100 
PROFESSOR W. C. WlLLIAMSOll ON THE ORGANISATION 
axis, tlie transverse section of which is a triangle with each of the three angles more 
or less prolonged in erpial measures. But in my fructification of Boivmanites the 
transverse section of this axis is very broadly robust, instead of being drawn out into 
three very long and slender radii. Not only so, but the extremity of each short arm is 
very broadly truncated ; a form that hitherto lias only been seen in my Boivmanites 
Dawsoni. In fig. 19 the central triangle a is composed of a cluster of tracheai 
grouped in no special order, except that the central ones are larger than those 
occupying the periphery of the triangle. In my fructification this central truncated 
triangle constitutes the sole vascular axis, a fact easy of explanation. Referring 
to my Memoir V., Plate 1, I have shown that in the very young twig of my Asfero- 
'phyllites, fig. 1, this triangle constituted the only tracheoeal bundle, but as the vege¬ 
tative twig grew in age and size, it developed exogenously zones of tracheae which 
were successively added, investing the triangular centre, as in figs, 2, 3, and 4, resulting 
in the conditions seen in Plate 2, figs. 9, 10, and 11 of the same memoir. The present 
figure 19 is as nearly as possible in the condition of the figure 9 just referred to. Its 
exogenously added tracheas are arranged in equally regular radial and concentric lines; 
the sizes of the individual tracheae enlarging as they were successively superimposed 
upon those previously developed. Portions of a layer of a cork-like cortex, c, are still 
preserved. A similar layer is seen, not in the young twigs of n\j AsterophylUtes 1 and 2, 
but in the older ones 9, 10, and 11. It also appears in the middle of the cortex, 
enclosed, between an inner and an outer parenchymatous zone in M. Renault’s 
Autun Splienopliylluin {loc. cit., Plate 1, figs. 4 and 5, g), as well as in my fig. 16. The 
French savant designates it a “ partie subereuse.”" 
That this stem of Boivmanites Dawsoni is constructed on the same plan as that of 
my Aster ophylUtes, and of M. Renault’s Splienopliyllum StejAianense is obvious 
enough, but to place all these three plants in the genus Spheiiopliyllum, because of 
this vegetative resemblance, as my friends M. Renault and Graf Solms-Laubach 
wish to do, would be wrong. I have already shown that the leaves of my 
AsterophylUtes are not those characteristic of Sphenophyllum ; and the fructification 
of my Boivmanites Dawsoni is altogether distinct from the strobili of Sphenophyllum. 
It must further be remembered that evidence is now being obtained from various 
quarters that there are Carboniferous plants, the branches of which bear both Astero- 
phyllitea.n and Sphenophylloid leaves. 
I therefore conclude once more that we must unite SphenopAiyUum and some 
forms of AsterophylUtes in one and the same genus ; it is equally clear that Boiv¬ 
manites, though its peculiar fructification demonstrates that it constitutes a perfectly 
* The facts here referred to illustrate conditions so frequently seen amongst these Carboniferous plants, 
in which the exogenous development of a xylem zone as well as the differentiation of separate zones of the 
cortex mark advance of age. The fructifications, being young and temporary organs, seem invariably 
to have a contial axis, the internal structure of which is identical with that of the youngest vegetative 
twigs of the parent plant. The Bowmanites described above is a good illustration of this truth. 
