The following convenient and useful classification of pines into four groups, proposed by Dr. H. Mayr, is based on the appearance of the pith ray as seen in a radial section of the spring wood of any ring:
Section I. Walls of the tracheids of the pith ray with dentate projections.
a. One to two large, simple pits to each tracheid on the radial walls of the cells of the pith ray.—Group 1. Represented in this country only by P. resinosa.
b. Three to six simple pits to each tracheid, on the walls of the cells of the pith ray.—Group 2. P. taeda, palustris, etc., including most of our "hard" and "yellow" pines.
Section II. Walls of tracheids of pith ray smooth, without dentate projections.
a. One or two large pits to each tracheid on the radial walls of each cell of the pith ray.—Group 3. P. strobus, lambertiana, and other true white pines.
b. Three to six small pits on the radial walls of each cell of the pith ray. Group 4. P. parryana, and other nut pines, including also P. balfouriana.