Sometimes our progress is not so easy; we may waver in what group or section to place the wood before us. In such cases we may try each of the doubtful roads until we reach a point where we find ourselves entirely wrong and then return and take up another line; or we may anticipate some of the later mentioned features and finding them apply to our specimen, gain additional assurance of the direction we ought to travel. Color will often help us to arrive at a speedy decision. In many cases, especially with conifers, which are rather difficult to distinguish, a knowledge of the locality from which the specimen comes is at once decisive. Thus, northern white cedar, and bald cypress, and the cedar of the Pacific will be identified, even without the somewhat indefinite criteria given in the key. 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 wood (Fig. 128). [Illustration: Fig. 128. "Non-porous" Woods. _A_, fir; _B_, "hard" pine; _C_, soft pine; _ar_, annual ring; _o.e._, outer edge of ring; _i.e._, inner edge of ring; _s.w._, summer wood; _sp.w._, spring wood; _rd._, resin ducts.] II. RING-POROUS WOODS--Pores numerous, usually visible on cross-section without magnifier. Annual rings distinct by a zone of large pores collected in the spring wood, alternating with the denser summer wood (Fig. 129). [Illustration: Fig. 129. "Ring-porous" Woods White Oak and Hickory. _a. r._, annual ring; _su. w._, summer wood; _sp. w._, spring wood; _v_, vessels or pores; _c. l._, "concentric" lines; _rt_, darker tracts of hard fibers forming the firm part of oak wood; _pr_, pith rays.] 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 annual ring, no zone of collected pores in spring wood (Fig. 130). [Illustration: Fig. 130. "Diffuse-porous" Woods. _ar_, annual ring; _pr_, pith rays which are "broad" at _a_, "fine" at _b_, "indistinct" at _d_.] 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, which do not grow by such additions. I.--NON-POROUS WOODS. (Includes all coniferous woods.) A. Resin ducts wanting.[1] 1. No distinct heart-wood. _a._ Color effect yellowish white; summer wood darker yellowish (under microscope pith ray without tracheids)..........FIRS. _b._ Color effect reddish (roseate) (under microscope pith ray with tracheids) ................................HEMLOCK. 2. Heart-wood present, color decidedly different in kind from sap-wood.