HICK AXD CASH : AFFINITIES UF LEPIDODENDROX. 
319 
following is the structure of the axis and appendages of ISfigmaria, so 
far as it is known. 
1. The Axis. Most specimens of Stigmarian axes, with struc- 
ture, have lost both cortex and appendages, and exhibit owly a cylin- 
drical mass of wood or xylem, enclosing a cavity tilled with mineral 
matter. 'Pransverse sections show this xylem whicli had an 
exogenous mode of development, to be made up of wedge-shaped 
masses of scalariform tracheides separated by primary and secondary 
medullary rays. The tracheides have wide lacuna, are nearly square 
in cross-section, and, except at the apex of the wedges, form regular 
radial rows. A primary vascular bundle at the apex of the wedges 
cannot be distinguished. The nature of the tissue originally 
enclosed within the xylem cylinder is somewhat doubtful, but 
Williamson believes it was wholly parenchymatous. According to 
Solms-Laubach a layer of bast or phloem surrounded the xylem, but 
this, as well as the inner parenchyma of the cortex, is very rarely 
preserved. The outer portion of the cortex which carries the apppen- 
dages is usually well-preserved, and consists of two layers : — 
i. An outer composed of thick-walled parenchyma which is 
probably the primary cortex, and 
ii. An inner composed of parench}aiiatous cells, elongated 
longitudinally, with pointed ends, and arranged in radial 
rows. These are probably the product of a secondary 
meristem zone or cambium layer lying at the outer periphery. 
2. The Ap2?endages. As in the case of the axis, the preserva- 
tion of all the tissues of the appendages is extremely rare. The only 
perfect specimens hitherto published, so far as we are aware, are 
those figured and described by Williamson.* From these it would 
seem that the appendages have an unmistakable root-like appear^ince, 
both externally and internally, and occasionally branch dichotomously. 
In the young condition there is a single vascular bundle, enclosed by 
a thick parenchymatous cortex. The bundle has a triangular or 
ovate form in cross section, the apex being occupied by the initial 
group of tracheides. From the accounts given by Williamson, Solms- 
Laubach, and Schenk it would seem that the bundle is sometimes 
♦Memoirs: part xi. ; Sbigmaria ficoides : Palasontographical Society's 
publications, 1886. 
