. Wood; a manual of the natural history and industrial applications of the timbers of commerce. Wood; Timber. 12 OF WOOD IN GENERAL such ceUular tissue, when its constituent cells are not more than three or four times long as they are broad, being technically known as parenchyma. ^ . ^ ^ a. As we have aheady seen, in addition to its function ol conduct- ing liquids, which necessitates these vessels or other conducting tissue, as it is termed physiologicaUy, the stem has to perform the mechanical function of bearing up a considerable weight—itself, its branches, leaves, etc. To enable it to do
. Wood; a manual of the natural history and industrial applications of the timbers of commerce. Wood; Timber. 12 OF WOOD IN GENERAL such ceUular tissue, when its constituent cells are not more than three or four times long as they are broad, being technically known as parenchyma. ^ . ^ ^ a. As we have aheady seen, in addition to its function ol conduct- ing liquids, which necessitates these vessels or other conducting tissue, as it is termed physiologicaUy, the stem has to perform the mechanical function of bearing up a considerable weight—itself, its branches, leaves, etc. To enable it to do this, both xylem and phloem are commonly accompanied by elongated elements, of which the chief characteristic is that their walls are much thickened and hard. The elements of this mechanical tissue are known as fibres, and from containing them the bundles are often termed fibro-vascular bundles (Figs. 7 and 8).. S S F T F T C Sv M JS Ccs Fig. 8.—Longitudinal section of a fibro-vascular bundle m Helianthus. (After Saclis) , /v ,. P, pith; Si spiral vessels of protoxylem ; F, wood-fibres ; T, traelie<J3; C, cambium; 8v, sieve-tubes , B, bast fibres; E, endodermis; Cx, cortex. The walls of cells, fibres, and vessels in the xylem acquire mechanical strength or resistance by undergoing a change known as Ugnification. This consists in their impregnation with a sub- stance known as Ugnin. Lignin consists of the same three elements as cellulose, viz. carbon, hydrogen, and oxygen, but in different proportions, its percentage composition being 49 per cent, of carbon, 6 of hydrogen, and 44 of oxygen. Its chemical constitution is, however, as yet unknown. It is harder and more elastic than cellulose, readily permeable by water, but not absor- bent, not, that is, retaining the water. It is more soluble in acids, such as chromic acid, than is cellulose, and is recognised by turning brown when treated with Schulze's solution, a mixture of zinc-. Please note that these images are extrac
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