SECONDARY XYLEM <><>l 



Ulmus saberosa, Celtis australis, Morus alba) in which the innermost layer 

 of the wall of the wood-fibres has a peculiar gelatinous or cartilaginous 

 consistency, and remains permanently unlignified. Leclerc du Sablon 

 and Schellenberg have shown that this layer consists of hemi-celluloses, 

 which are deposited late in the season and are converted into soluble 

 plastic material in the following spring. 353 



While wood-fibres are in general shorter than typical bast-elements, 

 they may nevertheless attain a length of "3 to 1*3 mm. According to 

 Sanio, their average length is 1"26 mm. in Prunus Lauroccrams, 1*03 

 mm. in Ulex curopacus, "8 mm. in Quercus pedunculated, "53 mm. in Salix 

 acutifolia and '46 mm. in Tilia parvifolia. 



The typical vessels and tracheides of the secondary wood, that is to 

 say, those which are not utilised for mechanical purposes, are scarcely 

 more thick-walled than the corresponding primary elements. They are 

 frequently provided with delicate spiral thickening fibres (Fig. 295, d, e), 

 which are, however, generally too feeble to be mechanically effective. 

 Strasburger believes that these thickenings serve to " facilitate the 

 passage of water between the gas bubbles of the Jamin's chain and the 

 cell-wall, the liquid travelling along the spiral fibres." Diffusion goes 

 on in most cases through closely crowded bordered pits, or occasionally 

 through the unthickened areas in a reticulately thickened wall (Crassu- 

 laccae, Opuntia, Ccreus). Where water-conducting elements abut against 

 conducting parenchyma (xylem-parenchyma or medullary-ray tissue), 

 they are furnished either with (one-sided) bordered or with large simple 

 pits. 



Among the Abietineae, and in other Conifers, as well as in certain 

 Dicotyledons (Hippopha'c rhamnoides, Salix fragilis, etc.), the tracheides 

 are frequently traversed by radial trabeculae of cellulose. The develop- 

 ment of these structures, which may extend through several annual 

 rings, is insufficiently known. It is likewise uncertain whether they 

 act as radial buttresses, or whether they merely represent abnormalities, 

 as Eaatz maintains. 



Xylem-parenchyma cells arise by the repeated transverse division 

 of cambial mother-cells ; after differentiation is completed, it is often 

 possible still to recognise the genetically related elements, owing to 

 their being arranged in spindle-shaped groups. The individual cells 

 are always elongated and prismatic ; in the vicinity of the larger vessels 

 they are often distinctly flattened. Their cell-walls are lignified, and 

 in general only moderately thickened. The pits are chiefly located on 

 the radial and transverse walls, and are always circular or elliptical in 

 outline. Xylem-parenchyma cells are always provided with living 

 contents, and in winter contain large quantities of reserve-materials in 

 the form of starch (most hard-wooded trees) or fatty oil (soft-wooded 



