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WOOD AND FOREST
_By_ WILLIAM NOYES, M.A.
Formerly Assistant Professor of Industrial Arts
Teachers College, Columbia University
NEW YORK CITY
[Illustration]
THE MANUAL ARTS PRESS
PEORIA, ILLINOIS
COPYRIGHT
WILLIAM NOYES
1912
_FIFTH EDITION, 1921_
_Printed in United States of America_
FOREWORD
This book has been prepared as a companion volume to the author's
_Handwork in Wood_.[1] It is an attempt to collect and arrange in
available form useful information, now widely scattered, about our
common woods, their sources, growth, properties and uses.
As in the other volume, the credit for the successful completion of
the book is to be given to my wife, Anna Gausmann Noyes, who has made
the drawings and maps, corrected the text, read the proof, and carried
the work thru to its final completion.
Acknowledgments are hereby thankfully made for corrections and
suggestions in the text to the following persons:
Mr. A. D. Hopkins, of the United States Department of Agriculture,
Bureau of Entomology, for revision of the text relating to Insect
Enemies of the Forest, in Chapter VI.
Mr. George G. Hedgcock, of the United States Bureau of Agriculture,
Bureau of Plant Industry, for revision of the text relating to the
fungal enemies of the forest, in Chapter VI.
Mr. S. T. Dana and Mr. Burnett Barrows, of the United States
Department of Agriculture, Forest Service, for revision of Chapters
IV, V, VI, VII, and VIII.
Professor Charles R. Richards, formerly Head of the Manual Training
Department of Teachers College, my predecessor as lecturer of the
course out of which this book has grown.
Professor M. A. Bigelow, Head of the Department of Botany of Teachers
College, for revision of Chapter I, on the Structure of Wood.
Mr. Romeyn B. Hough, of Lowville, N. Y., author of _American Woods_
and _Handbook of the Trees of the Northern States and Canada_, for
suggestions in preparing the maps in Chapter III.
The Forest Service, Washington, D. C., for photographs and maps
credited to it, and for permission to reprint the key to the
identification of woods which appears in Forest Service Bulletin No.
10, _Timber_, by Filibert Roth.
The Division of Publications, U. S. Department of Agriculture, for
permission to copy illustrations in bulletins.
The Macmillan Company, New York, for permission to reproduce Fig.
86, Portion of the Mycelium of Dry Rot, from _Timber and Some of its
Diseases_, by H. M. Ward.
Mrs. Katharine Golden Bitting, of Lafayette, Indiana, for the
photograph of the cross-section of a bud, Figure 5.
Finally and not least I hereby acknowledge my obligations to the
various writers and publishers whose books and articles I have freely
used. As far as possible, appropriate credit is given in the paged
references at the end of each chapter.
[Footnote 1: William Noyes, _Handwork in Wood_, Peoria, Ill.
The Manual Arts Press, 231 pp., $2.]
CONTENTS.
CHAPTER PAGE
General Bibliography 4
I The Structure of Wood 9
II Properties of Wood 41
III The Principal Species of American Woods 57
IV The Distribution and Composition of the
North American Forests 197
V The Forest Organism 211
VI Natural Enemies of the Forest 229
VII The Exhaustion of the Forest 251
VIII The Use of the Forest 271
Appendix 289
Index 304
GENERAL BIBLIOGRAPHY
Apgar, A. G., _Trees of the Northern United States_. N. Y.:
American Book Co., 224 pp. A small book dealing with the botany
of trees, giving descriptions of their essential organs, and
particularly valuable for the leaf key to the trees. It should
be supplemented by Keeler or Hough's Handbook.
Baterden, J. R., _Timber_. N. Y.: D. Van Nostrand Co., 1908,
351 pp. A description of the timbers of various countries,
discussion of timber defects, timber tests, etc.
Bitting, K. G., _The Structure of Wood_. _Wood Craft_, 5: 76,
106, 144, 172, June-Sept., '06. A very scholarly and valuable
series of articles on wood structure and growth. Excellent
microphotographs.
Britton, Nathaniel Lord, _North American Trees_. N. Y.: Henry
Holt & Co., 1908, 894 pp. A description of all the kinds of trees
growing independently of cultivation in North America, north of
Mexico, and the West Indies. The standard Botany of trees.
Boulger, G. S., _Wood_. London: Edward Arnold, 369 pp. A thoro
discussion of wood structure, with chapters on the recognition
and classification of woods, defects, preservation, uses, tests,
supplies, and sources of wood. Good illustrations.
Bruce, E. S., _Frost Checks and Wind Shakes_. _Forestry and
Irrigation_, 8: 159, April, '02. An original study of the
splitting of trees by sudden frost and thaw.
Bruncken, Ernest, _North American Forests and Forestry_. N. Y.:
G. P. Putnam's Sons. 265 pp. A comprehensive survey of American
Forestry conditions including the forest industries, fires,
taxation, and management. No illustrations.
Busbridge, Harold, _The Shrinkage and Warping of Timber_.
_Sci. Amer. Suppl._, No. 1500, Oct. 1, 1904. Good photographic
illustrations.
Comstock, J. H. and A. B., _A Manual for the Study of Insects_.
Ithaca, N. Y.: Comstock Publishing Co., 701 pp.
Valuable for reference in classifying insects injurious to wood.
Curtis, Carleton C., _Nature and Development of Plants_. N. Y.:
Henry Holt & Co., 1907, 471 pp. Chapter III is a very clear and
excellent discussion of the structure of the stem of plants
(including wood).
Encyclopedia Brittannica, Eleventh Edition, Cambridge: At the
University Press. Article: _Forests and Forestry_, Vol. 10, p.
645. Article: _Plants_, Anatomy of, Vol. 21, p. 741. Article:
_Timber_ Vol. 26, p. 978.
Felt, E. P., _The Gypsy and Brown Tail Moths_. N. Y. State Museum:
Bulletin 103, Entomology, 25. Valuable for colored illustrations
as well as for detailed descriptions.
Fernow, B. E., _Economics of Forestry_. N. Y.: T. Y. Crowell
& Co. 1902, quarto 520 pp. A treatment of forests and forestry
from the standpoint of economics, including a comprehensive
exposition of the forester's art, with chapters on forest
conditions, silviculture, forest policies, and methods of
business conduct, with a bibliography.
Fernow, B. E., _Report upon the Forestry Investigation of the U. S.
Department of Agriculture_, 1887-1898. Fifty-fifth Congress,
House of Representatives, Document No. 181. Quarto, 401 pp.
A review of forests and forestry in the U. S., of forest policies
of European nations, particularly of Germany, of the principles
of silviculture, of a discussion of forest influences, and a
section on timber physics.
Harwood, W. S., _The New Earth_. N. Y.: The Macmillan Co., 1906.
378 pp. A recital of the triumphs of modern agriculture.
Chap. X on modern forestry, describes what has been done in
different states in conservative lumbering.
Hough, Romeyn B., _American Woods_. Lowville, N. Y.: The
author. An invaluable collection in eleven volumes (boxes)
of sections of 275 species of American woods. There are three
sections of each species, cross, radial, and tangential,
mounted in cardboard panels. Accompanied by a list of
descriptions and analytical keys.
Hough, Romeyn B., _Handbook of the Trees of the Northern
States and Canada_. Lowville, N. Y.: The author. 470 pp.
A unique, elegant, and sumptuously illustrated book, with
photographs of tree, trunk, leaf, fruit, bud, and sometimes
wood, a map of the habitat of each species, and a full and
careful description of tree and wood. Intended for botanists,
foresters and lumbermen.
Johnson, J. B., _The Materials of Construction_. N. Y.: John
Wiley & Sons. 1898. 775 pp. Chapter XIII is identical with
Forestry Bulletin X, Roth's _Timber_.
Keeler, Harriet, _Our Native Trees_. N. Y.: Scribner's. 1900.
533 pp. A very attractive and popular book showing great
familiarity with the common trees and love of them. Numerous
photographs and drawings.
Lounsberry, Alice, _A Guide to the Trees_. N. Y.: Frederick
A. Stokes Co. 313 pp. A popular description of some 200 common
trees, with plentiful illustrations.
Pinchot, Gifford, _A Primer of Forestry_. Parts I and II, U.
S. Dept. of Agric. For. Serv. Bull. No. 24. 88 pp. and 88
pp. A concise, clear, and fully illustrated little manual of
forestry conditions, forest enemies, forestry principles and
practice abroad and in the U. S.
Pinchot, Gifford. _The Adirondack Spruce._ N. Y.: G. P. Putnam's
Sons. A technical account of the author's investigations on a
forest estate in Northern New York.
Price, O. W., _Saving the Southern Forests_. _World's Work_,
5: 3207, March, '03. A plea for conservative lumbering;
excellent illustrations.
Record, Samuel J., _Characterization of the Grain and Texture
of Wood_. Woodcraft, 15: 3, June, 1911.
Roth, Filibert, _A First Book of Forestry_. Boston: Ginn & Co.
291 pp. A book for young people, giving in an interesting form
many valuable facts about American forests and their care and
use. It includes a leaf key to the trees.
Sargent, Charles Sprague, _Forest Trees of North America_. U.
S. 10th Census, Vol. 9. Quarto, 612 pp. Part I deals with
the distribution of the forests, and gives a catalog and
description of the forest trees of North America, exclusive of
Mexico. Part II. Tables of properties of the woods of the U.
S. Part III. The economic aspects of the forests of the U. S.
considered geographically, and maps showing distributions and
densities. Exceedingly valuable.
Sargent, Charles Sprague, _Jesup Collection, The Woods of
the U. S._ N. Y.: D. Appleton & Co., 203 pp. A detailed
description of the Jesup Collection of North American Woods
in the American Museum of Natural History, N. Y. City, with
valuable tables as to strength, elasticity, hardness, weight,
etc. Condensed from Vol. IX of 10th U. S. Census.
Sargent, Charles Sprague, _Manual of the Trees of North
America_. Boston: Houghton, Mifflin & Co. 826 pp. A compact
mine of information, with some errors, about the known trees
of North America and their woods, summarized from Sargent's
larger work, "The Silva of North America." (See below.)
Sargent, Charles Sprague, _The Silva of North America_.
Boston: Houghton, Mifflin Co. A monumental and sumptuous work
of 14 quarto volumes, describing in great detail all the known
trees of North America and their woods, with beautiful line
drawings of leaves and fruits.
Shaler, Nathaniel S., _The United States of America_. Vol. 1,
pp. 485-517. N. Y.: D. Appleton & Co. Chapter IX is a popular
description of American forests and the Lumber Industry.
Snow, Chas. Henry, _The Principal Species of Wood_. N. Y.:
John Wiley & Sons. 203 pp. Descriptions and data regarding
the economically important varieties of wood, with excellent
photographs of trees and woods.
Strasburger, Noll, Schenck, and Schimper. _A Text Book of
Botany._ N. Y.: Macmillan & Co. 746 pp. Valuable for minute
information about the morphology of wood.
U. S. Tenth Census, Vol. IX. See Sargent.
U. S. Department of Agriculture, _Forest Service Bulletins_.
The character of these government pamphlets is well indicated
by their titles. No. 10 is an exceedingly valuable summary of
the facts about the structure and properties of wood, contains
the best available key to identification of common American
woods (not trees) and a concise description of each. It is
incorporated, as Chap. XIII, in Johnson's, "_The Materials for
Construction_." N. Y.: John Wiley & Sons. Nos. 13 and 22 are
large monographs containing much valuable information.
No. 10. Filibert Roth, _Timber_.
No. 13. Charles Mohr, _The Timber Pines of the Southern United
States_.
No. 15. Frederick V. Coville, _Forest Growth and Sheep Grazing
in the Cascade Mountains of Oregon_.
No. 16. Filibert Roth, _Forestry Conditions in Wisconsin_.
No. 17. George B. Sudworth, _Check List of the Forest Trees of
the United States_, 1898.
No. 18. Charles A. Keffer, _Experimental Tree Planting on the
Plains_.
No. 22. V. M. Spalding and F. H. Chittenden, _The White Pine_.
No. 24. Gifford Pinchot, _A Primer of Forestry_.
No. 26. Henry S. Graves, _Practical Forestry in the
Adirondacks_.
No. 41. Herman von Schrenck, _Seasoning of Timber_.
No. 45. Harold B. Kempton, _The Planting of White Pine in New
England_.
No. 52. Royal S. Kellogg, _Forest Planting in Western Kansas_.
No. 61. _Terms Used in Forestry and Logging_.
No. 65. George L. Clothier, _Advice for Forest Planters in
Oklahoma and Adjacent Regions_.
No. 74. R. S. Kellogg and H. M. Hale, _Forest Products of the
U. S._, 1905.
U. S. Department of Agriculture, _Forest Service Circulars_.
No. 3. George William Hill, _Publications for Sale_.
No. 25. Gifford Pinchot, _The Lumberman and the Forester_.
No. 26. H. M. Suter, _Forest Fires in the Adirondacks in
1903_.
No. 36. The Forest Service: _What it is, and how it deals with
Forest Problems_. Also _Classified List of Publications and
Guide to Their Contents_.
No. 37. _Forest Planting in the Sand Hill Region of Nebraska_.
No. 40. H. B. Holroyd, _The Utilization of Tupelo_.
No. 41. S. N. Spring, _Forest Planting on Coal Lands in
Western Pennsylvania_.
No. 45. Frank G. Miller, _Forest Planting in Eastern
Nebraska_.
No. 81. R. S. Kellogg, _Forest Planting in Illinois_.
No. 97. R. S. Kellogg, _Timber Supply of the United States_.
No. 153. A. H. Pierson, _Exports and Imports of Forest
Products, 1907_.
U. S. Department of Agriculture Year Books for:
1896. Filibert Roth, _The Uses of Wood_.
1898, p. 181. Gifford Pinchot, _Notes on some Forest
Problems_.
1899, p. 415. Henry S. Graves, _The Practice of Forestry by
Private Owners_.
1900, p. 199. Hermann von Schrenck, _Fungous Diseases of
Forest Trees_.
1902, p. 145. William L. Hall, _Forest Extension in the Middle
West_.
1902, p. 265. A. D. Hopkins, _Some of the Principal Insect
Enemies of Coniferous Forests in the United States_.
1902, p. 309. Overton, W. Price, _Influence of Forestry on the
Lumber Supply_.
1903, p. 279. James W. Toumey, _The Relation of Forests to
Stream Flow_.
1903, p. 313. A. D. Hopkins, _Insect Injuries to Hardwood
Forest Trees_.
1904, p. 133. E. A. Sterling, _The Attitude of Lumbermen
toward Forest Fires_.
1904, p. 381. A. D. Hopkins, _Insect Injuries to Forest Products_.
1905, p. 455. Henry Grinell, _Prolonging the Life of Telephone
Poles_.
1905, p. 483. J. Grivin Peters, _Waste in Logging Southern
Yellow Pine_.
1905, p. 636. Quincy R. Craft, _Progress of Forestry in 1905_.
1907, p 277. Raphael Zon and E. H. Clapp, _Cutting Timber in
the National Forests_.
U. S. Department of Agriculture, Division of Entomology
Bulletins:
No. 11. n. s. L. O. Howard, _The Gypsy Moth in America_.
No. 28. A. D. Hopkins, _Insect Enemies of the Spruce in the
Northeast_.
No. 32. n. s. A. D. Hopkins, _Insect Enemies of the Pine in
the Black Hills Forest Reserve_.
No. 48. A. D. Hopkins, _Catalog of Exhibits of Insect Enemies
of Forest and Forest Products at the Louisiana Purchase
Exposition, St. Louis, Mo._, 1904.
No. 56. A. D. Hopkins, _The Black Hills Beetle_.
No. 58. Part 1, A. D. Hopkins, _The Locust Borer_.
No. 58. Part II, J. L. Webb, _The Western Pine Destroying Bark
Beetle_.
U. S. Department of Agriculture, Bureau of Plant Industry,
Bulletins:
No. 32. Herman von Schrenck, _A Disease of the White Ash
Caused by Polyporus Fraxinophilus_, 1903.
No. 36. Hermann von Schrenck, _The "Bluing" and "Red Rot" of
the Western Yellow Pine_, 1903.
_Report of the Commissioner of Corporations on the Lumber
Industry_, Part I, _Standing Timber_, February, 1911. The
latest and most reliable investigation into the amount and
ownership of the forests of the United States.
Ward, H. Marshall, _Timber and some of its Diseases_.
London: Macmillan & Co., 295 pp. An English book that needs
supplementing by information on American wood diseases, such
as is included in the list of government publications given
herewith. The book includes a description of the character,
structure, properties, varieties, and classification of
timbers.
CHAPTER I.
THE STRUCTURE OF WOOD.
When it is remembered that the suitability of wood for a particular
purpose depends most of all upon its internal structure, it is plain
that the woodworker should know the essential characteristics of that
structure. While his main interest in wood is as lumber, dead material
to be used in woodworking, he can properly understand its structure
only by knowing something of it as a live, growing organism. To
facilitate this, a knowledge of its position in the plant world is
helpful.
All the useful woods are to be found in the highest sub-kingdom of
the plant world, the flowering plants or Phanerogamia of the botanist.
These flowering plants are to be classified as follows:
{ I. Gymnosperms. (Naked seeds.)
{ 1. Cycadaceae. (Palms, ferns, etc.)
{ 2. Gnetaceae. (Joint firs.)
{ 3. Conifers. Pines, firs, etc.
Phanerogamia, { II. Angiosperms. (Fruits.)
(Flowering plants) { 1. Monocotyledons. (One seed-leaf.)
{ (Palms, bamboos, grasses, etc.)
{ 2. Dicotyledons. (Two seed-leaves.)
{ a. Herbs.
{ b. Broad-leaved trees.
Under the division of naked-seeded plants (gymnosperms), practically
the only valuable timber-bearing plants are the needle-leaved trees
or the conifers, including such trees as the pines, cedars, spruces,
firs, etc. Their wood grows rapidly in concentric annual rings, like
that of the broad-leaved trees; is easily worked, and is more widely
used than the wood of any other class of trees.
Of fruit-bearing trees (angiosperms), there are two classes, those
that have one seed-leaf as they germinate, and those that have two
seed-leaves.
The one seed-leaf plants (monocotyledons) include the grasses, lilies,
bananas, palms, etc. Of these there are only a few that reach
the dimensions of trees. They are strikingly distinguished by the
structure of their stems. They have no cambium layer and no distinct
bark and pith; they have unbranched stems, which as a rule do not
increase in diameter after the first stages of growth, but grow only
terminally. Instead of having concentric annual rings and thus growing
larger year by year, the woody tissue grows here and there thru the
stem, but mostly crowded together toward the outer surfaces. Even
where there is radial growth, as in yucca, the structure is not
in annual rings, but irregular. These one seed-leaf trees
(monocotyledons) are not of much economic value as lumber, being used
chiefly "in the round," and to some extent for veneers and inlays;
_e.g._, cocoanut-palm and porcupine wood are so used.
The most useful of the monocotyledons, or endogens, ("inside growers,"
as they are sometimes called,) are the bamboos, which are giant
members of the group of grasses, Fig. 1. They grow in dense forests,
some varieties often 70 feet high and 6 inches in diameter, shooting
up their entire height in a single season. Bamboo is very highly
valued in the Orient, where it is used for masts, for house rafters,
and other building purposes, for gutters and water-pipes and in
countless other ways. It is twice as strong as any of our woods.
Under the fruit-bearing trees (angiosperms), timber trees are chiefly
found among those that have two seed-leaves (the dicotyledons) and
include the great mass of broad-leaved or deciduous trees such as
chestnut, oak, ash and maple. It is to these and to the conifers that
our principal attention will be given, since they constitute the bulk
of the wood in common use.
The timber-bearing trees, then, are the:
(1) Conifers, the needle-leaved, naked-seeded trees, such as pine,
cedar, etc. Fig. 45, p. 199.
(2) Endogens, which have one seed-leaf, such as bamboos, Fig. 1.
(3) Broad-leaved trees, having two seed-leaves, such as oak, beech,
and elm. Fig. 48, p. 202.
The common classifications of trees are quite inaccurate. Many of
the so-called deciduous (Latin, _deciduus_, falling off) trees are
evergreen, such as holly, and, in the south, live oak, magnolia and
cherry. So, too, some of the alleged "evergreens," like bald cypress
and tamarack, shed their leaves annually.
[Illustration: Fig. 1. A Bamboo Grove, Kioto, Japan.]
Not all of the "conifers" bear cones. For example, the juniper bears
a berry. The ginko, Fig. 2, tho classed among the "conifers," the
"evergreens," and the "needle-leaf" trees, bears no cones, has broad
leaves and is deciduous. It has an especial interest as being the sole
survivor of many species which grew abundantly in the carboniferous
age.
[Illustration: Fig. 2. Ginko Leaf.]
Also, the terms used by lumbermen, "hard woods" for broad-leaved trees
and "soft woods" for conifers, are still less exact, for the wood of
some broad-leaved trees, as bass and poplar, is much softer than that
of some conifers, as Georgia pine and lignum vitae.
Another classification commonly made is that of "endogens" (inside
growers) including bamboos, palms, etc., and exogens (outside growers)
which would include both conifers and broad-leaved trees.
One reason why so many classifications have come into use is that none
of them is quite accurate. A better one will be explained later. See
p. 23.
As in the study of all woods three sections are made, it is well at
the outset to understand clearly what these are.
The sections of a tree made for its study are (Fig. 3):
(1) Transverse, a plane at right angles to the organic axis.
(2) Radial, a longitudinal plane, including the organic axis.
[Illustration: Fig. 3.
A.
A, B, C, D, Transverse Section.
B, D, E, F, Radial Section.
G, H, I, J, Tangential Section.
B.
A, B, C, Transverse Section.
A, B, D, E, Radial Section.
B, C, E, F, Tangential Section.
]
(3) Tangential, a longitudinal plane not including the organic axis.
If a transverse section of the trunk of a conifer or of a broad-leaved
tree is made, it is to be noted that it consists of several distinct
parts. See Fig. 4. These, beginning at the outside, are:
(1) Rind or bark
(a) Cortex
(b) Bast
(2) Cambium
(3) Wood
(a) Sap-wood
(b) Heart-wood
(4) Pith.
[Illustration: Fig. 4. Diagram of Cross-section of Three Year Old Stem
of Basswood.]
(1) The rind or _bark_ is made up of two layers, the outer of which,
the "cortex," is corky and usually scales or pulls off easily; while
the inner one is a fibrous coat called "bast" or "phloem." Together
they form a cone, widest, thickest, and roughest at the base and
becoming narrower toward the top of the tree. The cortex or outer bark
serves to protect the stem of the tree from extremes of heat and cold,
from atmospheric changes, and from the browsing of animals. It is made
up of a tough water-proof layer of cork which has taken the place of
the tender skin or "epidermis" of the twig. Because it is water-proof
the outside tissue is cut off from the water supply of the tree, and
so dries up and peels off, a mass of dead matter. The cork and the
dead stuff together are called the bark. As we shall see later, the
cork grows from the inside, being formed in the inner layers of the
cortex, the outer layers of dry bark being thus successively cut off.
The characteristics of the tree bark are due to the positions and
kinds of tissue of these new layers of cork. Each tree has its own
kind of bark, and the bark of some is so characteristic as to make the
tree easily recognizable.
Bark may be classified according to formation and method of
separation, as scale bark, which detaches from the tree in plates,
as in the willows; membraneous bark, which comes off in ribbons and
films, as in the birches; fibrous bark, which is in the form of stiff
threads, as in the grape vine; and fissured bark, which breaks up
in longitudinal fissures, showing ridges, grooves and broad, angular
patches, as in oak, chestnut and locust. The last is the commonest
form of bark.
The bark of certain kinds of trees, as cherry and birch, has peculiar
markings which consist of oblong raised spots or marks, especially
on the young branches. These are called lenticels (Latin _lenticula_,
freckle), and have two purposes: they admit air to the internal
tissues, as it were for breathing, and they also emit water vapor.
These lenticels are to be found on all trees, even where the bark is
very thick, as old oaks and chestnuts, but in these the lenticels are
in the bottoms of the deep cracks. There is a great difference in
the inflammability of bark, some, like that of the big trees of
California, Fig. 54, p. 209, which is often two feet thick, being
practically incombustible, and hence serving to protect the tree;
while some bark, as canoe birch, is laden with an oil which burns
furiously. It therefore makes admirable kindling for camp fires, even
in wet weather.
Inside the cork is the "phloem" or "bast," which, by the way, gives
its name to the bass tree, the inner bark of which is very tough and
fibrous and therefore used for mat and rope making. In a living tree,
the bast fibers serve to conduct the nourishment which has been made
in the leaves down thru the stem to the growing parts.
(2) The _cambium_. Inside of the rind and between it and the wood,
there is, on living trees, a slimy coat called cambium (Med. Latin,
exchange). This is the living, growing part of the stem, familiar to
all who have peeled it as the sticky, slimy coat between the bark
and the wood of a twig. This is what constitutes the fragrant,
mucilaginous inner part of the bark of slippery elm. Cambium is a
tissue of young and growing cells, in which the new cells are formed,
the inner ones forming the wood and the outer ones the bark.
In order to understand the cambium and its function, consider its
appearance in a bud, Fig. 5. A cross-section of the bud of a growing
stem examined under the microscope, looks like a delicate mesh of thin
membrane, filled in with a viscid semi-fluid substance which is called
"protoplasm" (Greek, _protos_, first; _plasma_, form). These meshes
were first called "cells" by Robert Hooke, in 1667, because of their
resemblance to the chambers of a honeycomb. The walls of these
"cells" are their most prominent feature and, when first studied,
were supposed to be the essential part; but later the slimy, colorless
substance which filled the cells was found to be the essential part.
This slimy substance, called protoplasm, constitutes the primal stuff
of all living things. The cell walls themselves are formed from it.
These young cells, at the apex of a stem, are all alike, very small,
filled with protoplasm, and as yet, unaltered. They form embryonic
tissue, _i.e._ one which will change. One change to which an cell
filled with protoplasm is liable is division into two, a new partition
wall forming within it. This is the way plant cells increase.
[Illustration: Fig. 5. Young Stem, Magnified 18-1/2 Diameters, Showing
Primary and Secondary Bundles. _By Courtesy of Mrs. Katharine Golden
Bitting._
E, epidermis, the single outside layer of cells.
C, cortex, the region outside of the bundles.
HB, hard bast, the black, irregular ring protecting the soft bast.
SB, soft bast, the light, crescent-shaped parts.
Ca, cambium, the line between the soft bast and the wood.
W, wood, segments showing pores.
MR, medullary rays, lines between the bundles connecting the pith and
the cortex.
MS, medullary sheath, the dark, irregular ring just inside the
bundles.
P, pith, the central mass of cells.]
In young plant cells, the whole cavity of the chamber is filled with
protoplasm, but as the cells grow older and larger, the protoplasm
develops into different parts, one part forming the cell wall and in
many cases leaving cavities within the cell, which become filled with
sap. The substance of the cell wall is called cellulose (cotton and
flax fibers consist of almost pure cellulose). At first it has no
definite structure, but as growth goes on, it may become thickened
in layers, or gummy, or hardened into lignin (wood), according to the
function to be performed. Where there are a group of similar cells
performing the same functions, the group is called a tissue or, if
large enough, a tissue system.
When cells are changed into new forms, or "differentiated," as it is
called, they become permanent tissues. These permanent tissues of the
tree trunk constitute the various parts which we have noticed, viz.,
the rind, the pith and the wood.
The essentially living part of the tree, it should be remembered, is
the protoplasm: where there is protoplasm, there is life and growth.
In the stems of the conifers and broad-leaved trees--sometimes
together called exogens--this protoplasm is to be found in the buds
and in the cambium sheath, and these are the growing parts of the
tree. If we followed up the sheath of cambium which envelopes a stem,
into a terminal bud, we should find that it passed without break into
the protoplasm of the bud.
In the cross-section of a young shoot, we might see around the central
pith or medulla, a ring of wedge-shaped patches. These are really
bundles of cells running longitudinally from the rudiments of leaves
thru the stem to the roots. They are made of protoplasm and are called
the "procambium strands," Fig. 6.
[Illustration: Fig. 6. Three Stages in the Development of an Exogenous
Stem. P, pith; PB, primary bast; SB, secondary bast; C, cambium;
PR, pith ray; PW, primary wood; SW, secondary wood; PS, procambium
strands. _After Boulger._]
In the monocotyledons (endogens) these procambium strands change
completely into wood and bast, and so losing all their protoplasmic
cambium, become incapable of further growth. This is why palms can
grow only lengthwise, or else by forming new fibers more densely in
the central mass. But in the conifers and broad-leaved trees, the
inner part of each strand becomes wood and the outer part bast (bark).
Between these bundles, connecting the pith in the center with the
cortex on the outside of the ring of bundles, are parts of the
original pith tissue of the stem. They are the primary pith or
medullary rays (Latin, _medulla_, pith). The number of medullary rays
depends upon the number of the bundles; and their form, on the width
of the bundles, so that they are often large and conspicuous, as in
oak, or small and indeed invisible, as in some of the conifers. But
they are present in all exogenous woods, and can readily be seen with
the microscope. Stretching across these pith rays from the cambium
layer in one procambium strand to that in the others, the cambium
formation extends, making a complete cylindrical sheath from the bud
downward over the whole stem. This is the cambium sheath and is the
living, growing part of the stem from which is formed the wood on the
inside and the rind (bark) on the outside.
In the first year the wood and the bast are formed directly by the
growth and change of the inner and outer cells respectively of the
procambium strand, and all such material is called "primary;" but
in subsequent years all wood, pith rays, and bast, originate in the
cambium, and these growths are called "secondary."
[Illustration: Fig. 7. Sap-wood and Heart-wood, Lignum Vitae.]
(3) The _wood_ of most exogens is made up of two parts, a lighter
part called the sap-wood or splint-wood or alburnum, and a darker part
called the heart-wood or duramen, Fig. 7. Sap-wood is really immature
heartwood. The difference in color between them is very marked in some
woods, as in lignum vitae and black walnut, and very slight in
others, as spruce and bass. Indeed, some species never form a distinct
heart-wood, birch (_Betula alba_) being an example.
In a living tree, sap-wood and heart-wood perform primarily quite
different functions. The sap-wood carries the water from the roots to
the leaves, stores away starch at least in winter, and in other ways
assists the life of the tree. The proportional amount of sapwood
varies greatly, often, as in long-leaf pine, constituting 40 per cent.
of the stem.
As the sap-wood grows older, its cells become choked so that the sap
can no longer flow thru them. It loses its protoplasm and starch and
becomes heartwood, in which all cells are dead and serve only the
mechanical function of holding up the great weight of the tree and
in resisting wind pressures. This is the reason why a tree may become
decayed and hollow and yet be alive and bear fruit. In a tree that is
actually dead the sap-wood rots first.
Chemical substances infiltrate into the cell walls of heart-wood and
hence it has a darker color than the sap-wood. Persimmon turns black,
walnut purplish brown, sumac yellow, oak light brown, tulip and poplar
yellowish, redwood and cedar brownish red. Many woods, as mahogany and
oak, darken under exposure, which shows that the substances producing
the color are oxidizable and unstable. Wood dyes are obtained by
boiling and distilling such woods as sumach, logwood, red sanders,
and fustic. Many woods also acquire distinct odors, as camphor,
sandalwood, cedar, cypress, pine and mahogany, indicating the presence
of oil.
As a rule heart-wood is more valuable for timber, being harder,
heavier, and drier than sap-wood. In woods like hickory and ash,
however, which are used for purposes that require pliability, as in
baskets, or elasticity as in handles of rakes and hoes, sap-wood is
more valuable than heart-wood.
In a transverse section of a conifer, for example Douglas spruce, Fig.
8, the wood is seen to lie in concentric rings, the outer part of the
ring being darker in color than the inner part. In reality each
of these rings is a section of an irregular hollow cone, each cone
enveloping its inner neighbor. Each cone ordinarily constitutes a
year's growth, and therefore there is a greater number of them at
the base of a tree than higher up. These cones vary greatly in
_thickness_, or, looking at a cross-section, the rings vary in
_width_; in general, those at the center being thicker than those
toward the bark. Variations from year to year may also be noticed,
showing that the tree was well nourished one year and poorly nourished
another year. Rings, however, do not always indicate a year's growth.
"False rings" are sometimes formed by a cessation in the growth due
to drouth, fire or other accident, followed by renewed growth the same
season.
[Illustration: Fig. 8. Section of Douglas Fir, Showing Annual Rings
and Knots at Center of Trunk. _American Museum of Natural History, N.
Y._]
In a radial section of a log, Fig. 8, these "rings" appear as a series
of parallel lines and if one could examine a long enough log these
lines would converge, as would the cut edges in a nest of cones, if
they were cut up thru the center, as in Fig. 9.
[Illustration: Fig. 9. Diagram of Radial Section of Log (exaggerated)
Showing Annual Cones of Growth.]
In a tangential section, the lines appear as broad bands, and since
almost no tree grows perfectly straight, these lines are wavy, and
give the characteristic pleasing "grain" of wood. Fig. 27, p. 35. The
annual rings can sometimes be discerned in the bark as well as in the
wood, as in corks, which are made of the outer bark of the cork oak, a
product of southern Europe and northern Africa. Fig. 10.
[Illustration: Fig. 10. Annual Rings in Bark (cork).]
The growth of the wood of exogenous trees takes place thru the
ability, already noted, of protoplasmic cells to divide. The cambium
cells, which have very thin walls, are rectangular in shape, broader
tangentially than radially, and tapering above and below to a chisel
edge, Fig. 11. After they have grown somewhat radially, partition
walls form across them in the longitudinal, tangential direction,
so that in place of one initial cell, there are two daughter cells
radially disposed. Each of these small cells grows and re-divides, as
in Fig. 12. Finally the innermost cell ceases to divide, and uses its
protoplasm to become thick and hard wood. In like manner the outermost
cambium cell becomes bast, while the cells between them continue to
grow and divide, and so the process goes on. In nearly all stems,
there is much more abundant formation of wood than of bast cells. In
other words, more cambium cells turn to wood than to bast.
[Illustration: Fig. 11. Diagram Showing Grain of Spruce Highly
Magnified. PR, pith rays; BP, bordered pits; Sp W, spring wood; SW,
summer wood; CC, overlapping of chisel shaped ends.]
[Illustration: Fig. 12. Diagram Showing the Mode of Division of the
Cambium Cells. The cambium cell is shaded to distinguish it from the
cells derived from it. Note in the last division at the right that
the inner daughter cell becomes the cambium cell while the outer cell
develops into a bast cell. _From Curtis: Nature and Development of
Plants._]
In the spring when there is comparatively little light and heat, when
the roots and leaves are inactive and feeble, and when the bark, split
by winter, does not bind very tightly, the inner cambium cells produce
radially wide wood cells with relatively thin walls. These constitute
the spring wood. But in summer the jacket of bark binds tightly, there
is plenty of heat and light, and the leaves and roots are very active,
so that the cambium cells produce thicker walled cells, called summer
wood. During the winter the trees rest, and no development takes
place until spring, when the large thin-walled cells are formed again,
making a sharp contrast with those formed at the end of the previous
season.
It is only at the tips of the branches that the cambium cells grow
much in length; so that if a nail were driven into a tree twenty years
old at, say, four feet from the ground, it would still be four feet
from the ground one hundred years later.
Looking once more at the cross-section, say, of spruce, the inner
portion of each ring is lighter in color and softer in texture than
the outer portion. On a radial or tangential section, one's finger
nail can easily indent the inner portion of the ring, tho the outer
dark part of the ring may be very hard. The inner, light, soft portion
of the ring is the part that grows in the spring and early summer,
and is called the "spring wood" while the part that grows later in the
season is called "summer wood." As the summer wood is hard and heavy,
it largely determines the strength and weight of the wood, so that as
a rule, the greater the proportion of the summer growth, the better
the wood. This can be controlled to some extent by proper forestry
methods, as is done in European larch forests, by "underplanting" them
with beech.
In a normal tree, the summer growth forms a greater proportion of the
wood formed during the period of thriftiest growth, so that in neither
youth nor old age, is there so great a proportion of summer wood as in
middle age.
It will help to make clear the general structure of wood if one
imagines the trunk of a tree to consist of a bundle of rubber tubes
crushed together, so that they assume angular shapes and have no
spaces between them. If the tubes are laid in concentric layers, first
a layer which has thin walls, then successive layers having thicker
and thicker walls, then suddenly a layer of thin-walled tubes and
increasing again to thick-walled ones and so on, such an arrangement
would represent the successive annual "rings" of conifers.
_The medullary rays._ While most of the elements in wood run
longitudinally in the log, it is also to be noted that running at
right angles to these and radially to the log, are other groups of
cells called pith rays or medullary rays (Latin, _medulla_, which
means pith). These are the large "silver flakes" to be seen in
quartered oak, which give it its beautiful and distinctive grain, Fig.
32, p. 38. They appear as long, grayish lines on a cross-section, as
broad, shining bands on the radial section, and as short, thick lines
tapering at each end on the tangential section. In other words, they
are like flat, rectangular plates standing on edge and radiating
lengthwise from the center of the tree. They vary greatly in size in
different woods. In sycamore they are very prominent, Fig. 13. In oak
they are often several hundred cells wide (_i.e._, up and down in the
tree). This may amount to an inch or two. They are often twenty cells
thick, tapering to one cell at the edge. In oak very many are also
small, even microscopic. But in the conifers and also in some of the
broad-leaved trees, altho they can be discerned with the naked eye on
a split radial surface, still they are all very small. In pine there
are some 15,000 of them to a square inch of a tangential section. They
are to be found in all exogens. In a cross-section, say of oak, Fig.
14, it can readily be seen that some pith rays begin at the center
of the tree and some farther out. Those that start from the pith are
formed the first year and are called primary pith rays, while those
that begin in a subsequent year, starting at the cambium of that year,
are called secondary rays.
[Illustration: Fig. 13. Tangential Section of Sycamore, Magnified 37
Diameters. Note the large size of the pith rays, A, A (end view).]
The function of the pith rays is twofold. (1) They transfer formative
material from one part of a stem to another, communicating with both
wood and bark by means of the simple and bordered pits in them, and
(2) they bind the trunk together from pith to bark. On the other hand
their presence makes it easier for the wood to split radially.
The substance of which they are composed is "parenchyma" (Greek,
_beside_, to _pour_), which also constitutes the pith, the rays
forming a sort of connecting link between the first and last growth of
the tree, as the cambium cells form new wood each year.
[Illustration: Fig. 14 Cross-section of White Oak. The Radiating White
Lines are the Pith Rays.]
If a cambium cell is opposite to a pith ray, it divides crosswise
(transversely) into eight or ten cells one above another, which
stretch out radially, retaining their protoplasm, and so continue the
pith ray. As the tree grows larger, new, or secondary medullary rays
start from the cambium then active, so that every year new rays are
formed both thinner and shorter than the primary rays, Fig. 14.
Now suppose that laid among the ordinary thin-walled tubes were quite
large tubes, so that one could tell the "ring" not only by the thin
walls but by the presence of large tubes. That would represent the
ring-porous woods, and the large tubes would be called vessels, or
_tracheae_. Suppose again that these large tubes were scattered
in disorder thru the layers. This arrangement would represent the
diffuse-porous woods.
By holding up to the light, thin cross-sections of spruce or pine,
Fig. 15, oak or ash, Fig. 16, and bass or maple, Fig. 17, these three
quite distinct arrangements in the structure may be distinguished.
This fact has led to the classification of woods according to the
presence and distribution of "pores," or as they are technically
called, "vessels" or "tracheae." By this classification we have:
(1) _Non-porous_ woods, which comprise the conifers, as pine and
spruce.
(2) _Ring-porous_ woods, in which the pores appear (in a
cross-section) in concentric rings, as in chestnut, ash and elm.
(3) _Diffuse-porous_ woods, in which (in a cross-section) the rings
are scattered irregularly thru the wood, as in bass, maple and yellow
poplar.
In order to fully understand the structure of wood, it is necessary to
examine it still more closely thru the microscope, and since the three
classes of wood, non-porous, ring-porous and diffuse-porous, differ
considerably in their minute structure, it is well to consider them
separately, taking the simplest first.
[Illustration: Fig. 15. Cross-section of Non-porous Wood, White Pine,
Full Size (top toward pith).]
_Non-porous woods._ In examining thru the microscope a transverse
section of white pine, Fig. 18:
(1) The most noticeable characteristic is the regularity of
arrangement of the cells. They are roughly rectangular and arranged in
ranks and files.
(2) Another noticeable feature is that they are arranged in belts, the
thickness of their walls gradually increasing as the size of the cells
diminishes. Then the large thin-walled cells suddenly begin again,
and so on. The width of one of these belts is the amount of a single
year's growth, the thin-walled cells being those that formed in
spring, and the thick-walled ones those that formed in summer, the
darker color of the summer wood as well as its greater strength being
caused by there being more material in the same volume.
[Illustration: Fig. 16. Cross-section of Ring-porous Wood, White Ash,
Full Size (top toward pith).]
[Illustration: Fig. 17. Cross-section of Diffuse-porous Wood, Hard
Maple, full size (top toward pith).]
(3) Running radially (up and down in the picture) directly thru the
annual belts or rings are to be seen what looks like fibers. These are
the pith or medullary rays. They serve to transfer formative material
from one part of the stem to another and to bind the tree together
from pith to bark.
(4) Scattered here and there among the regular cells, are to be seen
irregular gray or yellow dots which disturb the regularity of the
arrangement. These are _resin ducts_. (See cross-section of white
pine, Fig. 18.) They are not cells, but openings between cells, in
which the resin, an excretion of the tree, accumulates, oozing out
when the tree is injured. At least one function of resin is to protect
the tree from attacks of fungi.
Looking now at the radial section, Fig. 18:
(5) The first thing to notice is the straightness of the long cells
and their overlapping where they meet endwise, like the ends of two
chisels laid together, Fig. 11.
(6) On the walls of the cells can be seen round spots called "pits."
These are due to the fact that as the cell grows, the cell walls
thicken, except in these small spots, where the walls remain thin and
delicate. The pit in a cell wall always coincides with the pit in
an adjoining cell, there being only a thin membrane between, so that
there is practically free communication of fluids between the two
cells. In a cross-section the pit appears as a canal, the length of
which depends upon the thickness of the walls. In some cells, the
thickening around the pits becomes elevated, forming a border,
perforated in the center. Such pits are called bordered pits. These
pits, both simple and bordered, are waterways between the different
cells. They are helps in carrying the sap up the tree.
(7) The pith rays are also to be seen running across and interwoven
in the other cells. It is to be noticed that they consist of several
cells, one above another.
In the tangential section, Fig. 18:
(8) The straightness and overlapping of the cells is to be seen again,
and
(9) The numerous ends of the pith rays appear.
In a word, the structure of coniferous wood is very regular and
simple, consisting mainly of cells of one sort, the pith rays being
comparatively unnoticeable. This uniformity is what makes the wood of
conifers technically valuable.
[Illustration: Fig. 18.]
The cells of conifers are called tracheids, meaning "like _tracheae_."
They are cells in which the end walls persist, that is, are not
absorbed and broken down when they meet end to end. In other words,
conifers do not have continuous pores or vessels or "_tracheae_," and
hence are called "non-porous" woods.
But in other woods, the ends of some cells which meet endwise
are absorbed, thus forming a continuous series of elements which
constitute an open tube. Such tubes are known as pores, or vessels,
or "tracheae," and sometimes extend thru the whole stem. Besides this
marked difference between the porous and non-porous woods, the porous
woods are also distinguished by the fact that instead of being made
up, like the conifers of cells of practically only one kind, namely
tracheids, they are composed of several varieties of cells. Besides
the tracheae and tracheids already noted are such cells as "wood
fiber," "fibrous cells," and "parenchyma." Fig. 19. Wood fiber proper
has much thickened lignified walls and no pits, and its main function
is mechanical support. Fibrous cells are like the wood fibers except
that they retain their protoplasm. Parenchyma is composed of vertical
groups of short cells, the end ones of each group tapering to a point,
and each group originates from the transverse division of one
cambium cell. They are commonly grouped around the vessels (tracheae).
Parenchyma constitutes the pith rays and other similar fibers, retains
its protoplasm, and becomes filled with starch in autumn.
[Illustration: Fig. 19. Isolated Fibers and Cells. _a_, four cells of
wood parenchyma; _b_, two cells from a pith ray; _c_, a single cell
or joint of a vessel, the openings, x, x, leading into its upper and
lower neighbors; _d_, tracheid; _e_, wood fiber proper. _After Roth._]
The most common type of structure among the broad-leaved trees
contains tracheae, trachaeids, 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.
[Illustration: 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.
[Illustration: Fig. 21.]
THE GRAIN OF WOOD.
The term "grain" is used in a variety of meanings which is likely to
cause confusion. This confusion may be avoided, at least in part,
by distinguishing between grain and texture, using the word grain to
refer to the arrangement or direction of the wood elements, and the
word texture to refer to their size or quality, so far as these affect
the structural character of the wood. Hence such qualifying adjectives
as coarse and fine, even and uneven, straight and cross, including
spiral, twisted, wavy, curly, mottled, bird's-eye, gnarly, etc., may
all be applied to grain to give it definite meaning, while to texture
the proper modifying adjectives are coarse and fine, even and uneven.
Usually the word grain means the pattern or "figure" formed by the
distinction between the spring wood and the summer wood. If the annual
rings are wide, the wood is, in common usage, called "coarse grained,"
if narrow, "fine grained," so that of two trees of the same species,
one may be coarse grained and the other fine grained, depending solely
on the accident of fast or slow growth.
The terms coarse grain and fine grain are also frequently used to
distinguish such ring-porous woods as have large prominent pores, like
chestnut and ash, from those having small or no pores, as cherry and
lignum vitae. A better expression in this case would be coarse and
fine textured. When such coarse textured woods are stained, the large
pores in the spring wood absorb more stain than the smaller elements
in the summer wood, and hence the former part appears darker. In the
"fine grained" (or better, fine textured,) woods the pores are absent
or are small and scattered, and the wood is hard, so that they are
capable of taking a high polish. This indicates the meaning of the
words coarse and fine in the mind of the cabinet-maker, the reference
being primarily to texture.
If the elements of which a wood are composed are of approximately
uniform size, it would be said to have a uniform texture, as in
white pine, while uniform grain would mean, that the elements, tho
of varying sizes, were evenly distributed, as in the diffuse-porous
woods.
The term "grain" also refers to the regularity of the wood structure.
An ideal tree would be composed of a succession of regular cones, but
few trees are truly circular in cross-section and even in those that
are circular, the pith is rarely in the center, showing that one side
of the tree, usually the south side, is better nourished than the
other, Fig. 14, p. 23.
The normal direction of the fibers of wood is parallel to the axis of
the stem in which they grow. Such wood is called "straight-grained,"
Fig. 22, but there are many deviations from this rule. Whenever the
grain of the wood in a board is, in whole or in part, oblique to the
sides of the board, it is called "cross-grained." An illustration of
this is a bend in the fibers, due to a bend in the whole tree or to
the presence of a neighboring knot. This bend makes the board more
difficult to plane. In many cases, probably in more cases than not,
the wood fibers twist around the tree. (See some of the logs in Fig.
107, p. 254.) This produces "spiral" or "twisted" grain.
[Illustration: Fig. 22. Straight Grained Long-leaf Pine (full size).]
Often, as in mahogany and sweet gum, the fibers of several layers
twist first in one direction and then those of the next few layers
twist the other way, Fig. 24. Such wood is peculiarly cross-grained,
and is of course hard to plane smooth. But when a piece is smoothly
finished the changing reflection of light from the surface gives a
beautiful appearance, which can be enhanced by staining and polishing.
It constitutes the characteristic "grain" of striped mahogany, Fig.
23. It is rarely found in the inner part of the tree.
[Illustration: Fig. 23. Mahogany, Showing Alternately Twisted Grain
(full size).]
[Illustration: Fig. 24. Spiral Grain in Cypress.
_After Roth._]
Sometimes the grain of wood is "cross," because it is "wavy" either in
a radial or a tangential section, as in maple, Fig. 25, and Fig. 26.
[Illustration: Fig. 25. Planed Surface of Wavy-Grained Maple (full
size).]
[Illustration: Fig. 26. Split Surface of Wavy-Grained Maple (full
size).]
"Curly grain" refers to the figure of circlets and islets and
contours, often of great beauty, caused by cutting a flat surface in
crooked-grained wood. See Fig. 27, curly long-leaf pine, and Fig.
28, yellow poplar. When such crookedness is fine and the fibers are
contorted and, as it were, crowded out of place, as is common in and
near the roots of trees, the effect is called "burl," Fig. 29. The
term burl is also used to designate knots and knobs on tree trunks,
Fig. 31. Burl is used chiefly in veneers.
[Illustration: Fig. 27. Curly Grained Long-leaf Pine (full size).]
[Illustration: Fig. 28. Curly Yellow Poplar (full size).]
[Illustration: Fig. 29. Redwood Burl (full size).]
[Illustration: Fig. 30. Bird's-eye Maple (full size.)]
[Illustration: Fig. 31. Burl on White Oak.]
Irregularity of grain is often caused by the presence of adventitious
and dormant buds, which may be plainly seen as little knobs on
the surface of some trees under the bark. In most trees, these
irregularities are soon buried and smoothed over by the successive
annual layers of wood, but in some woods there is a tendency to
preserve the irregularities. On slash (tangent) boards of such wood,
a great number of little circlets appear, giving a beautiful grain,
as in "Bird's-eye maple," Fig. 30. These markings are found to
predominate in the inner part of the tree. This is not at all a
distinct variety of maple, as is sometimes supposed, but the common
variety, in which the phenomenon frequently appears. Logs of great
value, having bird's-eyes, have often unsuspectingly been chopped up
for fire wood.
The term "grain" may also mean the "figure" formed by the presence of
pith rays, as in oak, Fig. 32, or beech, or the word "grain" may refer
simply to the uneven deposit of coloring matter as is common in sweet
gum, Fig. 33, black ash, or Circassian walnut.
[Illustration: Fig. 32. Figure Formed by Pith Rays in Oak (full
size).]
[Illustration: Fig. 33. Sweet Gum, Showing Uneven Deposit of Coloring
Matter (full size.)]
The presence of a limb constitutes a knot and makes great irregularity
in the grain of wood, Fig. 34. In the first place, the fibers on the
upper and lower sides of the limb behave differently, those on the
lower side running uninterruptedly from the stem into the limb,
while on the upper side the fibers bend aside making an imperfect
connection. Consequently to split a knot it is always necessary to
start the split from the lower side. On the other hand it is easier to
split around a knot than thru it. The texture as well as the grain of
wood is modified by the presence of a branch. The wood in and around a
knot is much harder than the main body of the trunk on account of the
crowding together of the elements. Knots are the remnants of branches
left in the trunk. These once had all the parts of the trunk itself,
namely bark, cambium, wood, and pith. Normally, branches grow from the
pith, tho some trees, as Jack pine and redwood, among the conifers,
and most of the broad-leaf trees have the power of putting out at any
time adventitious buds which may develop into branches. When a branch
dies, the annual layer of wood no longer grows upon it, but the
successive layers of wood on the trunk itself close tighter and
tighter around it, until it is broken off. Then, unless it has begun
to decay, it is successively overgrown by annual layers, so that
no sign of it appears until the trunk is cut open. A large trunk
perfectly clean of branches on the outside may have many knots around
its center, remnants of branches which grew there in its youth, as in
Fig. 34, and Fig. 8, p. 18. The general effect of the presence of a
knot is, that the fibers that grow around and over it are bent, and
this, of course, produces crooked grain.
Following are the designations given to different knots by lumbermen:
A _sound_ knot is one which is solid across its face and is as hard as
the wood surrounding it and fixed in position. A _pin_ knot is sound,
but not over 1/4" in diameter. A _standard_ knot is sound, but not
over 1-1/2" in diameter. A _large_ knot is sound, and over 1-1/2"
in diameter. A _spike_ knot is one sawn in a lengthwise position. A
_dead_, or, _loose_ knot is one not firmly held in place by growth or
position.
(4) _Pith._ At the center or axis of the tree is the pith or
_medulla_, Fig. 34. In every bud, that is, at the apex of every stem
and branch, the pith is the growing part; but as the stem lengthens
and becomes overgrown by successive layers of wood the pith loses its
vital function. It does not grow with the plant except at the buds.
It varies in thickness, being very small,--hardly more than 1/16", in
cedar and larch,--and so small in oak as to be hardly discernible;
and what there is of it turns hard and dark. In herbs and shoots it is
relatively large, Fig. 5, p. 15, in a three-year old shoot of elder,
for example, being as wide as the wood. In elder, moreover, it dies
early and pulverizes, leaving the stem hollow. Its function is one of
only temporary value to the plant.
[Illustration: Fig. 34. Section Thru the Trunk of a Seven Year Old
Tree, Showing Relation of Branches to Main Stem. A, B, two branches
which were killed after a few years' growth by shading, and which have
been overgrown by the annual rings of wood; C, a limb which lived four
years, then died and broke off near the stem, leaving the part to the
left of XY a "sound" knot, and the part to the right a "dead" knot,
which unless rotting sets in, would in time be entirely covered by the
growing trunk; D, a branch that has remained alive and has increased
in size like the main stem; P, P, pith of both stem and limb.]
THE STRUCTURE OF WOOD.
REFERENCES:[A]
Roth, _Forest Bull._ No. 10, pp. 11-23.
Boulger, pp. 1-39.
Sickles, pp. 11-20.
Pinchot, _Forest Bull._ No. 24, I, pp. 11-24.
Keeler, pp. 514-517.
Curtis, pp. 62-85.
Woodcraft, 15: 3, p. 90.
Bitting, _Wood Craft_, 5: 76, 106, 144, 172, (June-Sept. 1906).
Ward, pp. 1-38.
_Encyc. Brit._, 11th Ed., "Plants," p. 741.
Strasburger, pp. 120-144 and Part II, Sec. II.
Snow, pp. 7-9, 183.
[Footnote A: For general bibliography, see p. 4.]