THE STEM—ITS GENERAL STRUCTURE
There are two main types of stem structure in flowering plants, the
differences being based on the arrangement of bundles or strands of
tissue. These types are _endogenous_ and _exogenous_ (page 20). It
will require patient laboratory work to understand what these types
and structures are.
=Endogenous, or Monocotyledonous Stems.=—Examples of endogenous stems
are all the grasses, cane-brake, sugar-cane, smilax or green-brier,
palms, banana, canna, bamboo, lilies, yucca, asparagus, all the
cereal grains. For our study, a cornstalk may be used as a type.
[Illustration: FIG. 69.—CROSS-SECTION OF CORNSTALK, showing the
scattered fibro-vascular bundles. Slightly enlarged.]
A piece of _cornstalk_, either green or dead, should be in the hand
of each pupil while studying this lesson. Fig. 69 will also be of
use. Is there a swelling at the nodes? Which part of the internode
comes nearest to being perfectly round? There is a grooved channel
running along one side of the internode: how is it placed with
reference to the leaf? with reference to the groove in the internode
below it? What do you find in each groove at its lower end? (In a
dried stalk only traces of this are usually seen.) Does any bud on a
cornstalk besides the one at the top ever develop? Where do suckers
come from? Where does the ear grow?
[Illustration: FIG. 70.—DIAGRAM TO SHOW THE COURSE OF FIBRO-VASCULAR
BUNDLES IN MONOCOTYLEDONS.]
Cut a cross-section of the stalk between the nodes (Fig. 69). Does
it have a distinct bark? The interior consists of soft “pith” and
tough woody parts. The wood is found in _strands_ or _fibres_. Which
is more abundant? Do the fibres have any definite arrangement?
Which strands are largest? Smallest? The firm smooth _rind_ (which
cannot properly be called a bark) consists of small wood strands
packed closely together. Grass stems are hollow cylinders; and
the cornstalk, because of the lightness of its contents, is also
practically a cylinder. Stems of this kind are admirably adapted for
providing a strong support to leaves and fruit. This is in accordance
with the well-known law that a hollow cylinder is much stronger than
a solid cylinder of the same weight of material. Cut a thin slice of
the inner soft part and hold it up to the light. Can you make out a
number of tiny compartments or cells? These cells consist of a tissue
called _parenchyma_, the tissue from which when young all the other
tissues arise and differentiate. The numerous walls of these cells
may serve to brace the outer wall of the cylinder; but their chief
function in the young stalk is to give origin to other cells. When
alive they are filled with cell sap and protoplasm.
Trace the _woody strands_ through the nodes. Do they ascend
vertically? Do they curve toward the rind at certain places? Compare
their course with the strands shown in Fig. 70. _The woody strands
consist chiefly of tough fibrous cells that give rigidity and
strength to the plant, and of long tubular interrupted canals that
serve to convey sap upward from the root and to convey food downward
from the leaves to the stem and the roots._
[Illustration: FIG. 71.—DIAGRAM OF WOOD STRANDS OR FIBRO-VASCULAR
BUNDLES IN A ROOT, showing the wood (_x_) and bast (_p_) separated.]
Monocotyledons, as shown by fossils, existed before dicotyledons
appeared, and it is thought that the latter were developed from
ancestors of the former. It will be interesting to trace the
relationship in stem structure. It will first be necessary to learn
something of the structure of the wood strand.
=Wood Strand in Monocotyledons and Dicotyledons.=—Each wood strand
(or fibro-vascular bundle) consists of two parts—the bast and the
wood proper. The wood is on the side of the strand toward the
centre of the stem and contains large tubular canals that take the
watery sap upward from the roots. The bast is on the side toward
the bark, and contains fine tubes through which diffuses the dense
sap containing digested food from the leaves. In the root (Fig. 71)
the bast and the wood are separate, so that there are _two kinds of
strands_.
[Illustration: FIG. 72.—PART OF CROSS-SECTION OF ROOTSTOCK OF
ASPARAGUS, showing a few fibro-vascular bundles. An endogenous stem.]
In monocotyledons, as already said, the strands (or bundles) _are
usually scattered in the stem with no definite arrangement_ (Figs.
72, 73). In dicotyledons the strands, or bundles, _are arranged
in a ring._ As the dicotyledonous seed germinates, five bundles
are usually formed in its hypocotyl (Fig. 74); soon five more are
interposed between them, and the multiplication continues, in tough
plants, until the bundles touch (Fig. 74, right). The inner parts
thus form a ring of wood and the outer parts form the inner bark or
bast. A new ring of wood or bast is formed on stems of dicotyledons
each year, and the age of a cut stem is easily determined.
[Illustration: FIG. 73.—THE SCATTERED BUNDLES OR STRANDS, in
monocotyledons at _a_, and the bundles in a circle in dicotyledons at
_b_.]
[Illustration: FIG. 74.—DICOTYLEDONOUS STEM OF ONE YEAR AT LEFT WITH
FIVE BUNDLES, and a two-year stem at right. _o_, the pith; _c_, the
wood part; _b_, the bast part; _a_, one year’s growth.]
[Illustration: FIG. 75.—FIBRO-VASCULAR BUNDLE OF INDIAN CORN, much
magnified.
_A_, annular vessel; _A′_, annular or spiral vessel; _TT′_,
thick-walled vessels; _W_, tracheids or woody tissue; _F_, sheath
of fibrous tissue surrounding the bundle; _FT_, fundamental tissue
or pith; _S_, sieve tissue; _P_, sieve plate; _C_, companion cell;
_I_, intercellular space, formed by tearing down of adjacent cells;
_W′_, wood parenchyma.]
[Illustration: FIG. 76.—THE DICOTYLEDONOUS BUNDLE OR WOOD STRAND.
Upper figure is of moonseed:
_c_, cambium; _d_, ducts; 1, end of first year’s growth; 2, end of
second year’s growth; bast part at left and wood part at right.
Lower figure (from Wettstein) is sunflower: _h_, wood-cells; _g_,
vessels; _c_, cambium; _p_, fundamental tissue or parenchyma; _b_,
bast; _bp_, bast parenchyma; _s_, sieve-tubes.]
When cross-sections of monocotyledonous and dicotyledonous bundles
are examined under the microscope, it is readily seen why
dicotyledonous bundles form rings of wood and monocotyledonous cannot
(Figs. 75 and 76). The dicotyledonous bundle (Fig. 76) has, running
across it, a layer of brick-shaped cells called =cambium=, which
cells are a specialized form of the parenchyma cells and retain the
power of growing and multiplying. The bundles containing cambium
are called _open bundles_. There is no cambium in monocotyledonous
bundles (Fig. 75) and the bundles are called _closed bundles_.
Monocotyledonous stems _soon cease to grow in diameter_. The stem
of a palm tree is almost as large at the top as at the base. As
dicotyledonous plants grow, the _stems become thicker each year_, for
the delicate active cambium layer forms new cells from early spring
until midsummer or autumn, adding to the wood within and to the bark
without. As the growth in spring is very rapid, the first wood-cells
formed are much larger than the last wood-cells formed by the slow
growth of the late season, and the spring wood is less dense and of
a lighter colour than the summer wood; hence the time between two
years’ growth is readily made out (Figs. 77 and 78). Because of the
rapid growth of the cambium in spring and its consequent soft walls
and fluid contents, the bark of trees “peels” readily at that season.
[Illustration: FIG. 77.—WHITE PINE STEM, 5 years old. The outermost
layer is bark.]
[Illustration: FIG. 78.—ARRANGEMENT OF TISSUES IN TWO-YEAR-OLD STEM
OF MOONSEED.
_p_, pith; _f_, parenchyma. The fibro-vascular bundles, or wood
strands, are very prominent, with thin medullary rays between.]
=Medullary Rays.=—The first year’s growth in dicotyledons forms a
woody ring which almost incloses the pith, and this is left as a
small cylinder which does not grow larger, even if the tree should
live a century. It is not quite inclosed, however, for the narrow
layers of soft cells separating the bundles remain between them (Fig.
78), forming radiating lines called =medullary rays= or =pith rays=.
[Illustration: FIG. 79.—MARKINGS IN CELL WALLS OF WOOD FIBRE
_sp_, spiral; _an_, annular; _sc_, scalariform.]
=The Several Plant Cells and their Functions.=—In the =wood= there
are some parenchyma cells that have thin walls still, but have lost
the power of division. They are now _storage cells_. There are also
wood fibres which are thick-walled and rigid (h, Fig. 76), and serve
to _support_ the =sap-canals= or _wood vessels_ (or tracheids)
that are formed by the absorption of the end walls of upright rows
of cells; the canals pass from the roots to the twigs and even to
ribs of the leaves and serve to transport the root water. They are
recognized (Fig. 79) by the peculiar thickening of the wall on
the inner surface of the tubes, occurring in the form of spirals.
Sometimes the whole wall is thickened except in spots called _pits_
(g, Fig. 76). These thin spots (Fig. 80) allow the sap to pass to
other cells or to neighbouring vessels.
[Illustration: FIG. 80.—PITS IN THE CELL WALL.
Longitudinal section of wall at _b_, showing pit borders at _o_,
_o_.]
The =cambium=, as we have seen, consists of cells whose function is
_growth_. These cells are thin-walled and filled with protoplasm.
During the growing season they are continually adding to the wood
within and the bark without; hence the layer moves outward as it
deposits the new woody layer within.
[Illustration: FIG. 81.—SIEVE-TUBES, _s_, _s_;
_p_ shows a top view of a sieve-plate, with a companion cell, _c_,
at the side; _o_ shows sieve-plates in the side of the cell. In
_s_, _s_ the protoplasm is shrunken from the walls by reagents.]
[Illustration: FIG. 82.—THICK-WALLED BAST CELLS.]
=The bark= consists of inner or _fibrous bark_ or new bast (these
fibres in flax become linen), the _green or middle bark_ which
functions somewhat as the leaves, and the _corky or outer bark_. The
common word “bark” is seen, therefore, not to represent a homogeneous
or simple structure, but rather a collection of several kinds of
tissue, all separating from the wood beneath by means of cambium. The
new bast contains (1) the _sieve-tubes_ (Fig. 81) which transport the
sap containing organic substances, as sugar and proteids, from the
leaves to the parts needing it (_s_, Fig. 76). These tubes have been
formed like the wood vessels, but they have sieve-plates to allow
the dense organic-laden sap to pass with sufficient readiness for
purposes of rapid distribution. (2) There are also thick-walled _bast
fibres_ (Fig. 82) in the bast that serve for _support_. (3) There is
also some parenchyma in the new bast; it is now in part a storage
_tissue_. Sometimes the walls of parenchyma cells in the cortex
thicken at the corners and form _brace cells_ (Fig. 83) (collenchyma)
for _support_; sometimes the whole wall is thickened, forming _grit
cells_ or _stone cells_ (Fig. 84; examples in tough parts of pear, or
in stone of fruits). Some parts serve for secretions (milk, rosin,
etc.) and are called _latex tubes_.
[Illustration: FIG. 83.—COLLENCHYMA IN WILD JEWELWEED OR TOUCH-ME-NOT
(IMPATIENS).]
[Illustration: FIG. 84.—GRIT CELLS.]
The =outer bark= of old shoots consists of _corky_ cells that
_protect_ from mechanical injury, and that contain a fatty substance
(suberin) impermeable to water and of service to _keep in moisture_.
There is sometimes a cork cambium (or phellogen) in the bark that
serves to extend the bark and keep it from splitting, thus increasing
its power to protect.
=Transport of the “Sap.”=—We shall soon learn that the common word
“sap” does not represent a single or simple substance. We may roughly
distinguish two kinds of more or less fluid contents: (1) _the root
water_, sometimes called mineral sap, that is taken in by the root,
containing its freight of such inorganic substances as potassium,
calcium, iron, and the rest; this root water rises, we have found,
_in the wood vessels_,—that is, in the young or “sapwood” (p. 96);
(2) the _elaborated_ or _organized materials_ passing back and
forth, especially from the leaves, to build up tissues in all parts
of the plant, some of it going down to the roots and root-hairs;
this organic material is transported, as we have learned, _in the
sieve-tubes of the inner bast_,—that is, in the “inner bark.”
Removing the bark from a trunk in a girdle will not stop the upward
rise of the root water so long as the wood remains alive; but it will
stop the passage of the elaborated or food-stored materials to parts
below and thus starve those parts; and if the girdle does not heal
over by the deposit of new bark, the tree will in time _starve to
death_. It will now be seen that the common practice of placing wires
or hoops about trees to hold them in position or to prevent branches
from falling is irrational, because such wires interpose barriers
over which the fluids cannot pass; in time, as the trunk increases
in diameter, the wire girdles the tree. It is much better to bolt
the parts together by rods extending through the branches (Fig. 85).
These bolts should fit very tight in their holes. Why?
[Illustration: FIG. 85.—THE WRONG WAY TO BRACE A TREE. (See Fig.
118).]
=Wood.=—The main stem or trunk, and sometimes the larger branches,
are the sources of lumber and timber. Different kinds of wood have
value for their special qualities. The business of raising wood, for
all purposes, is known as _forestry_. The forest is to be considered
as a crop, and the crop must be harvested, as much as corn or rice is
harvested. Man is often able to grow a more productive forest than
nature does.
=Resistance to decay= gives value to wood used for shingles
(_cypress_, heart of _yellow pine_) and for fence posts (_mulberry_,
_cedar_, _post oak_, _bois d’arc_, _mesquite_).
=Hardness and strength= are qualities of great value in building.
_Live oak_ is used in ships. _Red oak_, _rock maple_, and _yellow
pine_ are used for floors. The best flooring is sawn with the
straight edges of the annual rings upward; tangential sawn flooring
may splinter. _Chestnut_ is common in some parts of the country,
being used for ceiling and inexpensive finishing and furniture.
_Locust_ and _bois d’arc_ (osage orange) are used for hubs of wheels;
bois d’arc makes a remarkably durable pavement for streets. _Ebony_
is a tropical wood used for flutes, black piano keys, and fancy
articles. _Ash_ is straight and elastic; it is used for handles for
light implements. _Hickory_ is very strong as well as elastic, and is
superior to ash for handles, spokes, and other uses where strength is
wanted. Hickory is never sawn into lumber, but is split or turned.
The “second growth,” which sprouts from stumps, is most useful, as it
splits readily. Fast-growing hickory in rich land is most valuable.
The supply of useful hickory is being rapidly exhausted.
=Softness= _is often important_. _White pine_ and _sweet gum_ because
of their softness and lightness are useful in box-making. “_Georgia_”
or _southern pine_ is harder and stronger than white pine; it is much
used for floors, ceilings, and some kinds of cabinet work. _White
pine_ is used for window-sash, doors, and moulding, and cheaper
grades are used for flooring. _Hemlock_ is the prevailing lumber in
the east for the framework and clapboarding of buildings. _Redwood_
and _Douglas spruce_ are common building materials on the Pacific
coast. _Cypress_ is soft and resists decay and is superior to white
pine for sash, doors, and posts on the outside of houses. _Cedar_
is readily carved and has a unique use in the making of chests for
clothes, as its odour repels moths and other insects. _Willow_ is
useful for baskets and light furniture. _Basswood_ or _linden_ is
used for light ceiling and sometimes for cheap floors. _Whitewood_
(incorrectly called poplar) is employed for wagon bodies and often
for house finishing. It often resembles curly maple.
[Illustration: FIG. 86.—THE MAKING OF ORDINARY BOARDS, AND ONE WAY OF
MAKING “QUARTERED” BOARDS.]
=Beauty of grain and polish= gives wood value for furniture, pianos,
and the like. _Mahogany_ and _white oak_ are most beautiful, although
red oak is also used. Oak logs which are first quartered and then
sawn radially expose the beautiful silver grain (medullary rays).
Fig. 86 shows one _mode of quartering_. The log is quartered on the
lines _a_, _a_, _b_, _b_; then succeeding boards are cut from each
quarter at 1, 2, 3, etc. The nearer the heart the better the “grain”:
why? Ordinary boards are sawn tangentially, as _c_, _c_. _Curly
pine_, _curly walnut_, and _bird’s-eye maple_ are woods that owe
their beauty of grain to wavy lines or buried knots. A mere stump of
curly walnut is worth several hundred dollars. Such wood is sliced
very thin for veneering and glued over other woods in making pianos
and furniture. If the cause of wavy grain could be found out and such
wood grown at will, the discovery would be very useful. _Maple_ is
much used for furniture. _Birch_ may be coloured so as very closely
to represent mahogany, and it is useful for desks.
=Special Products of Trees.=—Cork from the bark of the cork oak in
Spain, latex from the rubber, and sap from the sugar-maple trees,
turpentine from pine, tannin from oak bark, Peruvian bark from
cinchona, are all useful products.
SUGGESTIONS.—_Parts of a root and stem through which liquids rise._
=49.= Pull up a small plant with abundant leaves, cut off the root
so as to leave two inches or more on the plant (or cut a leafy
shoot of squash or other strong-growing coarse plant), and stand
it in a bottle with a little water at the bottom which has been
coloured with red ink (eosine). After three hours examine the root;
make cross sections at several places. Has the water coloured the
axis cylinder? The cortex? What is your conclusion? Stand some cut
flowers or a leafy plant with cut stem in the same solution and
examine as before: conclusion? =50.= Girdle a twig of a rapidly
growing bush (as willow) in early spring when growth begins (_a_)
by very carefully removing only the bark, and (_b_) by cutting away
also the sapwood. Under which condition do the leaves wilt? Why?
=51.= Stand twigs of willow in water; after roots have formed under
the water, girdle the twig (in the two ways) above the roots. What
happens to the roots, and why? =52.= Observe the swellings on trees
that have been girdled or very badly injured by wires or otherwise:
where are these swellings, and why? =53.= _Kinds of wood._ Let each
pupil determine the kind of wood in the desk, the floor, the door
and window casings, the doors themselves, the sash, the shingles,
the fence, and in the small implements and furniture in the room;
also what is the cheapest and the most expensive lumber in the
community. =54.= How many kinds of wood does the pupil know, and
what are their chief uses?
NOTE TO TEACHER.—The work in this chapter is intended to be mainly
descriptive, for the purpose of giving the pupil a rational
conception of the main vital processes associated with the stem, in
such a way that he may translate it into his daily thought. It is
not intended to give advice for the use of the compound microscope.
If the pupil is led to make a careful study of the text, drawings,
and photographs on the preceding and the following pages, he will
obtain some of the benefit of studying microscope sections without
being forced to spend time in mastering microscope technique. If
the school is equipped with compound microscopes, a teacher is
probably chosen who has the necessary skill to manipulate them and
the knowledge of anatomy and physiology that goes naturally with
such work; and it would be useless to give instruction in such
work in a text of this kind. The writer is of the opinion that
the introduction of the compound microscope into first courses
in botany has been productive of harm. Good and vital teaching
demands first that the pupil have a normal, direct, and natural
relation to his subject, as he commonly meets it, that the obvious
and significant features of the plant world be explained to him
and be made a means of training him. The beginning pupil cannot
be expected to know the fundamental physiological processes, nor
is it necessary that these processes should be known in order to
have a point of view and trained intelligence on the things that
one customarily sees. Many a pupil has had a so-called laboratory
course in botany without having arrived at any real conception of
what plants mean, or without having had his mind opened to any real
sympathetic touch with his environment. Even if one’s knowledge be
not deep or extensive, it may still be accurate as far as it goes,
and his outlook on the subject may be rational.
[Illustration: FIG. 87.—THE MANY-STEMMED THICKETS OF MANGROVE OF
SOUTHERNMOST SEACOASTS, many of the trunks being formed of aërial
roots.]