302 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
This statement is confirmed by my more recent researches, as is also another observation 
made in the same memoir, viz. that the crenulated outline described by Brongniart 
and Binney as characterizing the line of junction between the vascular medullary 
cylinder and ligneous zones of Sigillaria and Diploxylon is not a constant feature in the 
latter genus. In the variety which I described under the name of Diploxylon cyca- 
deoideum, believing it to be identical with Corda’s plant so named, I pointed out, as 
already stated, that the cells of the medullary rays had a barred or scalariform structure ; 
and I showed how these cells started from an interrupted layer of similar ones located 
between the inner and outer vascular cylinders. Nothing of the latter kind exists in 
the plant now described. The cells of the medullary rays have very thin and delicate 
walls, differing but little, save in form, from those of the innermost bark, with which 
latter those of the outermost extremities of the medullary rays become actually merged. 
The exogenous growth of the ligneous zone which I have so long recognized, but which 
has been objected to by some botanists, is now more clearly demonstrated than before. 
Decided as were my previous convictions on this point, they have received fresh strength, 
so that I am less than ever inclined to abandon them. We have in these plants the 
three distinct tissues of pith, wood, and bark, in addition to the vascular medullary 
cylinder, which latter I am still inclined to suspect may typically represent the medullary 
sheath of the true Exogens. The specimens described in the memoir demonstrate two 
facts bearing upon the question of the growth of these plants : — 1st, that the formative 
layer, whether we designate it cambium or give it some other name, has been substan- 
tially parallel with the exterior of the previously formed vascular tissues ; 2nd, that this 
layer has displayed an intermittent activity, periodic resumptions of vigorous growth 
alternating with times of rest. The facts detailed in the memoir clearly demonstrate 
that the ligneous zone was gradually built up by a succession of such growths. The 
pith, primarily small, ultimately attained to considerable dimensions through the fissi- 
parous multiplication of its cells. Possibly it may have been the pressure occasioned by 
this multiplication that caused the continued expansion of the medullary cylinder. 
But before attempting to discuss either the physiological questions suggested by this 
inquiry, or the homologous relations of the various tissues of the Lepidodendra to those 
of the living Lycopods, it will be necessary to call attention to a few features in the 
latter objects which require to be considered. 
Considerable variations exist in the structure of the living Lycopodia and Selaqinellce 
but an essential unity pervades the entire group. If we take a matured stem of a 
Selaginella Martensii as a simple type, we find in the centre a single large fibro-vascular 
bundle. In the transverse section this bundle is elliptical, consisting of a central line of 
vessels which are scalariform, spiral, and annular, all the three modifications occurring 
fell into it from the circumstance that the small size of these vessels was in exact correspondence with that of 
the innermost series of the exogenous growth, and very different from that of the large vessels constituting the 
hulk of the medullary cylinders. Having now traced the origin of this vascular cylinder, the question appears 
to he set at rest. 
