116 College of Forestry 
directly with one another, the adjoining ends being more or 
less completely absorbed so that the rows of cells or vessel 
segments finally form long, continuous tubes. The segments 
of the large vessels are barrel-shaped. After the vessels lose 
their sap and the air in them is rar ified, tyloses (very delicate 
partition-like walls) begin to form and block up the cavities, 
rendering the heartwood impervious, or nearly so,.to the 
passage of fluids. Tyloses consist of parenchymatous tissue 
which has been forced out of the turgescent adjacent thin- 
walled pith-ray cells or wood-parenchyma fibers oe the 
lumina or channels of the vessels. 
The individual cells of wood-parenchyma resale the 
pith-ray cells, but are grouped in vertical instead of hori- 
zontal rows. They predominate in the more porous parts 
of the annual rings and often closely surround the vessels 
and tracheids. In either case they remain extremely thin- 
walled. In the early wood they occur isolated or scattered. 
In the dense late wood, the wood parenchyma elements form 
conspicuous regular or branched and interrupted tangential 
bands which can be seen readily with the unaided eye. The 
pits are chiefly simple. Wood parenchyma cells usually con- 
tain starch and also a certain amount of tannin. The amount 
and nature of the contents of the parenchyma depends largely 
upon the age of the tree, the time of felling, and the part of 
the tree from which the wood was taken. Ch rystals of calcrum 
oxalate are of common occurrence in all parenchymatous 
tissue. As in sugar maple wood they are developed singly 
in the cells and belong to the tetragonal crystal system. In 
contiguous pith-ray cells they often form rows of 3—6, while 
in the wood parenchyma fibers they occur in much longer 
rows, particularly in the early wood, and are technically 
known as idioblasts. In such cases the whole cell becomes 
merely a repository for the crystal. Both the wood and ray 
parenchyma cells of the material examined were packed full 
of large starch grains, varying in shape from globoid to 
ellipsoid. 
As stated earlier, two kinds of wood prosenchyma are 
present — compactly arranged, thick-walled fibers and scat- 
