SECONDARY INCREASE IN THICKNESS OF STEMS AND ROOTS. 1 33 
of the later-formed wood-cells being thicker tends to the same result. In Dico- 
tyledons, in addition to this difference, the vernal wood contains a larger number 
of large pitted vessels than that formed in the autumn, where, indeed, they may be 
altogether wanting. 
The cause of the formation of annual rings lies, as I was the first to suggest, 
and as has been proved experimentally by de Vries, in the pressure, changing with 
the time of year, which the cortex, and especially the bark, exercises on the 
cambium and the young wood. This will be shown more in detail in Book III. 
Sect. 16. 
In most woods, but not all, whether the annual rings are conspicuous or not, 
there is a distinction, when the stem has attained a sufficient thickness, between the 
so-called Duramen and Alburnum. The Duramen consists of the older and inner 
layers of wood, and is of a dark-brown, yellow, red, or even black colour, and of a 
firmer harder texture ; the Alburnum, consisting of the last annual additions, forms 
a light-coloured or white, softer and more spongy tissue round this nucleus. The 
inner layers of alburnum are gradually transformed into duramen, the cambium 
depositing new layers of wood on the outside, and the cell-walls assuming a darker 
colour, from saturation with resin, colouring substances, &c. The distinction 
between alburnum and duramen is very clear and well-marked in the oak, walnut, 
cherry, elm, 'acacia' {Rohinia Pseud- Ac acid), lignum-vitae {Guaiacum officinale), log- 
wood {Hcematoxylon ca7npechianum^, brazil-wood {Ccesalpinia echinaia), &c. On the 
other hand no duramen is found in the silver fir, Scotch fir, lime, or Paidoimiia 
imperialis, Sec. 
(3) The Secondary Increase in Thickness of Roots in Gymnosperms and Dicoty- 
ledons differs from that which takes place in the stem only in the first production of 
the cambium-ring ; and this diff'erence consists merely in the fact that in roots 
the phloem does not lie outside the xylem-bundles (in the radial direction), but is 
disposed alternately with it on the periphery of the axial fibro-vascular cylinder. 
When the cambium is once formed, it behaves in precisely the same manner as 
it does in the stem. 
We may first investigate the process in a root which is from the first moder- 
ately thick, and has a true pith enclosed within its axial fibro-vascular cylinder 
or plerome, as in many primary roots, at least in their upper thicker portion. 
Fig. 106 represents a transverse section through the upper part of a root of the 
scarlet-runner {Phaseolus multiflorus) at the time when the cambium has been formed 
and has commenced its work ; is the plerome- or bundle-sheath belonging to the 
fundamental tissue, which immediately encloses the pericambium or outermost layer 
of the plerome p c. Within this may be observed the four primary xylem-bundles, 
consisting of vessels of which the oldest and smallest p lie on the outside, while the 
younger and larger g g are next to the pith ÄI. In the interval between each 
pair of xylem-bundles lies a broad phloem-bundle b, b, with true bast-fibres. 
The parts now described have been formed during the growth in length ; this 
has however, at the time when the section was taken, ceased for three or four 
days in this part of the root and increase in thickness has taken its place. The 
cells lying on the inner side of each phloem-bundle first divide repeatedly by 
tangential longitudinal walls ; a layer of cambium {c) is thus formed behind each 
