= 
cage oy wietiirat and Ph iolle vical Boing’ 
and it then constitutes the medullary rays. The vascular 
bundles grow for some time by means of the cambium which > 
lies between their xylem- and bast-portion. The xylem- 
portion consists of elongated wood-cells, which sometimes 
resemble an elongated parenchymatous, sometimes a fusiform — 
prosenchymatous tissue, and in addition of vessels and a 
thin-walled parenchyma that is but little or not at all ligni- 
fied ; while the bast-portion contains on its outer side a 
bundle or crescent} of bast-fibres, which sometimes consist 
of true prosenchymatous, sometimes of elongated paren- 
chymatous cells. In contact with it on the inside are thin- 
- walled sieve-tubes and elongated parenchymatous cells. In 
many plants bast-cells are also formed next the pith on the 
axial side of the vascular bundle; in others laticiferous 
vessels also occur, usually in the bast “portion of the vascular 
bundle, but also in the cortex. , 
The vascular bundles of woody dicotyledonete plants 
have, at the end of their first year of growth, the structure 
already described, but are already «distinguished by the 
regularity of their bast-bundles, as wellas by possessing ves- 
sels dispersed through the xylem. The stems of many tropical 
trees in which the growth is never subject to intermission, 
as the cocoa and coffee-bushes, remain in this condition, 
and only gradually increase in diameter by the formation of 
portions of the vascular bundles exactly resembling those 
already 1n existence. But in those tropical trees and shrubs 
which cast their leaves periodically and have closed buds, like 
the bread-fruit tree, the activity of the thickening-ring is 
renewed afresh on the recurrence of the active period of 
vegetation, as in our deciduous trees ; and fresh medullary 
rays and new xylem- and bast-portions: of the vascular 
bundles are formed in exact continuation of the similar parts 
already in existence. This process 1s repeated every year, 
a new layer of cells being annually separated on each side of 
the thickening-ring, which may even be recognisec at a later 
period as a sharply defined annual ring (Fig. 95, Il. p. 68). 
