LEAF-STRUCTURE 



341 



The root, which is generally diarch, shows quite the usual 

 structure, although characterised by the presence of a large 

 resin-canal within each protoxylem- group. 



The leaves of most Conifers are traversed by a single vein 



FIG. 197. Structure of the stem of the Scotch Fir (Pinus sylvestris.) 

 A, small part of the secondary wood in transverse section, showing 

 spring- (Sp.) and autumn- (Au) wood ; B, the same in tangential 

 longitudinal section showing a medullary ray ; C, the same in radial 

 longitudinal section, with part of a medullary ray ; D, single bor- 

 dered pit in section (on the left) and from the surface (on the right) ; 



E, small part of the secondary phloem in radial longitudinal section ; 



F, single sieve-tube in tangential longitudinal section, a, albuminoid 

 cells of medullary ray of phloem ; b, bordered pits ; s, storage cells 

 of medullary rays of wood ; Si., sieve-plates ; t, tracheidal cells of 

 medullary rays of wood. 



only, but, apart from that, their structure is somewhat varied. 

 Its range can, however, be gauged from a consideration of the 

 relatively simple dorsiventral leaf of the Yew (Taxus, Fig. 198) 

 and the more complex centric one of the Scotch Fir (Pinus 

 sylvestris, Fig. 199). In the former case the epidermis (Fig. 



