650 
PHANEROGAMS. 
place. But usually, even in annual stems (as Helianthus and Brassica) and invariably in 
woody stems and branches, several years old, the subsequent increase in thickness 
begins after the elongation of the internodes. A layer of cambium is formed in each 
bundle between the phloem, which is external, and the xylem which is turned towards 
the axis of the stem ; the cambium layers of the bundles, which are at first separated by 
the medullary rays and lie side by side in a ring, unite into a closed mantle of cambium, 
an interfascicular cambium being formed by divisions in the intermediate cells of the 
medullary rays, which bridge over the spaces between the layers of the cambium 
of the separate fibro-vascular bundles (see Fig. 93). The Cambium- ring thus formed 
produces on the outside layers of phloem, on the inside layers of xylem, while it is at 
the same time itself constantly increasing in circumference. All the tissue formed in the 
fibro-vascular bundles from the cambium-ring on the outside may be termed Secondary 
Bast, all the xylem formed on the inside Secondary Wood, in opposition to the Primary 
Bast and the Primary Wood, which consist of the isolated bundles of phloem and 
xylem of the foliar bundles which were already in existence before the formation of the 
cambium-ring. While the wood which is produced from this cambium-ring forms a 
hollow cylinder, the primary woody bundles project from the inside of the ring into the 
pith as ridges, and often cause it to present on transverse section the appearance of 
Fir;. 465. —The course of tlie buudltb in two internodes of Sainöiiats Illntlus: they lie in a cyHnder which is here 
flattened out ; each internode bears two opposite leaves, and each leaf receives from the stem a middle bundle h h and 
two strong lateral bundles s' s' ; the descending- limbs of the bundles split and interpose between the lower bundles ; 
there are in addition weaker bundles s" s" united by horizontal branches, from which bundles « n ascend into the 
stipules. {After Hanstein.) 
a star. The whole of these primary xylem-bundles are included in the term Medullary 
Sheath; and in the same sense one may adopt Nägeli's term of Cortical Sheath to express 
the whole of the primary bast-bundles at the periphery of the bundles. The growth in 
length of the medullary and cortical sheaths accompanies that of the internodes which 
takes place before the formation of the cambium-ring, and they therefore generally consist 
of very long elements ; — the medullary sheath of very long annular, spiral, and reticulated 
vessels intermixed with long woody fibres ; the cortical sheath containing bundles of long 
bast-fibres which become widely separated from one another by the increase in circum- 
ference of the stem, and which are often strongly thickened but long and flexible; in 
addition to these, long cambiform cells and elongated bast-vessels (latticed and sieve-tubes) 
occur in it. The structural elements of the secondary cortex are, like those of the 
secondary wood, shorter ; in the secondary wood there are no annular or spiral vessels, 
these being altogether replaced by shorter and broader vessels with bordered pits, sur- 
rounded by wood-fibres intermixed with woody parenchyma (see p. 116). The secondary 
cortex forms either a number of layers of thick-walled as well as thin-walled bast-fibres, 
and partially parenchymatous masses of phloem, or these last only, or the most various 
combinations of both. Finally the primary cortex and the epidermis are both generally 
supplanted by the formation of periderm and bark ; although they may sometimes 
undergo a considerable growth in thickness by increasing in diameter at the same time 
