45 
Long leaved <>k Xellow Pine, Pinus palustris, Mill. No. 3S0. 
The striking appearance of the structure of (Ins wood, as seen in the 
transverse section, consists in the si rongly marked zones of differenl cells 
in each layer of growth. At the present time satisfactory distinctive 
names have not been given to these two Glasses of wood-cells or tracheitis. 
I shall use ( he term thin - and t hick -walled tracheids to distinguish them. 
The former grow in the first part of the season, and the latter during the 
summer; but the line of demarkation between tliem is sharp, not being 
a gradual mergiug as in most of the other Conifers. In some trees, each 
layer is made up of about equal portions of the thin- and thick-walled tra- 
cheids, though these layers may vary in thickness from one-thirty-second 
to one-fourth of an inch ; in other trees, the layer of thin-walled tracheids 
is t lie thickest, and, as a rule, such wood is of less specific gravity than 
that with thicker layers. The size of the wood-cells in different individ- 
uals is not the same, and this feature also affects the specific gravity. 
In selecting a piece from which to make an illustration to show an 
entire annual ring, we are obliged to take one of less than medium 
thickness in order to get it on the page. In a luxuriant growth a 
larger one might be selected which would show only one class of tra- 
cheids. Measurements from these photomicrographs will give average 
results for this wood. The largest of these walls measures .003 by .0025 
of an inch, with a lumen of .0025 by .002 of an inch; the thick ones are 
.002 by .001 of an inch, with a lumen of .0000 by .0001. Many rows of 
cells of the same size are seen running parallel to the medullary rays, 
usually single, sometimes double, but rarely in threes. 
The thick-walled tracheids are nearly quadrangular, and some of 
them tabular, and the same number of concentric rows occupy only 
about one-half as much in width of the ring as would the same number 
of rows of thin- walled tracheids; the former give the hardness and 
great elasticity to the wood. It has been generally stated that the 
thick walled tracheids contain the most of the resinous matter; this 
applies only to the volume, as by weight I found that practically they 
contain equal amounts. The large openings which inclose the tissue 
of the resin canals are prominent features in the transverse section of this 
wood. Smaller canals occur in the medullary rays, and if vhey were 
wider they could be readily traced in the illustration; one appears in 
this photomicrograph. The two systems of resin-canals are connected. 
The heavier lines of cells running up and down the page are the me- 
dullary rays with their canals ; and, though numerous, the cells are not 
strong, the annual rings of wood being quite easily separated from each 
other when under severe service, or in the sun. Thick layers can be 
readily separated with a knife, from sections one-half to three fourths 
of an inch in width. The separation of the thick- from the thin-walled 
tracheids in the same layer is far more difficult, the latter breaking off 
in short sections. 
The lenticular markings on the tracheids are more abundant near the 
ends of the cells than in the center, and the appearance of swellings on 
