340 STEM-STRUCTURE 



cambial zone, also exhibits a very uniform, radially seriated 

 structure. Except for the narrow medullary rays and occasional 

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

 differentiated among themselves only in respect of the distinctions 

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

 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 ; in 

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

 groups of primary xylem, composed of spiral tracheids, project 

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

 primary rays. 



The structure just noted for the Scotch Fir is that typical 

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

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

 the Araucarias, and occasionally in other members of the group 

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

 bordered pits. 



Radial and tangential longitudinal sections exhibit the same 

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

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

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

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

 often bear simple pits in Pinus and its allies, although elsewhere 

 usually smooth. Several Abietineae, including the Scotch Fir, 

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

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

 the middle rows, which bear simple pits of exceptionally large 

 size, are more particularly concerned with storage, and contain 

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

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

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

 conducting function. Where the rays traverse the phloem, all 

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

 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 from 

 most of the fossil representatives of this group. 



