334 



BOTANY 



up of tracheids of very characteristic form, arranged in radiating series. These 

 tracheids have their radial walls marked with characteristic bordered pits 

 (Fig. 297, A), which are usually nearly round in outline, but may be somewhat 

 elongated, like those of the Cycads and Ferns. These pits are developed upon 

 opposite sides of the wall in adjacent tracheids, the bottoms being separated 

 by the thin membrane forming the original division-wall between the young 

 tracheids (Fig. 297, B). At intervals the tracheids are replaced by radially ex- 

 tended rows of cells, the medullary rays (rw). These consist of radiating plates 

 of cells, which are usually parenchymatous, but may be composed in part of 

 horizontal tracheids. 



FIG. 297. A, B, Pinus insignis. A, radial longitudinal section of the wood, show- 

 ing bordered pits upon the walls of the tracheids ; ra, medullary ray. B, tan- 

 gential section, cutting across the pit, p, and a medullary ray, ra (x 250). C, 

 sieve-tubes of P. sylvestris (x 500). (C, after STRASBURGER.) 



Outside the mass of the wood is the Cambium (cam.), a zone of meristematic 

 cells, which divide by periclinal walls, the cells upon the inner side becoming 

 transformed into wood-elements, those upon the outside adding to the phloem. 

 The most important elements of the latter are the sieve-tubes, which have 

 numerous lateral sieve-plates (Fig. 297, C). Elongated parenchyma cells and 

 fibrous cells (bast-fibres) also occur in the phloem, and the medullary rays are 

 continued into it. 



Bark. The outer or cortical part of the young stem is comp^ed largely of 

 green parenchyma. Later a zone of meristematic tissue, the Prr d j rm, is devel- 

 oped below the epidermis, and it is to the activity of this layer that the develop- 

 ment of the bark is due. Part of this is the Phellogen or " Cork-cambium." 



In both cortex and wood there occur numerous large resin-ducts which are 

 structurally much like the gum and mucilage ducts of the Cycads. The 

 secondary wood of Conifers, unlike that of dicotyledonous trees, is composed 

 exclusively of tracheids. 



