The simple pits are in sets and occur only at the points where the fiber touches the cells of a
medullary ray. (See fig. 21, also PI. J XIII, E, sp., and other figures of this plate and PI. XXIV.)
Above and below these simple pits occur very small bordered pits, communicating with those of
the short tranverse fibers or tracheids which form part of all medullary rays. (See PI. XXI,
D, h. p.)

As in all pines, the medullary or pith rays are of two kinds, the one small, 1 cell wide, and 1
to 10 — in large averages 5 to 7 — cells high; the other large, and each containing in the middle part
a transverse resin duct. (See Pis. XXI, XXIII, XXIV, and XXVI.) Of the former there occur
about 21 to 27 on each square millimeter (about 15,000 per square inch) of tangential section. Tlie
second class are much less abundant and scattered very irregularly, so that sometimes areas of
several square millimeters are found without any of these rays. Generally about one of these
rays occurs to every 1.5 or 2 square millimelers, or about 300 to 400 per square inch of tangential
section. In all rays the cell rows forming the upi)er and lower edge (see PI. XXI) are composed
of short fibers or tracheids (transverse tracheids), while the inner rows contain only parenchyma
cells. Occasionally small rays occur which are composed of tracheids only. (See PI. XXI, G.)
Frequently the rows of parenchyma are separated by one, rarely by two, series of tracheids (see
PI. XXIII, D, and PI. XXIV, D), giving rise to *< double" or "triple" rays.
