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 ; t. e. lamellae of cork cut out flakes oF 1;he 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 ofl" 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 

 Annual 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 trachei'des 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 



