130 MORPHOLOGY OF TISSUES. 
bundles themselves, they may conveniently be distinguished, the latter as Fascicular 
Cambium {B, /c)j the former as Interfascicular Cambium {B, ic). It may further 
be mentioned that three small groups of bast-fibres b b b lie in the phloem-portion 
of each bundle. 
The activity of the whole continuous cambium-ring now commences. The 
ring consists of rows of cells arranged radially; in each of these rows the cells which 
lie on the inner side are the origin of the secondary xylem, those which lie on the 
outer side of the secondary phloem ; while a middle layer of the cambial cells always 
remains capable of division, and thus maintains the continued formation of secondary 
xylem and phloem. In this manner the seedling stem increases considerably, in 
the course of a few weeks, both in diameter and rigidity, in consequence of the 
formation of a cylinder of secondary wood (xylem) surrounded by a layer of 
secondary ■ cortoir (phloem), understanding by the latter term all the layers of phloem 
developed out of the cambium-ring. Fig. 105, C is the transverse section of the 
tigellum of the seedling plant which has already increased con^derably in thick- 
ness; by the deposition of secondary phloem, the primary p£ioo'm^ (i?) has been 
compelled to grow tangentially ; its cells, as well as those of the epidermis, 
stretch in that direction, and become divided by radial longitudinal w^alls. At 
this time it is still possible to refer back the various parts of the secondary 
tissue to their origin ; the original xylem-portions of the bundles (x), which 
were developed before the increase in thickness, can still be recognised ; they 
appear as projections of the woody substance into the pith, and are collectively 
comprised under the term Medullary Sheath x x x. It is somewhat more difficult 
to make out the original phloem-portions of the bundles ; but we are assisted by the 
position of the groups of true bast-fibres b b b already mentioned ; they still remain 
unchanged, but are pushed much further asunder, because the intermediate soft bast 
has extended in the tangential direction, in consequence of the pressure of the 
secondary tissue from within. The radial diameter of each phloem-portion p 
has also increased, secondary layers, formed out of the fascicular cambium, having 
been added to the primary phloem from within. The primary xylem-portion x 
of each bundle is separated from its phloem-portion p by a thick layer of 
secondary xylem fh, which has been developed from the fascicular cambium. 
These portions may be distinguished as Fascicular Xylem from the Interfascicular 
Xylem (Fig. 105, C, ifli) formed out of the interfascicular cambium; a layer of Inter- 
fascicular Phloem ifp on the outside corresponds to each of these latter. The entire 
mass of secondary xylem consists therefore of fascicular and interfascicular xylem, 
the entire mass of secondary phloem similarly of fascicular and interfascicular 
portions. The primary xylem is wanting on the inner or medullary side of the 
interfascicular xylem, the primary phloem in the interfascicular phloem. 
With reference to the elementary constituents of the secondary or thickening- 
tissues, it is first to be noted that in this respect the fascicular and interfascicular 
formations are alike. The whole mass of secondary xylem, with the exception 
of the medullary rays or ' silver- grain,' consists, in Conifers of tracheides with bor- 
dered pits (Fig. 23, p. 25); in Dicotyledons of wood-prosenchyma, wood-parenchyma, 
and pitted vessels formed from short cells. In the secondary xylem no annular, 
spiral, or reticulated vessels are formed ; these arise only in the original xylem-portions 
