74 PHANEROGAMS. 



Scconthiry Wood ; in opposition to the Primary Cortex which consists only of funda- 

 mental tissue, and the Primary Wood consisting of the isolated bundles of xylem of 

 the foliar bundles which were already in existence before the formation of the cam- 

 bium-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 a star. 

 The whole of these primary xylem-bundles are included in the term Medullary Sheath ; 

 and in the same sense one may adopt Nageli's term of Cortical Sheath to express the 

 whole of the primary bast-bundles at the point of junction of the primary and secondary 

 cortex. The medullary and cortical sheaths increase in length with the internodes, and 

 therefore generally consist of very long elementary structures ; — 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 circumference of the stem, and which 

 are often strongly thickened but long and flexible ; in addition to these, long cam- 

 biform 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 w^ood, 

 shorter; in the secondary wood there are no annular or spiral vessels, these being 

 altogether replaced by shorter and broader vessels with bordered pits, surrounded by 

 w'ood-fibres intermixed with woody parenchyma (see p. 98). The secondary cortex 

 forms cither 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 com- 

 binations of both. Finally the primary cortex and the epidermis are both generally 

 supplanted by the formation of periderm and bark ; although these may sometimes 

 undergo a considerable growth in thickness by increasing in diameter at the same time 

 that longitudinal divisions are formed (as in Viscum, Helianthus annuus &c.). The 

 masses of xylem and phloem formed by the activity of the cambium-ring are pene- 

 trated lengthwise in radial direction by secondary medullary rays consisting of hori- 

 zontal cells which in the wood are not always lignified, and in the secondary cortex 

 are generally soft and parenchymatous. In the one case they are called xylem- 

 rays, in the other phloem-rays, and always have the power of taking up assimilated 

 food-materials. In proportion as the cambium-ring increases in size, the number 

 of these rays increases ; and the later layers of wood are always traversed by a 

 larger number of rays. They are one or more layers of cells in thickness, and 

 form thin vertical plates wedge-shaped at their upper and lower edges, which have 

 the appearance in a longitudinal section of ribbon-like structures (the 'silver-grain'). 

 In a tangential section the fibro-vascular bundles which run through the length of the 

 stem are seen to form a network of elongated meshes, through which they pass (especi- 

 ally clearly seen in decaying cabbage-stumps). The medullary rays, like the fibro-vascular 

 bundles, are added to by means *of the cambium-ring outwards and inwards ; and as 

 the ring increases in thickness, it produces new rays between the old ones. 



When the increase in thickness of a stem ceases periodically and is renewed 

 with each new period of vegetation, as in our w^oody plants, a layer of wood is 

 formed during each period of growth, (and usually also a secondary cortical layer) 

 •which is sharply marked off from those of the preceding and of the following year, 

 and is called an Anyiual Ring of the w-ood. These annual rings are usually distinctly 

 visible to the naked eye, because the mass of wood formed in the early part of each 

 period of vegetation has usually a different appearance from that formed in the autumn, 

 the latter being denser, the former less dense and generally with a greater number of 

 vessels. The wood form.ed in the spring consists also of wider cells than that produced 

 in the autumn, and the radial diameter of the cells is usually greater. The cells formed 

 in the autumn appear compressed radially and broad in the tangential direction ; their 

 cavities are smaller, and hence, other things being equal, the thickness of their wall is 

 greater. A given quantity of wood produced in the autum.n is therefore denser than 



