70 
PROFESSOR W. 0. WILLIAMSON ON THE ORGANIZATION 
emerges to reach, the bark. These bundles appear in all tangential sections of the roots. 
I have endeavoured to discover some regularity in their taxis, but have failed to do so. 
The clusters of rootlets were evidently given off alike from all parts of the circumference 
of the primary axis and its branches, but nothin quincuncial or any other regular order. 
Sometimes two or three of the vascular bundles appear in close approximation in one 
tangential section ; in others they are fewer in number and much further apart. It thus 
appears obvious that the clusters of rootlets are arranged irregularly on the periphery of the 
bark. On making a tangential section of the outer bark-layer, h (Plate IX. fig. 55), this 
latter tissue appears as a thick-walled parenchyma, composed of very uniform cells having 
a tolerably mean diameter of ‘0019. Where the vascular root-bundles pass outwards 
through this tissue, they do so enclosed in a sharply defined, round cylinder composed of 
the cells of the inner bark, and which ultimately expands to form the principal paren- 
chyma of the individual rootlets. This cylinder has usually a diameter of about *038 to 
•04, and is obviously the same structure as that seen in Plate VIII. fig. 48, g. Though the 
individual rootlets correspond in their general aspects with those of Stigmaria , they differ 
in several points of detail, but especially in their smaller size, and in the circumstance that 
all their component tissues are almost invariably preserved, whilst the middle parenchyma 
is rarely seen in those of Stigmaria. Plate IX. fig. 53 represents a longitudinal 
section of part of a rootlet. The root-bundle (n) consists of a few slender vessels, which 
are sometimes barred and at others reticulated. This bundle is surrounded by a sheath 
( o "), which is often very clearly defined, especially in the transverse sections (Plate 
VII. fig. 46, o). It consists of long, narrow, thin-walled, and parallel-sided cells. This 
is invested by the principal parenchyma (o'), the cells of which are somewhat lengthened 
in the longitudinal direction ; and the whole is enclosed in a well-marked, and often 
very sharply defined parenchymatous cortical investment (o), which only differs from 
that upon which it rests in the cells being yet more elongated and compressed. These 
rootlets further differ from those of Stigmaria in being of a darker colour, and more 
suggestive of a robust and less delicate texture. 
I have called attention to the peculiar aspect of the outer bark as seen in all vertical 
and transverse sections of it, and also to the fact that the largest and outermost cells of 
its inner parenchyma are frequently intersected by secondary cell-walls, which are 
parallel to each other as well as to the periphery of the axis. It appears obvious to me 
that we have here a meristem structure analogous to some peculiar bark-tissues 
described in two of my previous memoirs*. It also appears clear that the outer paren- 
chymatous cells of the inner bark were the source from whence additions were con- 
stantly being made to the inner surface of the outer bark, by the elongation of those 
cells in a radial direction, and the contemporaneous development within each cell of 
the numerous parallel tangential cell- walls already referred to. It obviously follows 
that each long column of parallel cells, seen in Plate VIII. figs. 47, Jc, & 49, Jc, with their 
common, strongly marked, radially arranged boundary lines, represent what had originally 
* Phil. Trans. 1872, Part I. Plate xxxi. figs. 54 & 57. Ibidem, Part II. p. 313. 
