25© GYMNOSPERMS 



Besides the sieve tubes, the phloem may contain companion cells, 

 parenchyma, thick-walled tracheids, and stone cells; but tracheids 

 and stone cells are rare in Coniferalcs, and companion cells do not 

 occur in gymnospcrms unless, perhaps, in some of the Gnctalcs. 



The cortex. — The term "cortex" is almost as indefinite as bark. 

 The primary cortex is limited externally by the epidermis, but does 

 not include it; centripetally, its innermost layer of cells is the endo- 

 dermis. In this sense the cortex is a morphological tissue, as definite 

 as xylem or phloem; but there is a secondary development which 

 soon obscures the primary cortex. As the stem grows, the cortex 

 does not keep pace with the stelar structures, and so becomes 

 stretched and cracked, and the outer portions peel ofT or scale ofT. 

 The embryonic layer forming the secondary "cortex" becomes 

 deeper and deeper, until it is finally formed by the phloem. WTicn 

 this stage has been reached, there is no structure which can be called 

 homologous with either the primary or secondary cortex. It is, from 

 the beginning to old age, a protective structure, and may be called 

 hark. 



The structure of the bark in adult trees is so characteristic that 

 many can be identified by this feature (fig. 257). This group includes 

 Liboccdrns and two species of Pinus; but if Abies magnifica and A. 

 concolor, which grow in the vicinity, could have been included in the 

 photograph, all five species could be recognized from their bark. 



In early stages the cells of the cortex contain chlorophyll, and do 

 considerable photosynthetic work. In' many genera there are resin 

 canals, and tannin is abundant. There are other substances in the 

 bark, some of them giving a characteristic odor, so that forest 

 rangers claim that some species can be recognized by the odor. It is 

 certain that the bark of Sequoia semperoircns has such strong anti- 

 septic qualities that in poultry houses, bedded with the shredded 

 bark of this species, the fowls escape some of their worst diseases. 



Resin canals. — Cells, canals, or cavities containing turpentine or 

 resin are found in all the Coniferales with the single exception of 

 Taxus. In some, they are abundant throughout the plant; in others, 

 they are absent from the wood, except that of the first year; and, in 

 others, they are entirely absent from any wood, but occur in the 

 cortex. 



