THE GYMNOSPERMAE : CONIFERALES AND TAXALES 669 



bundle, and small canals form a connected system throughout the wood and 

 the phloem. They also run horizontally along many of the medullary rays. 

 These ray canals have been traced continuously through more than a hundred 

 annual rings, but however long they are, they always begin where a ray touches 

 a vertical canal. This system of resin canals or passages is occasionally met 

 with in the higher plants, for example, in the Ivy, but it is more characteristic 

 of the Conifers. Each canal is bounded by a layer of glandular secreting cells 

 called an epithelium (Fig. 668). The canals contain Turpentine, which 



Sclerotic sheath 



Epithelium 



Fig. 668. — Piiiiis sykestris. Transverse section of 

 schizogenous resin canal in the cortex, with lining 

 epithelium and sclerotic sheath. 



flows out when any injury breaks the tissues open. The Pines have very 

 little power of forming callus tissue over wounds, so that the antiseptic 

 action of the Turpentine is valuable as a protection against the invasion 

 of the wounded tissues by Fungi or Bacteria. When exposed to the air 

 the Turpentine is oxidized to solid Resin which remains as a covering to the 

 wound until fresh bark can be formed. Distillation of Pine wood, leaves, 

 etc., is the chief source of natural Turpentine. The resin canals of Pi/ius 

 are sometimes blocked by the ingrowth of epithelial cells, forming vesicles 

 called tyloses. This apparently occurs as a result of injury. In Cedrus there 

 are normally no resin canals, but they occasionally appear where the wood 

 has been damaged. Jeffrey has claimed that the production of anomalous 

 structures as a result of injury throws light on the evolutionary history of the 

 species and has raised it to a principle under the name of " traumatic 

 reversion," an idea of doubtful significance. The example quoted, if 

 interpreted in this manner, would suggest that Cedrus w^as derived from 

 ancestors which had formerly possessed resin canals. 



