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.