340 STEM-STRUCTURE 



cambial zone, also exhibits a very uniform, radially seriated 

 structure. Except for the narrow medullary raj'S and occasional 

 resin-canals, it consists entirel}? of fibre-like tracheids. which are 

 differentiated among themselves only in respect of the distinctions 

 between spring- (Fig. 197, A, Sp.) and autumn-wood (An.) ' (see 

 p. 125). The tracheids bear a sin,gle 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, h.), 

 sections, when the pits themseh-es are seen in surface view ; in 

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

 groups of primarv xvlem, composed of spiral tracheids, project 

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

 primar\-' rays. 



The structure just noted for the Scotch Fir is that tj'pical 

 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. Pi mis pahistris), 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 Pinits and its allies, although elsewhere 

 usually smooth. Several AbietinccX', 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 (/.), which liear small bordered 

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

 conducting function. Where the rays tra^•crse the phloem, all 

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

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

 processes which are insinuated between the sieve-tubes. 



' Annual ruigs arc, however, absent fri)ni ,sunie .lyaiicdiia^, and from 

 most of the fossil representatives of thi.s ,L,'roiip. 



