Part 5 (1/2)

The most common type of structure among the broad-leaved trees contains trache, trachids, woody fiber, fibrous cells and parenchyma. Examples are poplars, birch, walnut, linden and locust. In some, as ash, the tracheids are wanting; apple and maple have no woody fiber, and oak and plum no fibrous cells.

This recital is enough to show that the wood of the broad-leaved trees is much more complex in structure than that of the conifers. It is by means of the number and distribution of these elements that particular woods are identified microscopically. See p. 289.

[Ill.u.s.tration: Fig. 20.]

_Ring-porous woods._ Looking thru the microscope at a cross-section of ash, a ring-porous wood, Fig. 20:

(1) The large round or oval pores or vessels grouped mostly in the spring wood first attract attention. Smaller ones, but still quite distinct, are to be seen scattered all thru the wood. It is by the number and distribution of these pores that the different oak woods are distinguished, those in white oak being smaller and more numerous, while in red oak they are fewer and larger. It is evident that the greater their share in the volume, the lighter in weight and the weaker will be the wood. In a magnified cross-section of some woods, as black locust, white elm and chestnut, see Chap. III, beautiful patterns are to be seen composed of these pores. It is because of the size of these pores and their great number that chestnut is so weak.

(2) The summer wood is also distinguishable by the fact that, as with the conifers, its cells are smaller and its cell walls thicker than those of the spring wood. The summer wood appears only as a narrow, dark line along the largest pores in each ring.

(3) The lines of the pith rays are very plain in some woods, as in oak. No. 47, Chap. III.

(4) The irregular arrangement and

(5) Complex structure are evident, and these are due to the fact that the wood substance consists of a number of different elements and not one (tracheids) as in the conifers.

Looking at the radial section, Fig. 20:

(6) If the piece is oak, the great size of the medullary rays is most noticeable. Fig. 32, p. 38. They are often an inch or more wide; that is, high, as they grow in the tree. In ash they are plain, seen thru the microscope, but are not prominent.

(7) The interweaving of the different fibers and the variety of their forms show the structure as being very complex.

In the tangential section, Fig. 20:

(8) The pattern of the grain is seen to be marked not so much by the denseness of the summer wood as by the presence of the vessels (pores).

(9) The ends of the pith rays are also clear.

In _diffuse porous woods_, the main features to be noticed are: In the transverse section, Fig. 21:

(1) The irregularity with which the pores are scattered,

(2) The fine line of dense cells which mark the end of the year's growth,

(3) The radiating pith rays,

(4) The irregular arrangement and,

(5) The complex structure.

In the radial section, Fig. 21:

(6) The pith rays are evident. In sycamore, No. 53, Chap. III, they are quite large.

(7) The interweaving of the fibers is to be noted and also their variety.

In the tangential section, Fig. 21:

(8) The grain is to be traced only dimly, but the fibers are seen to run in waves around the pith rays.

(9) The pith rays, the ends of which are plainly visible.