132 
MORPHOLOGY OF TISSUES. 
longitudinally are formed out of longitudinally elongated cambial cells, so the rays 
are formed out of cambial cells lying in rows and elongated in the radial direction. 
In the xylem the cells of the rays are usually lignified, and sometimes have very 
hard walls, as in the copper-beech, where they alone remain after the decay of the 
wood, and then have completely the appearance of constituents of the wood. At 
other times they continue thin-walled, unlignified, and different from the true 
woody tissue. The phloem-rays are usually thin-walled, parenchymatous, and at 
their outer end, where they pass through the primary phloem, they are frequently 
compelled, by the increase in size of the stem, to extend tangentially, when they 
become divided by radial longitudinal walls ; the phloem -bundles which lie between 
them becoming thus pushed further and further apart (Fig. 105, C). 
All the most essential points have now been spoken of in reference to the theory 
of the increase in thickness of the stems of Gymnosperms and Dicotyledons. But 
the formation of secondary xylem and phloem from the cambium, which we have 
at present followed only in the early period of growth, continues, in perennial 
plants, throughout their whole growth ; the wood and the secondary phloem are 
therefore constantly increasing in thickness ; but the increase of the xylem is 
generally considerably greater than that of the phloem. At an earlier or later period 
in the thickening of the stem periderm is formed in the primary cortex, which 
sometimes, as in the beech, copper-beech, birch, and cork-oak, follows the increase 
in size of the stem, and surrounds it as a continuous envelope of cork. But usually 
Bark is formed ; i. e. lamellae of cork cut out flakes of the primary, and subse- 
quently also of the secondary phloem, which then dry up^ and, accumulating on the 
surface, as in the pine, oak, &c., form the bark, or fall off periodically, as in the 
plane. The whole of the primary cortex (phloem) is then replaced by bark ; with 
the exception of the pith and medullary sheath, the stem consists then entirely of 
masses of secondary tissue which have all originated in the cambium; but even of 
the secondary phloem only the inner younger layer usually retains its vitality, the 
outer layers uniting sooner or later, in the production of bark. 
In tropical woody plants when several years old, the additions to the wood 
formed in each successive year are not generally distinguishable on a transverse 
or longitudinal section ; the entire mass of the wood is homogeneous. In woody 
plants, on the contrary, that grow in a climate in which the periods of growth 
are interrupted by a cold or wet season, as is the case with us, the annual additions 
to the wood may be recognised as sharply separated concentric layers, known as 
Anmial Rings. Their sharp separation from one another is caused by the Vernal 
Wood being of a looser texture than the Autumnal Wood. Every annual ring 
consists, therefore, on the inside of looser, on the outside of denser wood which 
' pass into one another insensibly without any sharp line of demarcation; while, 
on the contrary, the dense autumnal wood of the preceding ring is very sharply 
separated from the looser vernal wood of the succeeding ring. In Coniferse the 
distinction between the looser vernal and the denser autumnal wood consists only 
in the tracheides of the former having larger transverse diameters, while in the 
later wood, and especially that formed at the end of the period of growth, they 
are narrower. The same mass of vernal wood includes therefore a larger amount 
of cell-cavity, and is consequently looser, than the autumnal wood. The walls 
