54 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



I. CONIFERS. 



The wood (except immediately around the pith) contains no 

 true vessels, though resin-canals occur in many cases in the 

 autumn wood. Annual rings nearly always sharply marked, 

 from the denser autumn zone. Medullary rays very fine and 

 numerous. 



A. There are no resin-canals in the wood. 



(1) No true " heart" is to be distinguished : 



e.g. Silver Fir ; Abies Webbiana. 



(2) There is a distinct central " heart- wood " : 



e.g. Yew, Juniper, Deodar ; Wellingtonia. 



B. Resin-canals are present, at least in the autumn-wood. 



(3) No true " heart " is to be distinguished : 



e.g. Spruce ; Abies Smithiana* 



(4) There is a distinct central " heart-wood " : 



e.g. The Pines and the Larch. 



II. DICOTYLEDONS. 



Always have true vessels (except Drimys and one or two rare 

 forms), which differ considerably in size, number, and dis- 

 tribution. The wood is usually complex in structure, the 

 elements (cells, fibres, tracheides, &c.), being variously dis- 

 posed. Annual rings may be obvious, or indistinct, or even 

 absent, and marked in various ways. Medullary rays always 

 present, but differ much in number, size, &c. 



A. DICOTYLEDONS with no distinguishable annual rings ; but 

 there may be also partial zones of tissue (usually wood- 

 parenchyma) easily distinguished as incomplete bands which 



