720 | PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
for it was the identity of these structures on the stem of Lyginodendron and on the 
petiole known as “ Rachiopteris aspera” which first suggested that these organs 
might be different parts of one and the same plant—a suggestion which has meer 
amply conlirmed since by their discovery in direct continuity.* 
7. On certain Small Stems of the Lyginodendron type. 
In a previous memoir attention has been called to the peculiarities of certain very 
small stems which, from their general structure, appear to belong to Lyginodendron.t 
At that time the roots of Lyginodendron were not yet understood ; it now turns out 
that the smallest of the supposed stems, including al! those with the “ central axis 
solid,” were really roots. This applies to the following specimens, enumerated on 
p. 94 of the memoir cited’: C.N. 1885 C., 1885 A., 1883 and 1885. 
There remain, ‘however, a certain number of very small true stems, the nature of 
which is proved beyond doubt by the- presence of leaf-trace bundles in their cortex. 
Examples of such stems are shown in figs. 11 and 12 of Part XVIL, and in fig. 2 of 
Part IV. 
A clear distinction must be drawn between those small stems which differ in their 
primary § structure from the typical forms, and merely young stems, which are charac- 
terized simply by the absence or small development of the secondary tissues. One of 
these young stems is shown in transverse section in photograph 2, and has been 
already mentioned (pp. 710-and 714). Here there is a well-marked pith, and the bundles 
surrounding it are distinctly separated from one another. Other examples at about 
the same stage of growth are known, some of which are of considerable size. They 
present no special difficulties, being evidently stems of the normal type at an early 
stage of development. In these young specimens the outer cortex is mainly scleren- 
chyraatous, with only narrow iadial bands of parenchyma. The great development 
of the latter in older stems was, no doubt, largely due to dilatation induced by the 
secondary growth. 
Returning to the stems in which the primary structure is on a small scale and is 
peculiar in type, we may distinguish two categories. The one is represented by 
fig. 2in Memoir IV. and fig. 11 in Memoir XVIL., the other by fig. 12 in Memoir XVII. 
In the former type the primary xylem forms an almost complete ring, or, at least, 
is not differentiated into distinct bundles; in the second type, the bundles surrounding 
the pith are quite distinct and normal, but the whole is on an extremely small scale. 
In fact, the smallest stems we have, one of which is only 3 millims. in diameter, 
belong to this type. These smallest stems have secondary wood of considerable thick- 
ness (about 14 cells thick). Cambium and phloém are well preserved, and the leaf- 
traces are evident in the cortex. The phyllotaxis appears to have been 4. In one 
* Witttamson on “ Organization,” Part IV., p. 405; Part VI., p. 682; Part XVII, p. 91, 
+ Wintramson, “ Organization,” Part XVIL., p. 92, 
