I. FORMS AND GROWTH OF STEMS
MATERIAL.—Vigorous young hop or beau seedlings grown in pots; a
fresh dandelion stalk; a stem of pea, squash, cucumber, grape, or
passion flower vine, with tendrils.
APPLIANCES.—A bowl of fresh water; rods of different sizes and
smoothness for testing the hold of climbers.
EXPERIMENT 54. TO SHOW THE MOVEMENTS OF TWINING STEMS.—Raise a
young hop or bean seedling in the schoolroom and allow it to grow
about two decimeters—8 to 10 inches—in length before providing
it with a support. Does the stem form any coils? Bring it in
contact with a suitable upright support and watch for a day or
two. What happens? Notice whether it starts to coil from right to
left or from left to right and see if you can coax it to turn in
the opposite direction. When it has reached the end of its stake,
allow it to grow about five centimeters (two inches, approximately)
beyond, and watch the revolution of the tip. Cut a hole through the
center of a piece of cardboard about 14 centimeters (five to six
inches) in diameter, slip it over the loose end of the stem, and
fasten it to the stake in a horizontal position, with a pin. Note
the position of the stem tip at regular intervals and mark on the
cardboard; how long does it take to complete a revolution? Does it
continue to coil, or to coil as readily, after leaving its stake as
before? What would you infer from this as to the effect of contact
in stimulating it to coil?
Find out by experiment if it can climb well by means of a glass or
other smooth rod; by a fine wire; a broomstick; a large, smooth
post. See whether it does better on a horizontal or an upright
support.
EXPERIMENT 55. TO ILLUSTRATE THE COILING OF STEMS.—Run a gathering
thread in one side of a narrow strip of muslin and notice how the
ruffle thus drawn will curl into a spiral when allowed to dangle
from the needle. Can you think of any cause that might act on a
stem in the same way? Suppose, for instance, that one side should
grow faster than the other; what would be the effect? (54.)
Split the stem of a fresh dandelion, or other herbaceous scape,
longitudinally, and immerse it in a pan of fresh water for a few
minutes. Notice how the two halves curve outward, or even coil up
like the strip of muslin. This is due to the tension caused by the
more rapid absorption of the thinner walled cells of the internal
tissues. These, when relieved of the resistance of the thicker
walled outer tissues, swell on their free side, but are held back
on the other by the non-absorbent outer parts, as one side of the
muslin ruffle was held by the gathering thread.
EXPERIMENT 56. TO FIND OUT WHETHER THE DIRECTION OF STEM GROWTH IS
INFLUENCED BY LIGHT.—Place two rapidly growing young pea, bean,
sunflower, or squash plants, each with several well-developed
leaves, in a room or box with a light exposure on one side only.
After two or three days, notice the position of the stems in regard
to the light. Does either one show a more decided inclination
toward it than the other?
EXPERIMENT 57. IS THE LIGHT RELATION OF THE STEM INFLUENCED BY THE
LEAVES?—Cut the leaves from one of the plants used in Exp. 56,
covering the cut surfaces with vaseline to prevent “bleeding”;
reverse the positions of both with regard to the light, and watch
for two or three days. In which is the response to light the more
rapid? What does this indicate as one object of the stem in seeking
light? What is the best position of a stem, ordinarily, for getting
its leaves into the light?
[Illustration: FIG. 92.—Stems of red oak and hickory that have
grafted themselves.]
=90. Classification.=—Stems are classed according to (1) duration, as
annuals, biennials, and perennials; (2) with reference to hardness
or softness of structure, as herbaceous and woody; (3) in regard to
position and direction of growth, as erect, prostrate, climbing,
inclined, declined, underground, etc.
=91. Annuals= complete their life cycle in a single season and then
die down as soon as they have perfected their seed. Many of our most
troublesome weeds belong to this class and might be exterminated
by the simple expedient of mowing them down before their time of
flowering.
=92. Biennials=, as the name implies, live for two years. Their
energy during the first season is spent chiefly in laying by a store
of nourishment, usually in the tissues of fleshy roots (70). By this
means they get a good start in the second season and mature their
seeds early. Many of our common garden vegetables, such as turnips,
carrots, parsnips, and cabbage, belong to this class. Where is the
nourishment stored in the cabbage?
[Illustration: FIG. 93.—A biennial plant, mullein, in winter
condition with stem reduced to little more than a disk supporting
a rosette of leaves. Notice how close they cling to the earth, and
compare them with their fruiting condition a few months later as
shown in Fig. 237.]
=93. Perennials= are plants that live on indefinitely, like most of
our forest trees and woody-stemmed shrubs. Woody stems are usually
perennial and may live for hundreds and even thousands of years, as
those of the giant sequoias of California, and the famous chestnut of
Mt. Etna.
=94. Herbaceous stems= are more or less succulent and die down after
fruiting. They are usually annuals, though some kinds, like the
garden geraniums and the common St.-John’s-wort, show a tendency
to become woody, especially at the base, and live on from year to
year. Others, such as the hawkweed and dahlia, die down above ground
in winter, but are enabled to keep their underground parts alive
indefinitely, through the nourishment stored in them, and are thus
perennial below ground and annual above. Woody-stemmed annuals, such
as the cotton and castor oil plant, are not, properly speaking,
herbs. In the tropical countries to which they belong they are
perennial shrubs, or even small trees, but on being transplanted to
colder regions have been compelled to take on the annual habit as an
adaptation to climate.
[Illustration: FIG. 94.—Orange hawkweed with runners.]
[Illustration: FIG. 95.—Prostrate stem of Lycopodium with assurgent
branches.]
[Illustration: FIG. 96.—Diagram of stem growth: _ps_, surface of the
ground; _e_, erect position; _d_, declined; _a_, assurgent; _p_,
prostrate; _u_, vertical direction underground.]
=95. Direction and habit of growth.=—As to manner of growth, there
are many forms, from the upright boles of the beech and pine to the
trailing, prostrate, and creeping stems of which we have examples
in the running periwinkle, the prostrate spurge and the creeping
partridge berry (_Mitchella repens_), respectively. Trailing and
prostrate stems are very apt to become creepers by the development of
adventitious roots at their nodes wherever they come in contact with
the soil. The rooting stems of dewberries, the runners and stolons of
strawberries and currants, are familiar examples.
Between the extremes of prostrate and upright, stems may be inclined
or bent in various degrees. As shown in Fig. 96, there are two modes
of inclination: _assurgent_, _a_, from the prostrate, _p_, toward
the upright, _e_; and _declined_, _d_, from the upright toward the
prostrate. Below the surface, _ps_, occur only underground stems. Is
the prostrate habit an advantageous one for light exposure? Can you
think of any compensating advantages a plant might derive from it;
for example, in regard to warmth and moisture?
[Illustration: FIG. 97.—Twining stems: _A_, hop twining with the sun;
_B_, convolvulus twining against the sun.]
=96. Climbing stems.=—These are such as lift themselves from the
ground and attain the advantages of the upright position by clinging
to supports of various kinds—usually, in a state of nature, the
stems and boughs of other plants. The means of climbing may be:
(1) by merely leaning upon or propping themselves up by the aid of
the supporting object—examples, the rose, wistaria, star jessamine
(_Jasminum officinalis_); (2) by coiling their main axes spirally
around the support—hop, bean, morning-glory; (3) by means of
adventitious roots—poison ivy, common English ivy, trumpet vine
(_Tecoma radicans_); (4) by organs specially developed for the
purpose, called tendrils—gourd, cucumber, grape, passion flower.
[Illustration: FIG. 98.—Leaf of common pea, showing upper leaflets
reduced to tendrils.]
=97. Tendrils.=—The part assigned to do the work of climbing may be
a secondary branch, a flower stem, a leafstalk, a leaf, a leaflet,
or a group of leaflets (Fig. 98). Tendrils behave in general very
much like twining stems, except that they are more sensitive and
respond more quickly to any cause that may influence their movement.
While young, their tips revolve just as do the tips of twining stems,
until they meet with an object round which they can coil. When this
happens, not only the part in contact with the object coils, but
the free part between it and the main axis will usually respond by
twisting itself into a helix (Fig. 99). As the distance between the
base and tip of the tendril is shortened by coiling, the body of
the plant is drawn upward proportionally. It will be observed that
the helix is interrupted at one or more points, above and below
which the coils turn in opposite directions. This is because the
tendril is attached at both ends and cannot adjust itself to the
opposite strains of torsion. Twist with your fingers a piece of tape
so attached, and you will see that on one side of your hand it turns
from right to left and on the other from left to right.
=98. The cause of twining.=—Botanists are not fully agreed on this
point. The explanation most generally accepted at present is that
the twining of stems is due to the combined action of lateral and
negative geotropism (51). The first causes one side to grow more
rapidly than the other, thus forming a succession of coils, while
the second, by stimulating the upward growth of the axis, stretches
it into a spiral, and in this way draws it more tightly round the
support. For this reason twining stems do best on an upright support.
[Illustration: FIG. 99.—Stems of a passion flower transformed into
tendrils. (_After_ GRAY.)]
In tendrils, the twining is thought to be due not to gravity, but to
contact with a solid body, which, by inducing unequal development
on opposite sides of the tendril, causes it to turn about an
available object. The coiling of the free part of the twining
organ is in response to the stimulus transmitted from the part in
contact—_stimulus_, in this sense, denoting the influence of any
external agent that calls forth a responsive adjustment on the part
of the plant.
[Illustration: FIG. 100.—Showing the economy of labor and building
material effected by the climbing habit. Notice how the grapevine
coils like an anaconda around the tree boles, and overtops their
tallest branches. Compare the diameter of the vine with that of the
trees.]
=99. The object of the various habits of stem growth.=—To bring
the growing parts of the plant into the best possible relations
with light and air is one of the special functions of the stem, and
the various habits of growth described in this section have been
developed with reference to this function. In the case of prostrate
and underground stems other factors may intervene; can you name some
of the causes that might influence the position of the stem in such
cases?
Practical Questions
1. Why is the normal direction of most stems upright? (Exp. 56.)
2. Name a dozen woody-stemmed plants; a dozen with herbaceous stems.
3. Name all the plants you can think of that have prostrate stems,
or leaf rosettes that hug the earth, like mullein and dandelion.
Which of these are wintergreen plants? Which are hot-weather
growers?
4. Can you explain in what ways both hot-weather and cold-weather
plants may be advantaged by the habit of clinging close to the
earth? (94, 95.)
5. Is there any difference in the height of the stem of a dandelion
flower and a dandelion ball?
6. Of what advantage is this to the plant? (Exp. 17.)
7. Name all the means you can think of by which a stem may climb,
and give an example of each.
8. Why do we support peas with brush, and hops or beans with poles?
(98; Exp. 54.)
9. Are the vines of gourds, watermelons, squashes, and pumpkins
normally climbing or prostrate? How can you tell? (96, 97.)
10. Why does not the gardener provide them with poles or trellises
to climb on?
11. Do twining plants grow equally well on horizontal and upright
supports? (98; Exp. 54.)
12. If there is any difference, which do they seem to prefer?
13. Can you give any reasons for thinking that the climbing habit
might lead to parasitism? (83, 85, 87.)
14. What method of climbing would be most favorable to the
development of such a habit? (Suggestion: What mode of climbing
brings the stem into closest contact with its support?)
15. Name some plants the stems of which are used as food.
16. Name some from which gums and medicines are obtained.
17. Explain how it can benefit a plant to have its leaves, or some
of them, modified into tendrils. (99.)
18. In what way is the loss of the normal function of the leaves so
modified, compensated for? (Exp. 57.)
19. Suppose the vine shown in Fig. 100 had to lift itself without
the aid of a support; could it reach the same height and carry the
same weight of foliage and flowers with the same expenditure of
labor and building material?
II. MODIFICATIONS OF THE STEM
MATERIAL.—A shoot of asparagus; thorny branches of locust, plum, or
haw; a cactus plant; bulbs of lily and hyacinth or onion; tubers of
potato; rootstocks of iris, fern, or violet. If fresh specimens are
not accessible, dried rootstocks of the sweet flag and Florentine
iris may be obtained at the drug stores under the names of calamus
and “orris” root.
=100. How to recognize modified parts.=—Stems, like roots, are often
modified to serve other than their normal purpose, and in adapting
themselves to these new functions they sometimes undergo such changes
of form and structure that it would be impossible to recognize their
true nature from appearances alone. The safest tests in such cases
are: (1) by a comparison of the parts of the modified structure with
those of known organs of the same kind; and (2) by observing its
position in reference to other parts. For instance, we know that
the stem is the part of the plant which normally bears leaves and
flowers, and if either of these, or if the small scales which often
take the place of leaves, are found growing on any plant structure,
we may usually take for granted that it is a stem. Then, again, as
will be shown in the next chapter, buds and branches naturally appear
only at the nodes, in or near the _axil_, or inner angle made by a
leaf with the stem. Hence, if you see any growth springing from such
a position, you may generally conclude it to be a stem.
[Illustration: FIG. 101.—Stem-leaves (cladophylls) of a ruscus,
bearing flowers.]
=101. Stems as foliage.=—The connection between stem and leaf is so
intimate that we need not be surprised to find a frequent interchange
of function between them, the leaf, or some part of it, doing the
work of the stem (Fig. 98), the stem more often taking upon itself
the office of the leaf. A common example is the garden asparagus.
Examine one of the young shoots sold in the market, and notice that
it bears a number of small scales in place of leaves. On an older
shoot that has gone to seed, the green, threadlike appendages, which
are usually taken for foliage, will be found to spring each from the
axil of one of these scales. What, therefore, are we to conclude that
it is?
In the butcher’s-broom of Europe, the transformation has gone so far
that the branches of the stem have assumed the flattened appearance
of leaves (Fig. 101), but their real nature is evident both from
their position in the axils of leaf scales, and from the fact that
they bear flower clusters in the axil of a scale on their upper face.
Another example of this sort of modification is seen in the pretty
little _myrsiphyllum_ of the greenhouses (wrongly called smilax),
which is so much used for decoration. The delicate green blades are
merely altered stems, shortened and flattened to simulate leaves.
[Illustration: FIG. 102.—Thorn branches of _Holocantha Emoryi_, a
plant growing in arid regions.]
=102. Weapons of defense.=—Conspicuous examples of these are the
bristling thorns of the honey locust. Is their frequent branching
any indication of their real nature? Does it _prove_ anything, or
must you look for other evidence? What further indications might
you expect to find, if they are true branching stems? (100.) On old
haw, plum, crab, and pear trees, stems can be found in all stages
of transition, from stubby, ill-developed branches, to well-defined
thorns.
[Illustration: FIG. 103.—Melon cactus, showing greatly condensed stem
for the storage and preservation of moisture.]
=103. Storage of nourishment.=—This is one of the most frequent
causes of modification in both roots and stems. Of stems that grow
above ground, the sugar cane probably comes first in economic
importance on this account. In hot, arid regions, where the moisture
drawn from the earth would, during prolonged drought, be too rapidly
dissipated by an expanded surface of leaves, the whole plant, as
in the case of the cactus, is sometimes compacted into a greatly
thickened stem, which fills the triple office of leaf, stalk, and
water reservoir.
[Illustration: FIG. 104.—Rootstock of creeping panic grass.]
=104. The uses of underground stems.=—It is in these that the storage
of nourishment most frequently takes place, and the modifications
that stems undergo for this purpose are in some cases so great that
their real nature becomes apparent only after a careful examination.
But while the chief function of underground stems is the storage of
nourishment, they serve other purposes also. In plants requiring a
great deal of moisture, like the ferns, and in others growing in dry
places and needing to husband moisture carefully, like the blackberry
lily, underground stems may be useful in preventing the too rapid
evaporation that would take place through aërial stems. Defense
against frost, cold, heat, and other dangers, as well as quickness of
propagation, are also attained or assisted by this means.
=105. Rootstocks and rhizomes.=—From a prostrate stem like that
shown in Fig. 95 to a creeping rootstock like the one in Fig. 104,
the transition is so easy that we find no difficulty in accounting
for it. From the prostrate rootstock to the thickened storage
rhizome (Fig. 105) of such plants as the iris, puccoon, bulrush, and
Solomon’s-seal, is a longer step, but the bud with its leaf scales
at the growing tip, _a_, the remains of the flower stem at the node,
_b_, and the roots from the under surface sufficiently indicate its
nature. The peculiar scars from which the Solomon’s-seal takes its
name are caused by the falling away each year of the flowering stem
of the season after its work is done, leaving behind the node of the
underground stem from which it originated. In this way the rhizome
lives on indefinitely, growing and increasing at one end as fast as
it dies at the other. Test a little of the substance of the rhizome
with iodine. Of what does it consist? Of what use is it to the plant?
[Illustration: FIG. 105.—Rhizome of Solomon’s-seal: _a_, growing bud
at the tip; _b_, remains of the past season’s flower stem; _c_, _c_,
_c_, scars of old stems. (_After_ GRAY.)]
[Illustration: FIG. 106.—Potato tuber showing lenticels, _A_, _A_, or
pores for air on the surface; _S_, leaf scale, or scar.]
=106. The tuber.=—A still further thickening and shortening of
the rhizome gives rise to the tuber, of which the potato and the
Jerusalem artichoke are familiar examples. Can you give any evidence
to show that the potato is a modified stem? Find the point of
attachment of the tuber to its stem and stand it on this end, which
is its natural base. Notice that the eye sits in the axil of the
little scale that forms the eyelid. What does the scale represent?
What is the eye? (100.) Do the scales occur in any regular order—that
is, opposite, or alternating with, each other, like the leaves on
a stem? Look on the surface for a number of small, lens-shaped
dots (_A_, _A_, Fig. 106) scattered irregularly over it. These are
aërating pores called _lenticels_, and are found in most dicotyl
stems. Does their presence help to throw light on the real nature
of the tuber? If any sprouts occur on your specimen, where do they
originate? Where do buds and sprouts originate on plants above
ground? Make a sketch of the outside of a potato, showing the
lenticels, eyes, and scales, or the scars left by the scales in case
they have fallen away, as has probably happened, if your specimen is
an old one.
[Illustration: FIGS. 107, 108.—Transverse and longitudinal sections
of the potato: _A_, skin; _B_, cortical layer; _C_, outer pith layer;
_D_, inner pith layer.]
Cut a small slice from the stem end of two potatoes, stand them in
coloring fluid for four or five hours, then divide into cross and
vertical sections, as shown in Figs. 107, 108, and draw, labeling the
parts that you can make out. Through which has the liquid ascended
most rapidly? Test with iodine and find out in which part nourishment
is most abundant. It is this abundant store of food that makes
the potato such a valuable crop in cold countries like Norway and
Iceland, where the seasons are too short to admit of the slow process
of developing the plant from the seed.
[Illustration: FIG. 109.—Scaly bud of oak, enlarged.]
[Illustration: FIG. 110.—Scaly bulb of lily (GRAY).]
Compare a common potato with a sweet potato. Are there any eyes or
buds on the latter? Is there a scale below them? Do they occur in any
regular order? Do you see any lenticels? The common potato and the
sweet potato are both tubers; can you give some of the reasons why
the one is regarded as a modified branch, and the other as a root?
(100.) Compare their food contents; which contains most starch? Which
most sugar? How can you judge about the sugar without a chemical test?
=107. The bulb= is a form of underground stem reduced to a single
bud. Get the scaly bulb of a lily, and sketch it from the outside
and in cross and vertical section. Compare it with the scaly winter
buds of the oak and hickory, or other common deciduous tree. Make an
enlarged sketch of the latter on the same scale as the lily bulb, and
the resemblance will at once become apparent. The scales of the bulb
are, in fact, only thick, fleshy leaves closely packed around a short
axis that has become dilated into a flat disk. From the center of
the disk, which is the terminal node of this transformed stem, rises
the flower stalk, or _scape_, as it is called, of the season. After
blossoming, the scape perishes with its bulb, and their place is
taken by new ones which are developed from the axils of the scales,
thus revealing their leaflike nature.
[Illustration: FIG. 111.—Leaf of an onion divided lengthwise showing
the base enlarged into the coat of a bulb.]
That bulbs are only modified buds is further shown by the bulblets
that sometimes appear among the flowers of the onion, and in the leaf
axils of certain lilies. They never develop into branches, but drop
off and grow into new plants just as the subterranean bulbs do.
The bulbs of the onion and hyacinth are still further modifications,
in which the scales consist of the thickened bases of leafstalks that
are dilated until each one completely envelops the growing parts
within.
=108. Morphology= is the part of botany that treats of the origin,
form, and uses of the different organs of plants, and of the
modifications they undergo in adapting themselves to changes of
condition or function. Organs or parts that have the same origin
but have become adapted to different functions, like the flattened
stems of the butcher’s-broom or the bulb scales of the lily, are
said to be _homologous_; those that are different in origin but
adapted to the same function, as the sweet and common potatoes, are
_analogous_. In other words, homologous organs are morphologically
alike, but may be physiologically different; analogous organs are
alike physiologically, but differ morphologically.
=109. Economic value of stems.=—We probably get a greater variety
of economic products from the stem than from any other part of the
plant. Consider the vast amount of food stored in underground stems
like the potato; the resins, gums, and sugar found in the sap of
plants like the sugar cane, the pine, and India-rubber trees; the
medicines, dyes, and extracts obtained from the tissues; the valuable
fibers, such as flax, jute, and hemp, furnished by the bast; the wood
pulp for making paper; and the timber for building and furnishing
our houses that we get from the woody trunks of trees. When we think
of all these things, it seems hardly possible to overestimate the
importance of this part of the vegetable kingdom to man, or to exert
ourselves too strenuously to regulate and prevent the destruction of
these invaluable natural resources.
Practical Questions
1. Would you judge from the observations made in the foregoing
section, that the work of an organ determines its form, or that the
form determines its work? (99, 100, 108.)
2. Which is the more important, form or function?
3. Name some plants that are propagated by rootstocks; by runners
or stolons; by rhizomes; by tubers; by bulbs.
4. What is the advantage of propagating in this way over planting
the seed? (104, 106.)
5. Mention any other advantages that the various plants named may
gain from the development of their underground parts. (104.)
6. What makes the nut grass so troublesome to farmers in some parts
of the country?
7. Is its “nut” a root or a tuber? How can you tell? (106.)
8. Suggest some ways for destroying weeds that are propagated in
this way.
9. Could you get rid of wild onions in a pasture by mowing them
down? By digging them up? (107.)
10. Is it wise for farmers to neglect the appearance of such a weed
in their neighborhood, even though it does not infest their own
land?
11. Name any plants of your neighborhood, either wild or
cultivated, that are valued for their rhizomes; for their tubers.
12. What part of the plants named below do we use for food or other
purposes? Ginger, angelica, ginseng, cassava, arrowroot, garlic,
onion, sweet flag, iris, sweet potato, Cuba yam, artichoke.
13. Why are the true roots of bulbous and rhizome-bearing plants
generally so much smaller in proportion to the other parts than
those of ordinary plants? (89, 104.)
14. If the Canada thistle grows in your vicinity, examine the roots
and see if there is anything about them that will help to account
for its hardihood and persistency.
15. If you live in the region of the horse nettle (_Solanum
Carolinense_), explain how it is helped by its root system. (89.)
III. STEM STRUCTURE
A. MONOCOTYLS
MATERIAL.—Fresh cornstalks with several well-developed nodes, some
of which should have stood in coloring fluid from 1 to 3 hours. If
fresh specimens cannot be obtained from the fields, a number of
seedlings may be grown in boxes of rich earth and cared for by the
pupils either at home or in the schoolroom; they should be planted
4 or 5 weeks before needed. Asparagus and smilax sprouts may be
used, or the stem of any large grass, or of wheat and other grains,
but stalks of corn or sugar cane make the best subjects for study
where they can be obtained.
APPLIANCES.—A compound microscope will be needed for detailed
study. Prepared slides can be used, but it is better for students
to make their own sections where practicable.
[Illustration: FIG. 112.—Cross section of a cornstalk (reduced): _v_,
fibrovascular bundles; _c_, cortex; _p_, pith.]
=110. Gross anatomy of a monocotyl stem.=—Obtain a fresh
cornstalk,—preferably one that has begun to tassel,—and observe its
external characters. How are the internodes divided from one another?
What is the use of the very firm, smooth epidermis? Notice a hollow,
grooved channel running down one side between the _joints_, or nodes;
does it occur in all of them? Is it on the same side or on the
opposite sides of alternate internodes? Follow one of these grooves
to the node from which it originates; what do you find there? After
studying the internal structure of the stalk, you will understand why
this groove should occur on the side of an internode bearing a bud or
fruit.
Cut a cross section midway between two nodes, and observe the
composition of the interior; of what does the bulk of it appear to
consist? Notice the arrangement of the little dots, like the ends of
cut-off threads, that are scattered through the pith; where are they
most abundant, toward the center or the circumference?
[Illustration: FIG. 113.—Vertical section of cornstalk (reduced):
_g_, groove; _c_, cortex; _v_, fibrovascular bundles mingled with
parenchyma; _b_, bud; _n_, node.]
Make a vertical section through one of the nodes. Cut a thin slice
of the pith, hold it up to the light, and examine with a hand lens.
Observe that it is composed of a number of oblong cells packed
together like bricks in a wall. These are filled with protoplasm
and cell sap, and constitute what is known to botanists as the
_parenchyma_ or fundamental tissue from which all the other tissues
are derived. Apply the iodine test; in what parts does starch occur
most abundantly?
Draw out one of the woody threads running through the pith. Break
away a bit of the epidermis, and see how very closely they are packed
on its inner surface. Trace the course of the veins in the bases
of the leaves; find their point of union with the stem; with what
part of it do they appear to be continuous? Has this anything to do
with the greater abundance of fibers near the epidermis? Can you
follow the fibers through the nodes, or do they become confused and
intermixed with other threads there? (If a stalk of sugar cane can be
obtained, the ring of scars left by the vascular bundles as they pass
from the leaves into the stem will be seen beautifully marked just
above the nodes.)
If there is an eye or bud at the node, see if any of the threads go
into it. Can you account now for the depression that occurs in the
internode above the eye?
Make drawings of both cross and vertical sections, showing the points
brought out in your examination of the cornstalk.
=111. The vascular system.=—To find out the use of the threads that
you have been tracing, examine a piece of a living stem that has
stood in red ink for three to twenty-four hours. Notice the course
the coloring fluid has taken; what would you infer from this as to
the use of the woody fibers?
These threads constitute what is called the _vascular system_ of the
stem, because they are made up of _vessels_ or _ducts_, along which
the sap is conveyed from the roots to the leaves and back from the
leaves to the parts where it is needed after it has contributed to
the elaboration of food.
On account of this double line of communication which they have to
maintain, the vascular threads, or _bundles_, as they are technically
called, are double; one part composed of larger vessels, carrying
water up, the other consisting of smaller ones, bringing back the
food. Can you give a reason for their difference in size?
[Illustration: FIG. 114.—Longitudinal section through the stem of a
palm, showing the curved course of the fibrovascular bundles (GRAY,
_after_ FALKENBERG).]
=112. Woody monocotyls.=—Examine sections of yucca, smilax, or
of palmetto from the handle of a fan, and compare them with your
sketches of the cornstalk. In which are the vascular fibers most
abundant? Which is the toughest and strongest? Why? Trace the course
of the leaf fibers from the point of insertion to the interior. How
does it differ from that of the fibers in a cornstalk?
=113. Growth of monocotyl stems.=—After tracing the course of the
leaf veins at the nodes of the cornstalk, you will have no difficulty
in identifying these veins as part of the vascular system. In jointed
stems like those of the corn and sugar cane and other grasses, their
intercalation between the vascular bundles of the stem takes place,
as we have seen, at the nodes, forming the hard rings known as
joints; but in other monocotyls the fibers entering the stem from the
leaves usually tend first downward, toward the interior (Fig. 114),
then bend outward, toward the surface, where they become entwined
with others and form the tough, inseparable cortex that gives to
palmetto and bamboo stems their great strength. Generally, monocotyl
stems do not increase in diameter after a certain point, and as
they can contain only a limited number of vascular fibers, they are
incapable of supporting an extended system of leaves and branches.
Hence plants of this class, with a few exceptions, like smilax and
asparagus, are characterized by simple, columnar stems and a limited
spread of leaves. Such plant forms are admirably adapted by their
structure to the purposes of mechanical support. It is a well-known
law of mechanics that a hollow cylinder is a great deal stronger than
the same mass would be in solid form, as may easily be tested by the
simple experiment of breaking in your fingers a cedar pencil and a
joint of cane or a stem of smilax of the same weight. In stems that
may be technically classed as solid in structure, like the corn and
palmetto, the interior is so light compared with the hard epidermis
that the result is practically a hollow cylinder.
[Illustration: PLATE 4.—Forest of bamboo, showing the tall, straight,
branchless habit of monocotyl stems.]
=114. Minute study of a monocotyl stem.=—Place under the microscope
a very thin transverse section of a cornstalk. The little dots that
looked like the cut ends of threads to the naked eye will now appear
as the complex group of cells shown in Fig. 115. The same parts are
shown longitudinally in Fig. 116. As seen in cross section, their
arrangement suggests a grotesque resemblance to the face of an old
woman wearing a pair of enormous spectacles and surrounded by a cap
frill of netting with very wide meshes. These are parenchyma cells,
_f_, _f_, Fig. 115, and constitute the greater portion of the living
tissues.
[Illustration: FIG. 115.—Transverse section through the fibrovascular
bundle of a cornstalk: _a_, annular tracheid; _sp_, spiral tracheid;
_m_ and _m′_, ducts; _l_, air space; _v_, sieve tubes; _s_, companion
cells; _vg_, strengthening fibers; _cp_, bast; _f_, _f_, parenchyma.]
[Illustration: FIG. 116.—Vertical section of the same; _a_ and _a′_,
rings of a decomposed annular tracheid; _v_, sieve tubes; _s_,
companion cells; _cp_, bast; _l_, air space; _vg_, strengthening
tissue; _sp_, spiral duct.]
The two large openings, _m_, _m′_, that represent the spectacles, are
ducts for carrying water _up_ the stem. They are called pitted ducts
on account of the bordered pits which cover their outer surface. The
two smaller openings between and slightly below the pitted ducts are
also vessels for carrying liquids up the stem. The lower one, _a_,
is called the annular _tracheid_ because its tube is strengthened
by rings on the inside. The upper, smaller one, _sp_, is known as
the spiral tracheid, because its walls are reinforced by spiral
thickenings. Can you think what is the use of these strengthening
contrivances in the walls of conducting cells? (Suggestion: What is
the use of the spiral wire on a garden hose?) The large, irregular
opening below the ducts is an air space. What is its object? Why has
it no surrounding wall?
Next look above the ducts for a group of rhomboidal or hexagonal
cells, _v_, _v_, with smaller ones, _s_, between them. The larger of
these are _sieve tubes_, the smaller ones, _companion cells_. The
sieve tubes carry sap _down_ the stem after it has been made into
food by the leaves. They get their name from the sievelike openings
between the connecting walls of the cells which form them—as if a row
of pepper boxes with perforations at both top and bottom were placed
end to end, so as to form a long tube divided into compartments
by perforated walls. Can you give a reason why the cells of ducts
that carry elaborated nutriment should have a more open line of
communication than those carrying crude sap? [56 (2).] Which one of
the organic food substances was shown by Exp. 39 to be unable, or
nearly so, to pass through the cell wall by osmosis? [56 (4).] The
conducting cells are surrounded by a mass of strengthening fibers
separating them from the parenchyma, _f_, and constituting with
them a _fibrovascular bundle_. The larger vessels, _m_, _m′_, _a_,
and _sp_, compose the _xylem_, the harder, more woody part of the
bundle, and the smaller ones, _v_, _s_, the _phloëm_, or softer part.
Notice also that there is no parenchyma in contact with the xylem
and phloëm in the fibrovascular bundles of a monocotyl, to supply
material for new growth, but they are entirely surrounded by a sheath
of strengthening tissue, whence such bundles are said to be _closed_,
and are incapable of further growth by the addition of new cells.
[Illustration: FIG. 117.—Horizontal view of the sieve tube of a gourd
stem, showing perforations.]
[Illustration: FIG. 118.—Side view of the sieve tube of a gourd
stem: _pr_, protoplasm layer; _u_, albuminous contents, forming
mucilaginous strand.]
B. HERBACEOUS DICOTYLS
MATERIAL.—Young stems of sunflower, hollyhock, burdock, ragweed,
cocklebur, castor bean, or any large herbaceous plant. In schools
unprovided with compound microscopes, the minute anatomy can be
studied with some degree of profit by the aid of pictures.
=115. Gross anatomy.=—Examine the outside of a young stem of
sunflower, burdock, or other herbaceous dicotyl. Notice whether it
is smooth, or roughened with hairs, scales, ridges, or grooves. If
hairy, observe the nature of the hairs, whether bristly, downy,
sticky, etc. Notice the color of the epidermis, whether uniform, or
splotched or striped with other colors, as, for example, jimson weed,
and pigweed (amarantus). If there are any buds, branches, or flower
stems, notice where they originate; what is the angle between the
leaf and stem called? (100.)
Make a transverse cut through a portion of the stem that has stood
for a time in coloring fluid and examine with a lens. Four regions
can easily be distinguished: (1) the epidermis, _e_, Fig. 119; (2)
the primary cortex, _c_; (3) a ring of fibrovascular bundles, _f_;
and (4) a central cylinder of parenchyma, _p_. In some specimens
there will be a fifth region, the pith, which will appear in the
section as a white circular spot in the center of the parenchyma.
[Illustration: FIG. 119.—Transverse section of a very young stem
of burdock, showing fibrovascular bundles not completely united
into a ring: _e_, epidermis; _c_, primary cortex; _f_, a ring of
fibrovascular bundles; _p_, central cylinder of parenchyma.]
In specimens a little older than the one shown in Fig. 119, a narrow
circular line will be seen running through the ring of bundles nearly
midway between their inner and outer extremities, connecting them
into an unbroken circle around the central cylinder. This is the
_cambium_ layer, which supplies the vascular region with materials
for new growth, and thus enables dicotyl stems to increase in
diameter by the successive addition of fresh vascular rings from year
to year.
Examine in the same way a vertical section, and find the parts
corresponding to those shown in Fig. 119. Make enlarged sketches of
both sections, labeling the various parts observed.
=116. Minute structure of a dicotyl stem.=—Place successively under
a high power of the microscope thin transverse and longitudinal
sections of the stem just examined, or such other specimen as the
teacher may provide. Bring one of the fibrovascular bundles into the
field, and try to make out the parts shown in Figs. 120 and 121.
The corresponding parts in the two sections are indicated by the
same letters. Notice the cortex, _R_, on the outside and the pith,
_M_, on the inside; between these, the cambium, _C_, the _xylem_,
or woody tissue, included between the radiating lines _X_, and the
newer tissues composing the _phloëm_ between the lines _P_. The
cambium and pith, which includes the medullary rays so conspicuous
in perennial stems, are composed of live parenchyma cells, from
which alone growth can take place; they are the active part of the
stem. The xylem contains the large vessels, _t_ and _s_, that convey
water _up_ the stem, together with the wood fibers, _h_. These are
the permanent tissues. After completing their growth the cells of
the xylem gradually lose their protoplasm, and all vitality ceases.
Even the cell sap disappears, and sometimes the walls of the ducts
are disintegrated, leaving a mere air space like that shown at _l_
in Figs. 115 and 116. The dead cells and tissues, however, are by
no means useless. They constitute the heartwood that is so valuable
for timber, and serve an important purpose as a mechanical support
for the stem. The phloëm contains on its outer face a mass of hard
fibers, _b_, called bast, and toward the interior, the sieve tubes,
_sb_, with a number of smaller vessels that convey _down_ the stem
the sap containing the food made in the leaves. It is separated from
the cortex by the bundle sheath, _e_, and on its other side, from
the exterior face of the xylem by the cambium, _C_. In this position
the growing cambium adds new cells to the inner side of the phloëm,
and to the outer side of the xylem, so that the former grows on its
inner face and the latter on its outer. In perennial plants, as new
rings are added to the xylem from season to season, the older ones
die and are changed into heartwood, which thus gradually increases in
thickness till in some of the giant redwoods and eucalypti, it may
attain a diameter of thirty-five or forty feet. In the phloëm, on
the other hand, as new cells are added from within, the older ones
are gradually changed into hard bast, _b_, then into bark, and are
finally sloughed off and fall to the ground. It is this free line of
communication with the active cambium that enables dicotyl stems to
grow on indefinitely, the sheath, _e_, being formed on the exterior
face of the bundles only, leaving the other free, whence they are
said to be _open_.
[Illustration: FIGS. 120-121.—Transverse and longitudinal sections of
a fibrovascular bundle in the stem of a sunflower. The two sections
are lettered to correspond: _M_, pith (parenchyma); _X_, xylem
region; _P_, phloëm; _R_, cortex; _s_, spiral ducts; _s′_, annular
ducts; _t_, _t_, pitted ducts; _C_, cambium between the phloëm and
xylem regions; _sb_, sieve tubes; _b_, bast; _e_, bundle sheath;
_ic_, cambium (parenchyma) cells; _h_, wood fibers.]
Make drawings of cross and vertical sections of a dicotyl stem as
it appears under the microscope, labeling correctly all the parts
observed. Show the shape and relative size of the different cells.
Compare your drawings with those made in your study of monocotyl
stems, and write in your notebook the essential points of difference
between the two.
[Illustration: FIG. 122.—Internal structure of a pine stem, showing
longitudinal section of a fibrovascular bundle through a medullary
ray, _sm_, _sm′_; _s_, tracheids; _t_, bordered pits, surface view;
_c_, cambium; _v_, sieve tubes; _vt_, sieve pits, analogous to the
sieve plates in dicotyl stems.]
[Illustration: FIG. 123.—Internal structure of a pine stem, showing
transverse section of a tracheid: _i_, cell walls; _m_, intermediate
layer between walls of adjoining cells; _m′_, intercellular space
here occupied by substance of intermediate layer; _b_, bordered pit
in section at right angles to the surface; _t_, membrane for closing
the pit canal.]
=117. The stems of conifers=, the group of Gymnosperms to which the
pine belongs, do not differ greatly from those of dicotyls, the
chief difference being that the vascular bundles contain tracheids
only, corresponding to the smaller vessels of the phloëm, _s_ and
_s′_, shown in Fig. 121. These tracheids have large sunken places in
their walls, called bordered pits (Fig. 123), closed by a very thin
membrane through which water and dissolved food materials can more
readily percolate. In all other essentials, the internal structure of
pine stems is like that of dicotyls. (See Plate 5.)
C. WOODY STEMMED DICOTYL
MATERIAL.—Elm, basswood, mulberry, leatherwood, and pawpaw show the
bast well; sassafras, slippery elm, and (in spring) hickory and
willow show the cambium; grape and trumpet vine, the ducts. Some
of the specimens used should be placed in coloring fluid from 3 to
8 hours before the lesson begins. The rate at which the liquid is
absorbed varies with the kind of stem and the season. It is more
rapid in spring and slower in winter. If a cutting stands too long
in the fluid, the dye will gradually percolate through all parts of
it; care should be taken to guard against this.
[Illustration: FIG. 124.—Part of a young China tree shoot, showing,
_A_, lenticels; _B_, leaf scar; _C_, _C_, traces left by the broken
ends of fibrovascular bundles that passed from the stem into the
leaf. Natural size.]
=118. The external layer.=—While the primary structures, as shown
in the last section, are essentially the same in all dicotyl stems,
the continued yearly growth of perennials causes them to develop
a number of secondary structures and variations of detail that
differentiate them in a marked degree from soft-stemmed annuals. Take
a piece of a three-year-old shoot of cherry, horse chestnut, or any
convenient hardwood tree, and notice that the soft, green epidermis
has given place to a thicker, harder, and usually darker colored
bark. Notice the presence of lenticels (106) and their porous, corky
texture for the admission of air to the interior. They are slightly
raised above the surface of the bark, and are usually round, or more
or less elongated in different directions, according as they are
stretched vertically or horizontally by the growth of the axis. The
characteristic markings of birch bark, which make it so ornamental,
are due to the lenticels. In most trees they disappear on the older
parts, where the bark is constantly breaking away and sloughing off.
[Illustration: PLATE 5.—Stem of a conifer, _Sequoia gigantea_,
Mariposa Grove, California. The first branch, 6 feet in diameter,
leaves the parent trunk 125 feet above the ground. The photographer
sitting on one of the exposed roots affords a good standard for
comparison. The tree is noted for its massive limbs. The smaller
trees in the background show the characteristic mode of branching in
trees of this class.]
=119. Internal structures.=—Cut a transverse section through your
specimen, and notice under the epidermis a greenish layer of young
bark; beneath this a layer of rather tough, stringy bast fibers, and
beyond these a harder woody substance that constitutes the bulk of
the interior; within this, at the very center of the axis, we find
a cylinder of lighter texture, the pith, or medulla, occupying the
place of the soft parenchyma which fills this space in very young
stems.
Between the woody axis and the bark notice a more or less soft and
juicy ring.
=120. The cambium layer.=—This is not always easily distinguishable
with a hand lens, but is conspicuous in the stems of sassafras,
slippery elm, and aristolochia. If some of these cannot be obtained,
the presence of the cambium can be recognized by observing the
tendency of most stems to “bleed,” when cut, between the wood and
bark. The reason for this is because the cambium is the active part
of the stem, in which growth is taking place, and consequently it is
most abundantly supplied with sap. In spring, especially, it becomes
so full of sap that if a rod of hickory or elder is pounded, the
pulpy cambium is broken up and the bark may be slipped off whole from
the wood.
=121. Medullary rays.=—Observe the whitish, silvery lines that
radiate in every direction from the center, like the spokes of a
wheel from the hub. These are the medullary rays, and consist of
threads of pith that serve as lines of communication between the
“central cylinder” and the growing cambium layer. In old stems the
central pith frequently disappears and its office is filled by the
medullary rays, which become quite conspicuous.
[Illustration: FIGS. 125, 126.—Cross sections of twigs: 125, section
across a young twig of box elder, showing the four stem regions: _e_,
epidermis, represented by the heavy bounding line; _c_, cortex; _w_,
vascular cylinder; _p_, pith; 126, section across a twig of box elder
three years old, showing three annual growth rings, in the vascular
cylinder. The radiating lines (_m_), which cross the vascular region
(_w_), represent the pith rays, the principal ones extending from the
pith to the cortex (_c_). (_From_ COULTER’S “Plant Relations.”)]
=122. Structural regions of a woody stem.=—Sketch cross and vertical
sections of your specimen, as seen under the lens, labeling the
different parts. Refer to Figs. 125, 126, if you have any difficulty
in distinguishing the parts. In a year-old shoot (Fig. 125), the
structural regions correspond closely to those shown in Fig. 119,
except that the ring of fibrovascular bundles is here compact and
woody, and crossed by the radiating lines of the medullary rays. In
a three-year-old shoot (Fig. 126), the main divisions are the same,
but the soft parenchyma of the central cylinder is replaced by the
pith, and the vascular ring is composed of three layers corresponding
to the three years of growth. In general, mature dicotyl stems may be
said to include four well-defined regions: (1) the epidermis, or the
bark; (2) the cortex, made up of bast and certain other tissues; (3)
the cambium; (4) the woody vascular cylinder, made up of concentric
rings, each representing a year’s growth. The pith, or medulla,
constitutes a fifth region, but is obvious only in young stems.
Notice the little pores or cavities that dot the woody part in the
cross section; where are they largest and most abundant? How are the
rings marked off from one another? These pores are the sections of
ducts. They are very large in the grapevine, and a cutting two or
three years old will show them distinctly. Examine sections of a twig
that has stood in red ink from three to twelve hours, and observe
the course the fluid has taken. How does this accord with the facts
observed in your study of the conducting tissues in monocotyl and
herbaceous stems? (111, 115, 116.)
[Illustration: FIG. 127.—Diagram illustrating the annual growth of
dicotyledons.]
=123. The rings= into which the woody cylinder is divided mark the
yearly additions to the growth of the stem, which increases by the
constant accession of new material to the outside of the permanent
tissues (116). The cambium constantly advances outward, beginning
every spring a new season’s growth, and leaving behind the ring of
ducts and woody fibers made the year before. As the work of the plant
is most active and its growth most vigorous in spring, the largest
ducts are formed then, the tissue becoming closer and finer as the
season advances, thus causing the division into annual rings that is
so characteristic of woody dicotyl stems. Each new stratum of growth
is made up of the fibrovascular bundles that supply the leaves and
buds and branches of the season. In this way we see that the increase
of dicotyl trunks and branches is approximately in an elongated cone
(Fig. 127), the number of rings gradually diminishing toward the top
till at the terminal bud of each bough it is reduced to a single one,
as in the stems of annuals.
Sometimes a late autumn, succeeding a very dry summer, will cause
trees to take on a second growth, and thus form two layers of wood
in a single season. On this account we cannot always rely absolutely
upon the number of rings in estimating the age of a tree, though the
method is sufficiently exact for all practical purposes.
Practical Questions
1. Old Fort Moultrie near Charleston was built originally of
palmetto logs; was this good engineering or not? Why? (113.)
2. Explain the advantages of structure in a culm of wheat; a stalk
of corn; a reed. (113.)
3. Would the same quality be of advantage to an oak? Why, or why
not?
4. Is it of any advantage to the farmer that grain straw is so
light?
5. Explain why boys can slip the bark from certain kinds of wood in
spring to make whistles. (120.)
6. Why cannot they do this in autumn or winter? (123.)
7. Name some of the plants commonly used for this purpose.
8. Is the spring, after the buds begin to swell, a good time to
prune fruit trees and hedges? (120.)
9. What is the best time, and why?
10. Why are grapevines liable to bleed to death if pruned too late
in spring? (120, 123.)
11. Why are nurserymen, in grafting, so careful to make the cambium
layer of the graft hit that of the stock? (120.)
12. In calculating the age of a tree or bough from the rings of
annual growth, should we take a section from near the tip, or from
the base? Why? (123.)
IV. THE WORK OF STEMS
MATERIAL.—Leafy shoots of grape, balsam, peach, or other active
young stems; a cutting of willow, currant, or any kind of easily
rooting stem. Two bottles of water and some linseed or cottonseed
oil.
[Illustration: FIG. 128.—Experiment showing that moisture is thrown
off by the leaves of plants.]
EXPERIMENT 58. DO THE LEAVES HAVE ANY ACTIVE PART IN EFFECTING THE
MOVEMENT OF SAP IN THE STEM?—Take two healthy young shoots of the
same kind—grape, peach, corn, tropæolum, calla lily absorb rapidly.
Trim the leaves from one shoot and close the cut surfaces with
a little vaseline or gardener’s wax to prevent loss of water by
evaporation. Place the lower end of each in a glass jar or tumbler
filled to the same height with water. Cut off _under water_ a
half inch from the bottom of each shoot, to get a fresh absorbing
surface. This is necessary because exposure to air for even a
second greatly hinders absorption by permitting the entrance of air
into the severed ends of the ducts. Pour a little oil on the water
in both jars to prevent evaporation. (Do not use kerosene; it is
injurious to plants.) At the end of twenty-four hours, which vessel
has lost the more water? How do you account for the difference?
EXPERIMENT 59. WHAT BECOMES OF THE WATER THAT GOES INTO THE
LEAVES?—Cover the top of the vessel containing the leafy twig used
in the last experiment with a piece of cardboard, having first
cut a slit in one side, as shown in Fig. 128, so that it can be
slid into place without injuring the stem. Invert over the twig a
tumbler that has first been thoroughly dried, and leave in a warm,
dry place. After an hour or two, what do you see on the _inside_ of
the tumbler? Where did the moisture come from?
[Illustration: FIG. 129.—A twig which had been kept standing in water
after the removal of a ring of cortical tissue: _a_, level of the
water; _b_, swelling formed at the upper denudation; _c_, roots.]
EXPERIMENT 60. THROUGH WHAT PART OF THE STEM DOES THE SAP FLOW
UPWARD?—Remove a ring of the cortical layer from a twig of any
readily rooting dicotyl, such as willow, being careful to leave the
woody part, with the cambium, intact. Place the end _below_ the cut
ring in water, as shown in Fig. 129. The leaves above the girdle
will remain fresh. How is the water carried to them? How does this
agree with the movement of red ink observed in 115 and 122?
EXPERIMENT 61. THROUGH WHAT PART DOES THE SAP COME DOWN?—Next prune
away the leaves and protect the girdled surface with tin foil, or
insert it below the neck of a deep bottle to prevent evaporation,
and wait until roots develop. Do they come more abundantly from
above or below the decorticated ring?
=124. The three principal functions of the stem= are:—(1) to serve
as a mechanical support and framework for binding the other organs
together and bringing them into the best attainable relations with
light and air; (2) as a water carrier, or pipe line, for conveying
the sap from the roots to the parts where it is needed; and (3) as a
receptacle for the storage of foods.
=125. Movement of water.=—It has already been shown (71, 111) that
a constant interchange of liquid is taking place through the stem,
between the roots, where it is absorbed from the ground, and the
leaves, where it is used partly in the manufacture of food. Just
what causes the rise of sap in the stem is one of the problems of
vegetable physiology that botanists have not yet been able to solve.
There are, however, certain forces at work in the plant, which,
though they may not account for all the phenomena of the movement,
undoubtedly influence them to a great extent. From experiments 58-61,
we can obtain an idea of what some of these forces may be.
[Illustration: FIG. 130.—The stump of a large oak that was injured by
lightning many years ago. The interior is completely decayed, leaving
only a hollow shell of living tissue, from which branches continue to
put forth leaves year after year.]
=126. Direction of the current.=—These experiments show that the
upward movement of crude sap toward the leaves is mainly through the
ducts in the woody portion of the stem, while the downward flow of
elaborated sap from the leaves takes place chiefly through the soft
bast and certain other vessels of the cortical layer. The action of
the leaves in giving off part of the water absorbed, as shown in Exp.
59, probably has also an important influence on the course of sap
movement. If loss of water takes place in any organ through growth or
other cause, the osmotic flow of the thinner sap from the roots will
set in that direction.
=127. Ringing fruit trees.=—The course of the sap explains why
farmers sometimes hasten the ripening of fruit by the practice of
_ringing_. As the food material cannot pass below the denuded ring,
the parts above become gorged, and a process of forcing takes place.
The practice, however, is not to be commended, except in rare cases,
as it generally leads to the death of the ringed stem. The portion
below the ring can receive no nourishment from above, and will
gradually be so starved that it cannot even act as a carrier of crude
sap to the leaves, and so the whole bough will perish.
[Illustration: FIG. 131.—Diagram showing general movement of sap.]
=128. Sap movement not circulation.=—It must not be supposed that
this flow of sap in plants is analogous to the circulation of the
blood in animals, though frequently spoken of in popular language
as the “circulation of the sap.” There is no central organ like the
heart to regulate its flow, and the water taken up by the roots does
not make a continual circuit of the plant body as the blood does of
ours, but is dispersed by a process of general diffusion, partly into
the air through the leaves and partly through the plant body as food,
wherever it is needed. Figure 131 gives a good general idea of the
movement of sap in trees, the arrows indicating the direction of the
movement of the different substances.
=129. Unexplained phenomena.=—Though the forces named above
undoubtedly exert a powerful influence over sap movement, their
combined action has not been proved capable of lifting the current
to a height of more than 200 feet, while in the giant redwoods of
California and the towering blue gums of Australia, it is known to
reach a height of more than 400 feet. The active force exerted by the
cell protoplasm has been suggested as an efficient cause, but as the
upward flow takes place through the cells of the xylem, which contain
no protoplasm (116), this explanation is inadequate, and we must be
content, in the present state of our knowledge, to accept the fact as
one which science has yet to account for.
Practical Questions
1. Why will a leafy shoot heal more quickly than a bare one? (125,
126; Exp. 58.)
2. Why does a transverse cut heal more slowly than a vertical one?
(126, 127.)
3. Why does a ragged cut heal less rapidly than a smooth one?
4. Why does the formation of wood proceed more rapidly as the
amount of water given off by the leaves is increased? (126; Exp.
59.)
5. Why do nurserymen sometimes split the cortex of young trees in
summer to promote the formation of wood? (116, 118.)
6. What is the advantage of scraping the stems of trees?
7. Explain the frothy exudation that often appears at the cut ends
of firewood, and the singing noise that accompanies it. [120, 124
(2).]
8. Of what advantage is it to high climbing plants, like grape and
trumpet vine (_Tecoma_), to have such large ducts? (111, 116, 122.)
9. Why is the process of layering more apt to be successful if the
shoot is bent or twisted at the point where it is desired to make
it root? (127; Exps. 60, 61.)
10. Why do oranges become dry and spongy if allowed to hang on the
tree too long? (72, 126; Exps. 60, 61.)
11. Why will corn and fodder be richer in nourishment if, at
harvest, the whole stalk is cut down and both fodder and grain are
allowed to mature upon it? (126, 127; Exps. 60, 61.)
12. Is the injury done to plants by freezing due, as a general
thing, to mechanical, or to chemical action? (33.)
13. Why in pruning a branch is it best to make the cut just above a
bud? (Exps. 60, 61.)
14. Why is the rim of new bark, or callus, that forms on the upper
side of a horizontal wound, thicker than that on the lower side?
(126, 127; Exps. 60, 61.)
15. Why is it that the medicinal or other special properties of
plants are found mostly in the leaves and bark, or in the parts
immediately under the bark? (120, 126.)
16. Why does twisting the footstalk of a bunch of grapes, just
before ripening, make them sweeter? (127.)
[Illustration: PLATE 6.—A white oak, one of the monarchs of the
dicotyl type. The owner of the ground on which this noble tree stands
left a clause in his will bequeathing it in perpetuity a territory
of 8 feet in every direction from its base. Refer to 89 and decide
whether such an amount of standing room is sufficient to secure the
preservation of this beautiful object.]
17. Is it a mere superstition to drive nails into the stems of plum
and peach trees to make them bear larger or more abundant fruit?
(126, 127.)
18. Why is a living corn stalk heavier than a dry one? (124.)
19. Why is a stalk of sugar cane heavier than one of corn?
Suggestion: Which is the heavier, pure water, or water holding
solids in solution?
V. WOOD STRUCTURE IN ITS RELATION TO INDUSTRIAL USES
MATERIAL.—Select from the billets of wood cut for the fire, sticks
of various kinds; hickory, ash, oak, chestnut, maple, walnut,
cherry, pine, cedar, tulip tree, all make good specimens. Red
oak shows the medullary rays well. Get sticks of green wood, if
possible, and have them planed smooth at the ends. Collect also,
where they can be obtained, waste bits of dressed lumber from a
carpenter or joiner. If nothing better is available, any pieces of
unpainted woodwork about the schoolroom will furnish subjects for
study.
=130. Detailed structure of a woody stem.=—Select a good-sized billet
of hard wood, and count the rings of annual growth. How old was the
tree or the bough from which it was taken? Was its growth uniform
from year to year? How do you know? Are the rings broader, as a
general thing, toward the center or the circumference? How do you
account for this? Is each separate ring of uniform thickness all the
way round? Mention some of the circumstances that might cause a tree
to grow less on one side than on the other. Are the rings of the same
thickness in all kinds of wood? Which are the more rapid growers,
those with broad or with narrow rings? Do you notice any difference
in the texture of the wood in rapid and in slow growing trees? Which
makes the better timber as a general thing, and why?
=131. Heartwood and sapwood.=—Notice that in some of your older
specimens (cedar, black walnut, barberry, black locust, chestnut,
oak, Osage orange, show the difference distinctly) the central part
is different in color and texture from the rest. This is because
the sap gradually abandons the center (116, 123) to feed the outer
layers, where growth in dicotyls takes place; hence, the outer
part of the stem usually consists of sapwood, which is soft and
worthless as timber, while the dead interior forms the durable
heartwood so prized by lumbermen. The heartwood is useful to the
plant principally in giving strength and firmness to the axis. It
will now be seen why girdling a stem,—that is, chipping off a ring of
the softer parts all round, will kill it, while vigorous and healthy
trees are often seen with the center of the trunk entirely hollow.
[Illustration: FIG. 132.—Cross section through a black oak, showing
heartwood and sapwood. (_From_ PINCHOT, U. S. Dept. of Agr.)]
[Illustration: FIG. 133.—Vertical section through a black oak.
(_From_ PINCHOT, U. S. Dept. of Agr.)]
[Illustration: FIGS. 134-136.—Diagrams of sections of timber: 134,
cross section; 135, radial; 136, tangential. (_From_ PINCHOT, U. S.
Dept. of Agr.)]
=132. Different ways of cutting.=—In studying the vertical
arrangement of stems, two sections are necessary, a radial and a
tangential one. The former passes along the axis, splitting the stem
into halves (Fig. 135); the latter cuts between the axis and the
perimeter, splitting off a segment from one side (Fig. 136). The
appearance of the wood used in carpentry and joiner’s work is due
largely to the manner in which the planks are cut.
=133. The cross cut.=—The section seen at the end of a log (Figs.
132, 134) is called by carpenters a cross cut. It passes at
right angles to the grain of the wood, and severs what important
structures? (116, 119, 122.) Examine a cross cut at the end of a
rough plank, or the top of a stump or an old fence post, and tell why
this kind of cut is seldom used in carpentry.
[Illustration: FIG. 137.—Tangential section of mountain ash, showing
ends of the medullary rays.]
=134. The tangent cut= is so called because it is made at right
angles to the radius of a log. Repeat the geometrical principle upon
which such a cut is described as “tangential.” It passes through
the medullary rays and the annual rings diagonally (Fig. 136), and
is the cheapest way of cutting timber, since the entire log is made
into planks and there is no waste except the “slabs” and “edgings,”
as shown in Fig. 138. The cut ends of the medullary rays appear on
the surface as small lines or slits (Fig. 137), and give to this kind
of plank its peculiar graining. The wavy or “watered” appearance of
the annual rings (Figs. 133, 136, 140, 141), so often seen in cheap
furniture and in the woodwork of cheaply constructed houses, is
caused by the tangential cut, which strikes them at various angles.
[Illustration: FIG. 138.—Diagram to show the common method of sawing
a log. The circles represent rings of annual growth: _R_, _R_,
diameter of the log; _r_, _r_, _r_ and _t_, _t_, _t_, boards cut
perpendicular to it, giving for the two or three central ones radial,
for the others, tangential, cuts. The waste portions are the “slabs”
and “edgings,” shown in the dark segments at _R_, _R_, and the small
triangular blocks, _e_, _e_, _e_.]
[Illustration: FIG. 139.—Diagram illustrating the “quartered” cut:
_d_, _d_ and _d′_, _d′_, radial cuts (diameters) by which the log is
“quartered”; _c_, center of the log; _r_, _r_, radii passing through
the middle of each quarter, parallel to which the planks _t_, _t_,
_t_ are cut. The circles represent rings of annual growth.]
=135. The radial, or quartered cut=, familiar to most of us in the
“quartered oak” of commerce, passes through the center of the log
and cuts the rings of annual growth perpendicularly, giving it the
“striped” appearance (Fig. 135) seen in the best woodwork. It gets
its name from the practice of dealers in first sawing a log into
quarters and then cutting parallel to the radius passing through the
middle of each quarter, as shown in Fig. 139. In this way each cut
strikes the rings perpendicularly, but except in the case of very
large logs, only narrow planks can be obtained in this manner. A
better way of treating small logs is shown in Fig. 138, where the
three central planks, _r_, _r_, _r_, on and near the diameter, will
give the “quartered” effect, while the rest can be used for the
cheaper tangential cuttings. Examine a piece of quartered board, or a
log of wood that has been split down the center, and notice that the
medullary rays appear as silvery bands or plates (Figs. 140, 141).
This is because the cut runs parallel to them. It is the medullary
rays chiefly that give to commercial woods their characteristic
graining. Knots, buds, and other adventitious causes also influence
it in various degrees.
[Illustration: FIG. 140.—Sections of sycamore wood: _a_, tangential;
_b_, radial; _c_, cross. (_From_ PINCHOT, U. S. Dept. of Agr.)]
[Illustration: FIG. 141.—Section of white pine wood. (_From_ PINCHOT,
U. S. Dept. of Agr.)]
[Illustration: FIG. 142.—Section of tree trunk showing knot.]
=136. The swelling and shrinking of timber.=—The capacity possessed
by certain substances of bringing about an increase of volume by
the absorption of liquids is termed _imbibition_. Care must be
taken not to confound imbibition with capillarity. (Exp. 53.) When
liquids are carried into a body by capillary attraction, they merely
fill up vacant spaces already existing between small particles of
the substance, and therefore do not cause any swelling or increase
in size. When imbibition takes place, the _molecules_, or chemical
units of the liquid, force their way between those of the imbibing
substance, and thus, in making room for themselves, bring about an
increase in volume of the imbibing body. To this cause is due the
alternate swelling and shrinking of timber in wet and dry weather.
[Illustration: FIGS. 143-144.—Diagrams of tree trunks, showing knots
of different ages: 143, from tree grown in the open; 144, from tree
grown in a dense forest.]
=137. Knots.=—Look for a billet with a knot in it. Notice how the
rings of growth are disturbed and displaced in its neighborhood.
If the knot is a large one, it will itself have rings of growth.
Count them, and tell what its age was when it ceased to grow. Notice
where it originates. Count the rings from its point of origin to the
center of the stem. How old was the tree when the knot began to form?
Count the rings from the origin of the knot to the circumference
of the stem; how many years has the tree lived since the knot was
formed? Does this agree with the age of the knot as deduced from its
own rings? As the tree may continue to live and grow indefinitely
after the bough which formed the knot died or was cut away, there
will probably be no correspondence between the two sets of rings,
especially in the case of old knots that have been covered up and
embedded in the wood. The longer a dead branch remains on a tree the
more rings of growth will form around it before covering it up, and
the greater will be the disturbance caused by it. Hence, timber trees
should be pruned while very young, and the parts removed should be
cut as close as possible to the main branch or trunk. Sometimes knots
injure lumber very much by falling out and leaving the holes that are
often seen in pine boards. In other cases, however, when the knots
are very small, the irregular markings caused by them add greatly to
the beauty of the wood. The peculiar marking of bird’s-eye maple is
caused by abortive buds buried in the wood.
Practical Questions
1. Is the swelling of wood a physical or a physiological process?
2. Does wood swell equally with the grain and across it?
(Suggestion: test by keeping a block under water for 10 to 20 days,
measuring its dimensions before and after immersion.)
3. In building a fence, what is the use of “capping” the posts?
(133.)
4. In laying shingles, why are they made to touch, if the work is
done in wet weather, and placed somewhat apart, if in dry weather?
(136.)
5. What is the difference between timber and lumber? Between a
plank and a board? Between a log, stick, block, and billet?
6. Why does sapwood decay more quickly than heartwood? (131.)
7. Explain the difference between osmosis, diffusion, capillarity,
and imbibition. (9, 56, 57, 136; Exp. 53.)
VI. FORESTRY
=138. Practical bearings.=—This part of our subject is closely
related to lumbering and forestry. The business of the lumberman is
to manufacture growing trees into merchantable timber, and to do this
successfully he must understand enough about the structure of wood
to cut his boards to the best advantage, both for economy and for
bringing out the grain so as to produce the most desirable effects
for ornamental purposes.
[Illustration: PLATE 7.—Timber tree spoiled by standing too much
alone in early youth. Notice how the crowded young timber in the
background is righting itself, the lower branches dying off early
from overshading, leaving tall, straight, clean boles. (_From_
PINCHOT, U. S. Dept. of Agr.)]
[Illustration: FIG. 145.—After the forest fire.]
=139. Forestry has for its object=: (1) the preservation and
cultivation of existing forests; (2) the planting of new ones, or
the reforestation of tracts from which the timber has been destroyed.
Forests may be either _pure_, that is, composed mainly of one kind of
tree, as a pine or a fir wood; or _mixed_, being made up of a variety
of different growths, as are most of our common hardwood forests.
[Illustration: FIG. 146.—Oyster fungus on linden.]
=140. Enemies of the forest.=-The first step in the preservation of
our forests is to know the dangers to be guarded against. The chief
of these are: (1) fires; (2) the ignorance or recklessness of man in
cutting for commercial purposes; (3) fungi; (4) injurious insects;
(5) sheep, hogs, and other animals that eat the seeds and the young,
tender growth.
=141. How to protect the forests.=—The annual destruction of forests
by fires probably exceeds that from all other causes combined. The
only effectual safeguard against this danger is watchfulness on the
part of _everybody_. We can each one of us help in this work by at
least being careful ourselves never to kindle a fire in the woods
without taking every precaution against its spreading. A single
match, or the glowing stump of a cigar, carelessly thrown among dry
leaves or grass, may start a conflagration that will destroy millions
of dollars’ worth of standing timber.
To prevent the spread of fungi, dead trees should be removed, and
broken or decayed branches trimmed off and the cut surfaces painted.
Birds which destroy insects should be protected; sheep and hogs
should be kept out, and dead leaves left on the ground to cover the
roots and fertilize the soil with the humus created by their decay.
Finally, none but mature trees should be cut for industrial purposes,
and the cutting ought to be done in such a way that the young
surrounding growth will not be injured by the falling trunks.
=142. The usefulness of forests.=—Aside from the value of their
products, forests are useful in many other ways. They influence
climate beneficially by acting as windbreaks, by giving off moisture
(Exp. 58), by shading the soil, and thus preventing too rapid
evaporation. Their roots also help to retain the water in the soil,
and by this means tend to prevent the washing of the land by heavy
rains and to restrain the violence of freshets.
=143. Forests and water supply.=—It is especially important that
the watershed of any region should be well protected by forests,
to prevent contamination of the streams and to insure an unfailing
supply of water by checking the escape of the rainfall from the soil.
Practical Questions
1. Explain the difference between a forest, grove, copse, wood,
woodland.
2. In pruning a tree why ought the branch to be cut as close to the
stock as possible? (137.)
3. Name the principal timber trees of your neighborhood. What gives
to each its special value?
4. Name six trees that produce timber valuable for ornament; for
toughness and strength.
5. Which is the better for timber, a tree grown in the open, or one
grown in a forest, and why? (Plate 7.)
6. What are the objects to be attained in pruning timber trees?
Orchard and ornamental trees?
7. Is the outer bark of any use to a tree, and if so, what?
8. Why should pruning not be done in wet weather? [140 (3), 141.]
9. Why should vertical shoots be cut off obliquely? [133, 140 (3),
141.]
Field Work
(1) Make a study of the various climbing plants of your
neighborhood with reference to their modes of ascent, and the
effect, injurious, or other, upon the plants to which they attach
themselves. Note the origin and position of tendrils, and try to
make out what modification has taken place in each case. Consider
the twining habit in reference to parasitism, especially in the
case of soft-stemmed twiners when brought into contact with
soft-stemmed annuals. Observe the various habits of stem growth:
prostrate, declined, ascending, etc., and decide what adaptation to
circumstances may have influenced each case.
(2) Notice the shape of the different stems met with, and learn
to recognize the forms peculiar to certain of the great families.
Observe the various appliances for defense and protection with
which they are provided, and try to find out the meaning of the
numerous grooves, ridges, hairs, prickles, and secretions that are
found on stems. Always be on the alert for modifications, and learn
to recognize a stem under any disguise, whether thorn, tendril,
foliage, water holder, rootstock, or tuber.
(3) Note the color and texture of the bark of the different trees
you see and learn to distinguish the most important kinds:
(_a_) scaly—peeling off annually in large plates, as sycamore,
shagbark-hickory;
(_b_) fibrous—detached in stiff threads and fibers, as grape;
(_c_) fissured—split into large, irregular cracks by the growth
of the stem in thickness, as oak, chestnut, and most of our large
forest trees;
(_d_) membranous—separating in dry films and ribbons, as common
birch (_Betula alba_).
Observe the difference in texture and appearance of the bark on
old and young boughs of the same species. Try to account for the
varying thickness of the bark on different trees and on different
parts of the same tree. Notice the difference in the timber of
the same species when grown in different soils, at different ages
of the tree, and in healthy and weakly specimens. Find examples
of self-pruning trees (Plate 7), and explain how the pruning was
brought about.
(4) Select a small plot, about a fourth of an acre, of any wooded
tract in your neighborhood, and make a study of all the trees and
shrubs it contains. Make a list of the different kinds, with the
number of each. Take note of those that show themselves, by vigor
and abundance of growth, best adapted to the situation. These are
the “climax” or dominant vegetation of the plot. Find out, if you
can, to what cause their superiority is due.
[Illustration: PLATE 8.—The American elm—a perfect type of
deliquescent branching.]