Trees are divided into two general divisions known as Exogens and Endogens.[8] [p007]
FOOTNOTES
[1] Fernow, Introduction to U. S. Forestry Bul. No. 17.
[2] Dr. Fernow credits 495 trees to United States (Introduction to U. S. Forestry Bul. No. 17); Prof. Sargent, counting species only and excluding varieties, gives 422 (Silva of North America).
[3] "The principal timbers of commerce in the United States are the species known popularly as pine, fir, oak, hickory, hemlock, ash, poplar, maple, cypress, spruce, cedar, and walnut." ("The Lumber Trade of the United States," Treas. Dept., Bureau of Statistics.)
[4] Fossils show that many species covered wider ranges than at present.
[5] Roth, U. S. Forestry Bul. No. 10, pp. 64-71. Also von Schrenk, U. S. Dept. Agriculture, Bureau Plant Industry Bul. No. 14, pp. 12-16.
[6] Only because imperfections are more likely in larger pieces. Large and small pieces of equally perfect wood are equally strong. (See Publications U. S. Forestry Div. and J. B. Johnson's "Materials of Construction," p. 462.)
[7] It should be noted that the selection and preparation of specimens require the exercise of more judgment than the simple testing of specimens, if the conclusions are to be such that they can be generalized from.
[8] This division coincides with that by which they are separated into Dicotyledons and Monocotyledons.