35 
a cell or tin' thickness of its wall can be obtained. For this amplifica- 
tion the field of the objective covers an area of the specimen equal to 
one-tenth of an inch in diameter, which is enlarged to It) inches, the 
photomicrograph being taken from the portion thus viewed with the 
same amplification retained. To show the entire structure of the wood 
a photomicrograph of three different sections is required, ie. } transverse, 
tangential, and radial. In the transverse section the cells are cut across, 
showing some in their Largest diameter, while others are cut at varying 
portions of their length, and. therefore, show different diameters. All 
of these transverse sections are so arrauged that the top of the page 
points to the center of the tree, and the bottom of the page to the out- 
side of the bark. The lines running down the page indicate the me- 
dullary rays, which extend outward through eacli layer of wood as it i-s 
formed. The views of t he other sections will be understood from the 
special description under each species. 
Diagram illustrating sections: a, trails verso section ; &, radial section ; c, tangential section. 
White oak, Quercus alba, L. No. 251.* 
The photomicrograph of the transverse section of the White Oak is 
from an animal ring of medium thickness, and a striking feature in its 
appearance is the concentric circle of ducts in the early spring growth 
of wood; in the thick rings sometimes there are two or three rows of 
ducts, the third one being of smaller size than the others. The first 
row forms in the spring as the leaves are opening. The largest duct in 
the illustration is round and .013 of an inch in diameter; some are 
smaller, and others flattened or elliptical. Except when first forming 
these ducts are never open, as usually stated, but are filled with deli- 
cate tissue.! This tissue is so delicate and its cells so large that in 
cutting the sections it is more or less removed by the section-knife. 
It is, however, quite durable, for in decaying ties this tissue in the 
ducts is still intact after many of the surrounding wood-cells are de- 
composed. During growth, fluids pass through this tissue by osmotic 
* The botanical names are those used by Prof. Sargent in bis report on the "Forest 
Trees of North America," Vol. IX., of the Tenth Census; the numbers refer to those 
used in that work. 
t Tyloses, a peculiar cell-structure found in the ducts of many of our timber trees. 
