42 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



obvious differences are to be noticed, and the tree is 

 said to have no " heart," but to consist entirely of 

 "sap-wood. I will not stop to discuss the physio- 

 logical significance of these cases, but simply quote, 

 as examples of woods that can be distinguished almost 

 by their " heart-wood " alone, Ebony, where it is black, 

 Guaiacum (green), Ccesalpinia Sappan (red), Logwood 

 (purple), and numerous instances suggest themselves 

 where the characters of the "heart-wood " are useful. 



Yet another accessory feature is the occurrence of 

 certain peculiar discoloured spots or patches in certain 

 woods, which are always suggestive and sometimes 

 distinctive : these are known as " medullary spots " or 

 " pith flecks," and usually look like small patches of 

 rust in the substance of the wood. They are not at 

 all uncommon, and may be seen in the birches, haw- 

 thorn, species of Pyrus, Salix, &c. : their nature needs 

 further investigation, but we are only concerned here 

 with the fact of their occurrence. 



In a certain sense we must regard the resin-canals 

 of some pines, firs, &c., and some Anacardiacea^ as 

 useful accessory characters : many Conifers especially 

 being distinguished by their presence. These resin- 

 canals have nothing to do with the true vessels of the 

 wood of Dicotyledons, of which more will be said 

 presently 



