WOUNDS 



233 



produce it (resiniferous cells), are disposed in the wood both 

 vertically and horizontally that is, radially. I was the first to 

 show that the latter, 

 which are known as 

 medullary-ray canals, 

 communicate freely at 

 certain points with the 

 vertical canals. This 

 is owing to the fact 

 that at those places 

 where the two sets of 

 canals come into con- 

 tact the parenchyma- 

 tous epithelial cells, 

 instead of remaining 

 coherent to each other, 

 become widely sepa- 

 rated (Fig. 135, e). 



By means of these 

 intercellular spaces 

 the resin of the ver- 

 tical canals can with 

 ease gain access to 

 the radial canals, and 

 should the latter be 

 opened by a wound 

 on the outside of the 

 tree the resin is en- 

 abled to flow freely 

 out to the surface. 

 This explains the 

 abundant outpouring 

 which takes place 

 when conifers arc par- 

 tially barked in order 



FIG. 135. Manner of communication between a ver- 

 tical resin-duct, a, and a duct in a medullary ray, 

 b, in the Norway spruce. The epithelial cells of 

 both canals are for the most part empty and 

 furnished with very thick walls ; the walJs be- 

 tween adjoining epithelial cells being abundantly 

 pitted, cc. Only a small proportion of these cells 

 retain thin walls, protoplasm, and a nucleus, and 

 seive for the preparation of turpentine, dd. At 

 the point where the back of the vertical canal 

 facing the reader, #, comes into contact with the 

 horizontal canal behind, l>, the epithelial cells of 

 both canals are provided with very delicate walls, 

 and are separated by large intercellular spaces, 

 e e, the latter providing the means for the passage 

 of the turpentine from the one canal to the other. 



to procure the resin. 



Under the oxidising influence of the air the resin that oozes from 

 the wounded surface soon forms a hard incrustation ; and of 

 course the partial volatilization of the turpentine also contri- 



