zone of 

 xylem and bast developed from a cambium layer. The 

 secondary xylem is developed with great regularity in radial 

 series, but a marked difference in the size of the tracheae is very 

 observable at definite points. Those filling the concave sides 

 have larger openings than those developed from the truncated 

 angles, which latter form curved radiating series of tracheides 

 that are easily distinguished from those formed on the sides of 

 the triangular stele. Professor Williamson and Dr. Scott have 

 designated the former interfascicular and the latter fascicular 

 wood. 1 The leaf traces spring from the apices of the triangle 

 of the primary bundle. The whole was surrounded by a thick 

 bark. The bark of the older stems contained several layers of 

 phyllogen, or cork cambium, situated at different levels in the 

 cortex, which gave it a scaley structure. 



Such is tlie general organization of the stem of Sphenophyllum, 

 but minor differences occur in the various species. 



In the two British species whose internal structure is known, 

 Sphtnophyllum insigne, Will. sp. (= Aster ophyllites insignis, 



1 Phil. Trans., Vol. CLXXXV, (B.), p. 924. 1895. 



