Chapter IV: The Stem (1)

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?

=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?

=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.

=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?

=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.

=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.

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.

=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.

=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.

=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.

=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.

=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?

=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.

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.

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.

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.

=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?

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?

=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.

=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.

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.

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.

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_.

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.

=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.

=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.

=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.

=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.)

=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.

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?

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.

=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.