KEY TO THE MORE IMPORTANT WOODS OF NORTH AMERICA. 



I. Non-porous woods — Pores not visible or conspicuous on cross-section, 

 even with magnifier. Annual rings distinct by denser (dark colored) bands 

 of summer Avood (Fig. 128). 



II. RiNG-POROUS WOODS — Pores numerous, usually visible on cross-section 

 without magnifier. Anniuil rings distinct by a zone of large pores collected 

 in the spring wood, alternating with the denser summer wood (Fig. 129). 



III. DiFFUSE-POROUS WOODS — Pores numerous, usually not plainly visible 

 on cross-section without magnifier. Annual rings distinct by a fine line of 

 denser summer wood cells, often quite indistinct; pores scattered thru an- 

 nual ring, no zone of collected pores in spring wood (Fig. 130). 



Note. — The above described three groups are exogenous, i. e., they grow by 

 adding annually wood on their circumference. A fourth group is formed by the 

 endogenous woods, like yuccas and palms, uhich do not grow by such additions. 



I. — Nox-PoROL's Woods. 



(Includes all coniferous woods.) 

 A. Resin ducts wanting.' 



1. No distinct heart-wood. 



a. Color effect yellowish white: summer wood darker yellowish (under 

 microscope pith ray without tracheids) Firs. 



additional notes for DISTINCTIONS IN THE GROUP. 



Spruce is hardly distinguishable from fir, except by the existence of the 

 resin ducts, and microscopicallj' by the presence of tracheids in the medullary 

 rays. Spruce may also be confounded with soft pine, except for the heart- 

 wood color of the latter and the larger, more frequent, and more readily visi- 

 ble resin ducts. 



In the lumber j'ard, hemlock is usually recognized by color and the silvery 

 character of its surface. Western hemlocks partake of this last character to 

 a less degree. 



Microscopically the white pine can be distinguished by having usually 

 only one large pit, while spruce shows three to five very small pits in the 

 parenchyma cells of the pith ray communicating with the tracheid. 



The distinction of the pines is possible only by microscopic examination. 

 The following distinctive features may assist in recognizing, when in the 

 log or lumber pile, those usually found in the market: 



'To discover the resin ducts a very smooth surface is necessary, since resin ducts are 

 frequently seen only with difficulty, appearing- on the cross-section as fine whiter or darker 

 spots normally scattered singly, rarely in jfroups, usually in the summer wood of the an- 

 nual ring-. They are often much more easily seen on radial, and still more so on tang-ential 

 sections, appearinijr there as fine lines or dots of open structure of different color or as in- 

 dentations or i)in scratches in a loniritudiual direction. 



294 



