LEAF-STRUCTURE 



34^ 



The root, which is gcncrall}' cliarch, shows quite the 

 structure, although characterised by the presence of a 

 resin-canal within each protoxylem-group. 



The leaves of most Conifers are tra^x'rsed by a single vein 



usual 

 large 



Fig. 197. — Structure of the stem of the Scotch Fir (Pmus sylvesiris.) 

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

 spring- (Sp.) and autumn- {-!«) 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 {Pimis 

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



