218 
PEOFESSOE W. C. WILLIAMSON ON THE OEGANIZATION 
section made in the plane of this tongue, shows us that the vessels composing it are 
deflected outwards (Plate XXX. fig. 43, n 1 ) at right angles to their previous course. In 
the section fig. 43 the cells of the part of the medullary ray bounding the remoter side 
of this mass of deflected vessels are seen at f". Fig. 47 is an oblique transverse section 
passing through the inferior keel-like edge (n) of this vascular mass at the innermost part 
of its course, and exhibiting the derivation of its component tissues from the two vascular 
wedges (e, e) bounding the medullary ray through which it ploughs its way outwards. 
Plate XXIX. fig. 44 is a radial section from the external portion of the ligneous cylinder, 
where we still find even the outermost of the vessels (e) contributing their share to this 
vascular root-bundle (n), the letters/* in this section indicating the external portion of 
the medullary ray just previous to its becoming merged with the inner bark ( g ). It 
follows that an enormous number of the vessels directly vertical and superior to each 
primary medullary ray have their lower extremities bent outwards ; but when we 
examine the ultimate bundles (Plate XXXI. fig. 52, n') that leave the exterior of the 
woody cylinder and pass through the bark to the rootlets, we find that the number of 
vessels composing them is very limited, rarely reaching twenty. Hence it is evident that 
the greater part of the deflected tubes never reach the rootlets, but successively disappear 
in the tongue-like projections seen in Plate XXIX. fig. 44, n, & Plate XXX. fig. 43, n. 
With the exception of the portion of the exterior noticed by Mr. Binney, the cortical 
layer of Stigmaria has not yet been described ; but a series of specimens in my cabinet, 
and in those of my coadjutors, enable me to fill up this hiatus in the history of the plant. 
I have already pointed out that the exterior of the ligneous cylinder is invested by a 
thin layer of very delicate cellular tissue (fig. 44, </), the cells of which are somewhat 
elongated vertically (Plate XXX. fig. 49, g)*. In other respects they are almost identical 
with those of the pith, and wholly so with those of the medullary rays, save in the 
direction of their longer axes. In a large majority of the examples which I have seen 
this tissue is the only representative of the bark that is preserved ; but I have several 
specimens which, when combined, give me its entire substance. The thin layer of 
delicate cells seen in fig. 49, which is not above ’015 in thickness, soon passes into a 
thin stratum of equally delicate parenchyma, which in its turn passes into a very thick 
layer consisting of an irregular and variable mixture of prosenchyma and parenchyma, 
but principally the former. In transverse sections these tissues are seen arranged in 
very narrow but regular radiating lines, each of which usually has a breadth of about 
•00085. The appearance in this section is that of a coniferous w 7 ood with very delicate 
fibres; but on making either a radial or a tangential section, the tissues forming this 
part of the bark usually appear as represented in Plate XXXI. fig. 50. It will be seen 
from this figure that whilst some of the cells have pointed and overlapping extremities, 
others have slightly oblique, and others, again, square ends. The tissue has evidently 
* It is impossible to overlook the close resemblance which this tissue bears to that seen investing the vascular 
bundles of the living Lycopods, and to which Nageli and Sachs have given the appropriate name of procambium, 
—May 6, 1872. 
