PHYSIOLOGY AND STRUCTURE OF TREES AND SHRUBS. 119 
very distinctly seen. The one cannot be mistaken for the other 
under any circumstances. Then if we examine a longitudinal 
radial, or a longitudinal tangential section of any of the trees, 
such as Plates 45 to 55,a much greater difference is observed 
than by transverse sections of the same trees. These ten plates 
show, in a marked degree, the very great difference in the 
medullary plates. These medullary plates or rays are a most 
important feature in the individuality of each species. The rays 
in the Oak, Plate 48, range from a few tubes to many hundreds 
of thousands in one ray-plate. In the Pine tribe, however, the 
medullary plates are composed of a single line of cylinders, 
varying from two to thirty cylinders in a plate. An exception 
to that rule is the Wellingtonia gigantea, the medullary rays or 
cylinders of which are very inconstant, ranging from three to 
one hundred cylinders in a plate, and from a single line to 
three lines, or irregular, as in hardwood, as shown on Plate 53. 
Among the Pine tribe, however, in both the genera Abzes and 
Pinus, the medullary tubes consist of single lines only; but at 
regular intervals there is a very peculiar break in this order of 
things, as is shown by a tangential section of Pinus Strobus on 
Plate 54. The same is seen on Plate 55, which is a tangential 
section of Norway Spruce. This plate also shows pitted cells in the 
longitudinal tangential section. Another distinguishing feature in 
the wood of the pine is the resinous canals. None of the Cedar 
tribe, with which we may group both the Cypress and Arborvite, 
have resinous canals, nor have the Araucarias. It is stated that 
the Wellingtonia gigantea has no resinous canals, but that is 
not strictly accurate. Although present, they are very incon- 
stant, and are produced in groups of many hundreds in a single 
row in the early spring growth. Plate 31, which is a transverse 
section of the stem of the Wellingtonia gigantea, shows an 
example of this peculiarity. 
Tt will be thus seen that the various classes of timber trees 
and shrubs are easily recognised, the one from the other, by 
the general structure of their wood—first, by the appearance of 
a transverse section; second, by the arrangement of the 
vascular tissue ; and third, by the medullary plates or rays, To 
demonstrate this, let us carefully examine Plates 58 to 64, and 
it will be seen that their identification is a comyaratively simple 
matter. No doubt, timber can also be identified by the external 
appearances, viz, the general contour, leaves, bark, blossom, and 
