340 STEM-STRUCTURE 



cambial zone, also exhibits a very uniform, radially seriatet 

 structure. Except for the narrow medullary rays and occasiona 

 resin-canals, it consists entirely of fibre-like tracheids, which ar< 

 differentiated among themselves only in respect of the distinction: 

 between spring- (Fig. 197, A, Sp.) and autumn-wood (Au.) l (se< 

 p. 125). The tracheids bear a single row of large circular bor 

 dered pits on their radial walls, as can be recognised in trans 

 verse, but more readily in radial longitudinal (Fig. 197, C, b.) 

 sections, when the pits themselves are seen in surface view ; ii 

 the autumn-wood the tangential walls are also pitted. Thi 

 groups of primary xylem, composed of spiral tracheids, projed 

 into the small pith and are separated from one another by thi 

 primary rays. 



The structure just noted for the Scotch Fir is that typica 

 of most Conifers, but resin-canals are absent from the wood ii 

 certain genera (being often replaced by resin-cells), whilst ii 

 the Araucarias, and occasionally in other members of the grouj 

 (e.g. Pinus palustris), the tracheids bear two or more rows o 

 bordered pits. 



Radial and tangential longitudinal sections exhibit the sam< 

 arrangement of the medullary rays as in Dicotyledons (Fig. 197) 

 In some Conifers certain rays, which are relatively wide, an 

 traversed by resin-canals connecting those of the pith and cortex 

 As a general rule the rays consist of uniform cells, whose wall: 

 often bear simple pits in Pinus and its allies, although elsewher* 

 usually smooth. Several Abietineae, including the Scotch Fir 

 show a complex differentiation of the rays, best seen in radia 

 longitudinal sections. In the region of the wood the cells o 

 the middle rows, which bear simple pits of exceptionally largi 

 size, are more particularly concerned with storage, and contaii 

 copious starch (Fig. 197, C, s.) ; on the other hand, the dead anc 

 empty cells of the marginal rows (t.), which bear small borderec 

 pits and often exhibit peg-like ingrowths of the walls, have i 

 conducting function. Where the rays traverse the phloem, al 

 the cells have thin walls and dense cytoplasm, but those at th< 

 margin (Fig. 197, E, a.) are often drawn out into finger-like 

 processes which are insinuated between the sieve-tubes. 



1 Annual rings are, however, absent from some Araucarias, and frorr 

 most of the fossil representatives of this group. 



