384 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
the growth of the stem has distinctly separated the medullary vascular buttresses widely 
apart, we distinctly see the intermediate arrangement of the peripheral bundles. 
Examination of a large series of these transverse sections further shows that the 
undulating outline of the ligneous zone, best seen in Plate XXII. fig. 4, and which is 
always more or less present in uncompressed stems, is due to the relationships just 
pointed out. That outline is centrifugal opposite to where the medullary bundles are 
located ( c ), and bulges inwards or becomes centripetal where the cortical bundles are 
placed (z). The latter might have been introduced to compensate for the absence of the 
former for the purpose of equalizing the resisting power of the walls of the entire, com- 
bined, vascular cylinder. I shall shortly point out the existence of a peculiar series of 
exogenous additions made to many of these cortical bundles. 
Prosenchymatous Layer . — Transverse sections of the stem at once arrest the attention 
of even casual observers by the remarkable aspect of this layer, which is well represented 
in Plate XXII. fig. 1, Jc. It bears a strong resemblance to a clock-face, upon which the 
figures have been inscribed in Roman numerals. On magnifying more highly the dark 
bands which give this aspect to the section, as is done to three of them in Plate XXIV. 
fig. 10, Jc', we discover that they are due to irregularly defined radiating laminae of 
prosencliyma, the walls of the component cells of which are thickened by internal 
deposits, converting them into true fibres. These fibrous bands alternate with lenticular 
masses of ordinary cellular tissue, which displays different forms of cells, not only in 
different plants, but in different parts of the same plant. On making tangential sections 
of this portion of the bark (Plate XXIV. figs. 1 3 & 15, & Plate XXV. fig. 14, Jc'), we learn 
that these fibrous laminae not only undulate as they ascend vertically through the stem, but 
bundles of fibres regularly detach themselves obliquely from one lamina to unite them- 
selves with those of a contiguous one, thus mapping out the section, by dark continuous 
lines, into a series of lenticular spaces (7c"). At the first glance these lenticular or 
rhomboidal areolae might be mistaken for those characterizing the bark of a Lepidoden- 
dron ; but closer examination shows that there is no real identity between the two 
structures. The fibrous laminae bear a much closer resemblance to the structure of the 
liber in Dicotyledonous plants, especially as seen in the lace-bark tree (Lagetta lintearia) 
and other Tliymelaceae. In tangential sections of young stems, the areolae described by 
these fibrous layers are longer in proportion to their breadth (Plate XXY. fig. 14, Jc", & 
Plate XXIY. fig. 15, Jc") and also less regular in their arrangement than is the case with 
older ones. Plate XXIY. fig. 13 represents part of the bark of a stem, the ligneous 
cylinder of which, apart from its bark-bundles, has been rather more than two inches in 
circumference. We here see that the areolae (Jc") have not only become less lenticular 
and more rhomboid in their form, but have also arranged themselves with greater 
regularity in pseudo-spiral lines than is the case with many other stems. In these 
respects, however, individual specimens vary very greatly. 
The thickness of this prosenchymatous layer of the bark, as a whole, is usually, in young- 
stems, from T to -145 : in some very young ones it did not exceed ’033. The fibres of 
