Chapter X: 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.

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?

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._

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_.

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.

_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.]

_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.

_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=.

_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.

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.

_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.]

=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_.

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?

=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.

=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.