THE PRINCIPAL SPECIES OF WOOD. 



the generic name including all species of oak. Alba, rubra, 

 and others are specific names denoting the said species. 

 'Quercus alba and Quercus rubra are completed terms. Genera 

 are not fixed but differ with authorities, so that the abbreviated 

 iicvn'e of the botanist responsible for the classification adopted 

 is often added, as Quercus alba Linn, and Ulmus fulva 

 Michx. 



A species is a collection of individuals that might well have 

 sprung from some single root. A genus is a collection of re- 

 lated species. Genera are gathered into families. Families 

 and genera differ with authorities. A variety includes indi- 

 viduals differing slightly from accepted species. Its name 

 when existing is part of the specific name. " Quercus robur 

 var. pedunculata " specifies a variety (pedunculata) of " red ' 

 or strong (robur) oak (Quercus). A variety of one botanist is 

 sometimes a distinct species of another. 



The size and character of the trunk, and the range, 

 locality, or distribution of the tree, have much to do with the 



utility of the wood, since large 

 or perfect timbers cannot be 

 derived from species charac- 

 terized by small or crooked 

 "trees, and since wood is always 

 more used if it is widely dis- 

 tributed so as to be easily 

 available.* 



Wood is made up of cell- 

 structures; as, the true fibre, 

 which originates from several 

 cells; the tracheid (tra-ke-id), 

 which originates from one; the 

 vessel, which is a short, wide 

 tube joined vertically end to 



end 'with others of its kind; the pith-ray; the resin-duct, and 

 others, all of which are often popularly referred to as fibres. 



FIG. i. SOME WOOD ELEMENTS. 



* Fossils show that many species covered wider ranges than at present. 



