If one compares a heavy piece of pine with a light specimen it will be seen at once that the heavier one contains a larger proportion of late wood than the other, and is therefore considerably darker.  The late wood of all species is denser than that formed early in the season, hence the greater the proportion of late wood the greater the density and strength.  When examined under a microscope the cells of the late wood are seen to be very thick-walled and with very small cavities, while those formed first in the season have thin walls and large cavities.  The strength is in the walls, not the cavities.  In choosing a piece of pine where strength or stiffness is the important consideration, the principal thing to observe is the comparative amounts of early and late wood.  The width of ring, that is, the number per inch, is not nearly so important as the proportion of the late wood in the ring.

It is not only the proportion of late wood, but also its quality, that counts.  In specimens that show a very large proportion of late wood it may be noticeably more porous and weigh considerably less than the late wood in pieces that contain but little.  One can judge comparative density, and therefore to some extent weight and strength, by visual inspection.

The conclusions of the U.S.  Forest Service regarding the effect of rate of growth on the properties of Douglas fir are summarized as follows: 

“1.  In general, rapidly grown wood (less than eight rings per inch) is relatively weak.  A study of the individual tests upon which the average points are based shows, however, that when it is not associated with light weight and a small proportion of summer wood, rapid growth is not indicative of weak wood.

“2.  An average rate of growth, indicated by from 12 to 16 rings per inch, seems to produce the best material.

“3.  In rates of growths lower than 16 rings per inch, the average strength of the material decreases, apparently approaching a uniform condition above 24 rings per inch.  In such slow rates of growth the texture of the wood is very uniform, and naturally there is little variation in weight or strength.

“An analysis of tests on large beams was made to ascertain if average rate of growth has any relation to the mechanical properties of the beams.  The analysis indicated conclusively that there was no such relation.  Average rate of growth [without consideration also of density], therefore, has little significance in grading structural timber."[18] This is because of the wide variation in the percentage of late wood in different parts of the cross section.

[Footnote 18:  Bul. 88:  Properties and uses of Douglas fir, p. 29.]

Experiments seem to indicate that for most species there is a rate of growth which, in general, is associated with the greatest strength, especially in small specimens.  For eight conifers it is as follows:[19]

[Footnote 19:  Bul. 108, U. S. Forest Service:  Tests of structural timbers, p. 37.]