CARBONIFEROUS LTCOPODS AND SPHENOPHTLLS. 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 tlie 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 



focm of a triangle %vith 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 %\ath great regularity in radial 



series, but a marked difference in the size of the trachea 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.* 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, wliich gave it a scaley structure. 



Such is the 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, 

 Spheno2}hyllum insigne, Will. sp. (= Asterophyllites insignis, 



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



