700 PROPERTIES, HARVESTING AND DISPOSAL OF RESIN. 



its formation ; in the bark, the growth in the diameter of the 

 tree increases by tension the diameter of the resin-cells till 

 rhitidome is formed. 



The course of the horizontal and vertical resin-ducts in the 

 wood is described on p. 37 ; Mayr has proved that each 

 horizontal duct arises from a vertical duct, so that there is 

 always a connection between these two systems of ducts. 

 (Fig. 869). There is also a connection between horizonal and 



-G, 



Fig. 369. Border of two 

 annual zones in spruce. 

 ft Commencement of 

 last year's resin-duct, 

 ft Interruption of the 

 pith, c End of present 

 year's resin-duct ; the 

 horizontal ducts (e and 

 f) spring from the ver- 

 tical duct <l. 



Fig. 370.- The intercellular spaces of the vertical 

 duct ft and of the horizontal duct ft, showing 

 the connection between them. 



vertical ducts, wherever these ducts meet accidentally, as 

 between (/) and (d). (Fig. 369), when both canals show 

 large intercellular spaces (dotted lines in Fig. 369). 



In pines, after the year of their formation, the cells of the 

 ducts remain thin-walled ; in spruces, larches and Douglas-firs, 

 more and more of these thin- walled cells are converted 

 gradually into thick-walled, normal parenchymatous cells. 

 Shortly before the transition of sapwood into heartwood, all 

 the thin- walled cells that contain plasma form tyl loses, 

 which block the ducts (Fig. 370). 



