i6o 



LECTURE X. 



As in the primary vascular bundles, on the existence of which the formation of 

 the cambium-ring and the entire growth in thickness depends, we can also distinguish 

 two main groups of tissue-elements in the products of the cambium. The whole of the 

 secondary cortex arising on the outer side of the cambium-ring is in the main a continued 

 development of the phloem of the vascular bundle ; and the wood-mass arising on the 

 inner side of the cambium-ring is in like manner a further development of the xylem of 

 the original vascular bundle. We here neglect the medullary rays for the moment. In 



the cortex, as well as in the wood, we 

 have again, in addition to vascular ele- 

 ments (sieve-tubes and wood vessels re- 

 spectively), also parenchymatous tissue 

 (xylem- and phloem-parenchyma) and 

 elastic fibres (wood and bast-fibres). 



We will, in the first place, consider 

 the composition of the wood arising 

 from the cambium-ring. In the Coni- 

 fers, this consists entirely of elongated 

 tracheVdes, or with a very small admix- 

 ture of parenchyma. These are long 

 fibres, pointed above and below, on the 

 radial side walls of which are remark- 

 ably large, isolated, bordered pits, which 

 produce a very characteristic appear- 

 ance (Fig. 1 66). In the Dicotyledons, on 

 the other hand, such trachei'des usually 

 enter, it is true, into the composi- 

 tion of the wood; together with these, 

 however, three other elements occur. 

 First the wood - vessels, which tra- 

 verse the wood -bundle as continuous 

 tubes, isolated or in groups, and usually 

 with bordered pits. More rarely they 

 are reticulately thickened (annular and 

 ordinary spiral vessels do not occur 

 in the secondary wood, though the 

 Cactacese possess trachei'des with the 

 walls thus sculptured), and then follow 

 parenchymatous cells — the wood-parenchyma. These cells occur either in the 

 form of spindle-shaped fibres pointed above and below, but with living contents ; 

 or they resemble these but are chambered by several transverse septa. Sometimes 

 forming the main mass of the wood, sometimes less conspicuous in the mass, 

 are the proper wood-fibres, or libriform fibres. These are narrow, thick-walled, 

 elongated cells, obliquely pointed above and below ; the wall-structure of which 

 may exhibit all transitions from that of trachei'des with bordered pits to that of 

 bast fibres. Of these histological elements of the wood, the one or the other may 

 predominate, or exist in small quantity, or be wholly wanting. Generally they 



Fig. z66.—Pi!iits sylvestris. Radial longitudinal section 

 the wood of a vigorous branch; cb cambial wood-cell; a- 

 wood cells; ^/'/"bordered pits of the wood-cells in order 

 St large pits where cells of the medullary rays are in contact 

 wood cells (X 550)- 



