Chapter VI: The Leaf (1)

I. THE TYPICAL LEAF AND ITS PARTS

MATERIAL.—Leaves of different kinds showing the various modes of
attachment, shapes, texture, etc. For stipules, leaves on very
young twigs should be selected, as these bodies often fall away
soon after the leaves expand. The rose, Japan quince, willow,
strawberry, pea, pansy, and young leaves of beech, apple, elm,
tulip tree, India rubber tree, magnolia, knotweed, furnish good
examples of stipules. For the different orders of leaf arrangement,
lilac, maple, spurge, trillium, cleavers (Galium) show the opposite
and whorled kinds. Elm, basswood, grasses; alder, birch, sedges;
peach, apple, cherry, show respectively for each group the three
principal orders of alternate arrangement.

=165. Parts of the leaf.=—Examine a young, healthy leaf of apple, quince, or elm, as it stands upon the stem, and notice that it consists of three parts: a broad expansion called the _blade_; a leaf stalk or _petiole_ that attaches it to the stem; and two little leaflike or bristle-like bodies at the base, known as _stipules_. Make a sketch of any leaf provided with all these parts, and label them, respectively, blade, petiole, and stipules. These three parts make up a perfect or typical leaf, but as a matter of fact, one or more of them is usually wanting.

=166. Stipules.=—The office of stipules, when present, is generally to subserve in some way the purposes of protection. In many cases, as in the fig, elm, beech, oak, magnolia, etc., they appear only as protective scales that cover the bud during winter, and fall away as soon as the leaf expands. When _persistent_, that is, enduring, they take various forms according to the purposes they serve. But under whatever guise they occur, their true nature may be recognized by their position on each side of the base of the petiole, and not in the _axil_, or angle formed by the leaf with the stem. (149.)

=167. Leaf attachment.=—The normal use of the petiole is to secure a better light exposure for the leaves, but, like other parts, it is subject to modifications, and is often wanting altogether. In this case the leaf is said to be _sessile_, that is, _seated_, on the stem, and the leaf bases are designated by various terms descriptive of their mode of attachment. The meaning of these terms, when not self-explanatory, can best be learned by a comparison of living specimens with Figs. 184-187.

=168. Arrangement of leaves on the stem.=—The mode of attachment is something quite distinct from the mode of leaf arrangement on the stem, or _phyllotaxy_, as it is termed by botanists. It was seen in 148 that this takes place in two different ways, the alternate and opposite. These two kinds of arrangement represent the principal forms of leaf disposition on the stem, the different varieties of each depending on the manner in which the leaves are distributed.

Where three or more occur at a node, as in the trillium and cleavers (_Galium_), they constitute a whorl, which is only a variant of the opposite arrangement. There is no limit to the number of leaves that may be in a whorl except the space around the stem to accommodate them.

The phyllotaxy of alternate leaves is more complicated. The different forms are characterized by the angular distance between the points of leaf insertion around the stem. In the elm, basswood, and most grasses, they are distributed in two rows or ranks on opposite sides of the stem, each just half way round the circumference from the one next in succession (Fig. 189), the third in vertical order standing directly over the first. In most of our common trees and shrubs five leaves are passed in making two turns round the stem, the sixth leaf in vertical order standing over the first. This is called the five-ranked arrangement, and is the most common order among dicotyls.

=169. Relation between the shape and arrangement of leaves.=—Phyllotaxy is of importance chiefly on account of its influence on the light relation of leaves. A compact, close-ranked arrangement tends to shut off the light from the lower nodes, and hence, in plants where it prevails, the leaves are apt to be long and narrow in proportion to the frequency of the vertical rows. The yucca, oleander, Canada fleabane and bitterweed (_Helenium tenuifolium_), illustrate this relation.

On the other hand, when the leaves are large and rounded in outline, as those of the sunflower, hollyhock, and catalpa, they are usually separated by longer internodes, or their blades are cut and incised so that the sunlight easily strikes through to the lower ones.

=170. Other external characteristics= to be observed in leaves are:—

(1) General Outline: whether round, oval, heart-shaped, etc. (Figs. 191-197).

(2) Margins: whether unbroken (_entire_), or variously toothed and indented. (Figs. 198-202.)

(3) Texture: whether thick, thin, soft, hard, fleshy, leathery, brittle.

(4) Surface: smooth, shining, dull, wrinkled, hairy, or otherwise roughened.

Not only do leaves of different kinds exhibit these characteristics in varying degrees, but young and old leaves, or those on young and old plants of the same kind, often differ from each other in color, size, shape, texture, mode of attachment, and the like, to such a degree (Figs. 203, 204) that one not familiar with them in both stages would hardly recognize them as belonging to the same species. The young leaves of eucalyptus, mulberry, and some oaks afford conspicuous examples of such differences, and they exist between the cotyledons and mature leaves of most plants.

Can you see any benefit, in the case of the plant whose leaves you are studying, that could be derived from such of the characteristics named above as they may exhibit?

Practical Questions

1. Tell the nature and use of the stipules in such of the following
plants as you can find: tulip tree; fig; beech; apple; willow;
pansy; garden pea; Japan quince (_Pyrus Japonica_); sycamore; rose;
paper mulberry (_Broussonetia_).

2. How would you distinguish between a chinquapin, a chestnut, a
chestnut oak, and a horse-chestnut tree by their leaves alone? By
their bark and branches? Between a hickory, ash, common elder, box
elder, ailanthus, sumach? Between beech, birch, elm, hackberry,
alder?

(Any other sets of leaves may be substituted for those named, the
object being merely to form the habit of distinguishing readily the
differences and resemblances among those that bear some general
likeness to one another.)

3. From the study of these or similar specimens, would you conclude
that resemblances in leaves are confined to those of closely
related kinds?

4. Name some causes independent of botanical relationship that
might influence them. (169, 170; Exps. 48, 57.)

5. Do you find, as a general thing, more leaves with stipules or
without?

6. Is their absence from a mature leaf always a sign that it is
really exstipulate? (166.)

7. Can you trace any line of development through intervening
forms from a merely sessile leaf, like that of the pimpernel or
specularia, to a peltate one? (Figs. 184-187, and observation of
living specimens.)

8. Does the leaf determine the position of the node, or the node
the position of the leaf?

9. Strip the leaves from a twig of one order of arrangement and
replace them with foliage from a twig of a different order; for
instance, place basswood upon white oak, birch upon lilac, elm upon
pear, honeysuckle upon barberry, etc. Is the same amount of surface
exposed as in the natural order?

10. What disadvantage would it be to a plant if the leaves were
arranged so that they stood directly over one another? (169.)

11. Why are the internodes of vigorous young shoots, or scions,
generally so long? (150.)

12. If the upward growth of a stem or branch is stopped by pruning,
what effect is produced upon the parts below, and why? (152, 153.)

13. Give some of the reasons why corn grows so small and stunted
when sown broadcast for forage? (60, 63, 169.)

14. What is the use of “chopping” (_i.e._ thinning out) cotton?

II. THE VEINING AND LOBING OF LEAVES

MATERIAL.—Leaves of any monocotyl and dicotyl will show the
difference between parallel and net-veining. To illustrate the
palmate and pinnate kinds, the leaves of grasses and arums may be
used for monocotyls, and for dicotyls, those of ivy, maple, grape,
elm, peach, cherry, etc.; for division, examine lobed and compound
leaves of as many kinds as are attainable. A specimen showing each
kind of veining should be placed in coloring fluid a short time
before the lesson begins. The leafstalks of celery and plantain
are excellent for showing the relation between the leaf veins and
vascular system of the plant.

=171. Parallel and net veining.=—Compare a leaf of the wandering Jew, lily, or any kind of grass, with one of grape, ivy, or willow. Hold each up to the light, and note the veins or little threads of woody substance that run through it. Make a drawing of each so as to show plainly the direction and manner of veining. Write under the first, _parallel-veined_, and under the second, _net-veined_. This distinction of leaves into parallel and net-veined corresponds with the two great classes into which seed-bearing plants are divided, monocotyls, as a general thing, being characterized by the first kind, and dicotyls by the second.

=172. Pinnate and palmate veining.=—Next, compare a leaf of the canna, calla lily, or any kind of arum, with one of the elm, peach, cherry, etc. What resemblances do you notice between the two? What differences? Which is parallel-veined and which is net-veined? Make a drawing of each, and compare with the first two. Notice that in leaves of this kind, the petiole is continued in a large central vein, called the _midrib_, from which the secondary veins branch off on either side like the pinnæ of a feather; whence such leaves are said to be _pinnately_, or _feather_ veined, as in Figs. 206, 207. In the cotton, maple, ivy, etc., on the other hand, the petiole breaks up at the base of the leaf (Fig. 208) into a number of primary veins or ribs, which radiate in all directions like the fingers from the palm of the hand; hence, such a leaf is said to be _palmately_ veined. Net-veined leaves—the plantain (Fig. 209), wild smilax, beech, dogwood—are sometimes ribbed in a way that might lead an inexperienced observer to confound them with parallel-veined ones, but the reticulations between the ribs show that they belong to the net-veined class.

=173. Veins as a mechanical support.=—Hold up a stiff, firm leaf of any kind, like the magnolia, holly, or India rubber, to the light, having first scraped away a little of the under surface, and examine it with a lens. Compare it with one of softer texture, like the peach, maple, or clover. In which are the veins the closer and stronger? Which is the more easily torn and wilted? Tear a blade of grass longitudinally and then cross-wise; in which direction does it give way the more readily? Tear apart gently a leaf of maple, or ivy, and one of elm or other pinnately veined plant; in which direction does each give way with least resistance? What would you judge from these facts as to the mechanical use of the veins?

=174. Effect upon shape.=—By comparing a number of leaves of each kind it will be seen that the feather-veined ones tend to assume elongated outlines (Figs. 197, 207); the palmate-veined ones, broad and rounded forms (Figs. 195, 208). Notice also that the straight, unbroken venation of parallel-veined leaves is generally accompanied by smooth, unbroken margins, while the irregular, open meshes of net-veined leaves are favorable to breaks and indentations.

=175. Veins as water carriers.=—Examine a leaf from a stem that has stood in red ink for an hour or two. Do you see evidence that it has absorbed any of the liquid? Cut across the blade and examine with a lens. What course has the absorbed liquid followed? What use does this indicate for the veins, besides the one already noted? Observe the point of insertion on the stem, and examine the scar with a lens: do you see any evidence of a connection between the leaf veins and the fibrovascular bundles of the stem? (111, 125, 126.) Notice where and how the veins end. Are they of the same size all the way, or do they grow smaller toward the tip? Are they separate and distinct, or are they connected throughout their ramifications, like the veins and arteries of the human body? How do you know? Do you see any of the coloring fluid in the small reticulations between the veins? How did it get there?

=176. The nature and office of veins.=—We learn from 173 and 175 that the veining serves two important purposes in the economy of the leaf: first, as a skeleton or framework, to support the expanded blade; and second, as a system of water pipes, for conveying the sap out of which its food is manufactured. In other words the veins are a continuation of the fibrovascular bundles into the leaves, by means of which the latter are put in communication with the body of the plant.

=177. The relation between veining and lobing.=—Compare the outline of a leaf of maple or ivy with one of oak or chrysanthemum. Do you perceive any correspondence between the manner of lobing or indentation of their margins, and the direction of the veins? (Figs. 210, 211.) To what class would you refer each one?

The lobes themselves may be variously cut, as in the fennel and rose geranium, thus giving rise to twice-cleft, thrice-cleft (Fig. 212), four-cleft, or even still more intricately divided blades.

=178. Compound leaves.=—Compare with the specimens just examined a leaf of horse-chestnut, clover, or Virginia creeper, and one of rose, black locust, or vetch. Notice that each of these last is made up of entirely separate divisions or leaflets, thus forming a _compound leaf_. Notice also that the two kinds of compound leaves correspond to the two kinds of veining and lobing, so that we have palmately and pinnately compound ones. In pinnate leaves the continuation of the common petiole along which the leaflets are ranged is called the _rhachis_.

Practical Questions

1. In selecting leaves for decorations that are to remain several
hours without water, which of the following would you prefer, and
why: smilax or Madeira vine (_Boussingaultia_); ivy or Virginia
creeper; magnolia or maple; maidenhair or shield fern (_Aspidium_)?
(173.)

2. Would you select very young leaves, or more mature ones, and why?

3. Can you name any parallel-veined leaves that have their margins
lobed, or indented in any way?

4. Which are the more common, parallel-veined or net-veined leaves?

5. Why do the leaves of corn and other grains not shrivel
lengthwise in withering, but roll inward from side to side? (173.)

6. Can you name any palmately veined leaves in which the secondary
veins are pinnate? Any pinnately veined ones in which the secondary
veins are palmate?

7. Lay one of each kind before you; try to draw a pinnate leaf with
palmate divisions. Do you see any reason now why these so seldom
occur in nature?

8. Name some advantages to a plant in having its leaves cut-lobed
or compound. (169.)

9. Mention some circumstances under which it might be advantageous
for a plant to have large, entire leaves. (169; Plate 9.)

10. How would the floating qualities of the leaves of the pond lily
be affected if their blades were cut-lobed or compound?

11. Do the leaves of the red cedar and arbor vitæ contribute to
their value as shade trees?

12. Name some of the favorite shade trees of your neighborhood; do
they, as a general thing, have their leaves entire, or lobed and
compound?

13. Which of the following are the best shade trees, and why: pine,
white oak, mimosa (_Albizzia_), sycamore, locust, horse-chestnut,
fir, maple, linden, China tree, cedar, ash?

14. Which would shade your porch best, and why: cypress vine,
grape, gourd, morning-glory, wistaria, clematis, smilax, kidney
bean, Madeira vine, rose, yellow jasmine, passion flower?

III. TRANSPIRATION

MATERIAL.—Leafy twigs of actively growing young plants. Sunflower,
corn, peach, grape, calla, and arums in general transpire rapidly;
thick-leaved evergreens and hairy or rough species, like mullein
and horehound more slowly. For Exp. 63, small-leaved, large-leaved,
and thick-leaved kinds will be needed.

APPLIANCES.—Glass jars and bottles with air-tight stoppers; a
little vaseline, oil, gardener’s wax, thread, cardboard, and a pair
of scales.

EXPERIMENT 62. TO SHOW WHY LEAVES WITHER.—Dry two self-sealing jars
thoroughly, by holding them over a stove or a lighted lamp for a
short time to prevent “sweating.” Place in one a freshly cut leafy
sprig of any kind, leaving the other empty. Seal both jars and set
them in the shade. Place beside them, but without covering of any
kind, a twig similar to the one in the jar. Both twigs should have
been cut at the same time, and their cut ends covered with wax or
vaseline, to prevent access of air. Look at intervals to see if
there is any moisture deposited on the inside of either jar. If
there is none, set them both in a refrigerator or cover with a wet
cloth and allow to cool for half an hour, and then examine again.
In which jar is there a greater deposit of dew? How do you account
for it? Take the twig out of the jar and compare its leaves with
those of the one left outside; which have withered the more, and
why?

EXPERIMENT 63. TO MEASURE THE RATE AT WHICH WATER IS GIVEN OFF BY
LEAVES OF DIFFERENT KINDS.—Fill three glass vessels of the same
size with water and cover with oil to prevent evaporation. Insert
into one the end of a healthy twig of peach or cherry; into the
second a twig of catalpa, grape, or any large-leaved plant, and
into the third, one of magnolia, holly, or other thick-leaved
evergreen, letting the stems of all reach well down into the water.
Care must be taken to select twigs of approximately the same size
and age, since the absorbent properties of very young stems are
more injured by cutting and exposure than those of older ones. All
specimens should be cut under water as directed in Exp. 58. Weigh
all three vessels, and at the end of twenty-four hours, weigh
again, taking note of the quantity of liquid that has disappeared
from each glass. This will represent approximately the amount
absorbed by the leaves from the twigs to replace that given off.
Which twig has lost most? Which least? Note the condition of the
leaves on the different twigs; have they all absorbed water about
as rapidly as they have lost it? How do you know this? Pluck the
leaves from each twig, one by one, lay them on a flat surface
that has been previously measured off, into square inches or
centimeters, and thus form a rough estimate of the area covered by
each specimen. Make the best estimate you can of the number of
leaves on each tree, and calculate the number of kilograms of water
it would give off at that rate in a day.

EXPERIMENT 64. THROUGH WHAT PART OF THE LEAF DOES THE WATER GET
OUT?—Take some healthy leaves of tulip tree, grape, tropæolum,
or any large, soft kind attainable. Cover with vaseline the
_leafstalk_ and _upper_ surface of one; the stalk and _under_
surface of a second; the stalk and _both_ surfaces of a third, and
leave a fourth one untreated. Suspend all four in a dry place by
means of a thread attached to the petioles so that both surfaces
may be equally exposed. The leaves must be all of the same species,
and as nearly as possible of the same age, size, and vigor, and
care must be taken that none of the vaseline is rubbed off in
handling. Examine at intervals of a few hours. Which of the leaves
withers soonest? Which keeps fresh longest? From what part would
you conclude, judging by this experiment, that the water escapes
most rapidly?

=179. Transpiration, nutrition, and growth.=—We learn from the foregoing, and from Exps. 58 and 59, that plants give off moisture very much as animals do by perspiration. The two processes must not be classed together, however, for they are physiologically different. The action, in plants, is called _transpiration_. It is usually assumed that a large amount of water must pass through the plant in order to bring to it the necessary supply of food material; but since the entrance of mineral salts is brought about by osmosis, conditioned by the living cells of the root; and since osmosis of salts may take place in a direction opposite to that of the greater movement of water, it follows that the entrance of salts is independent of transpiration.

Inasmuch, however, as a certain amount of water is necessary to bring the living cells into a condition of turgor (7) so that they may grow, it follows that there is a relation between transpiration and growth. If transpiration exceeds absorption for any length of time, the tissues will be depleted of their moisture, as is shown by the wilting of crops in dry, hot weather; and if the unequal movement continues long enough, the plant will die. Hence, a knowledge of the laws governing this important function is necessary to all who are interested in cultivating agricultural products.

=180. Magnitude of the work of transpiration.=—Few people have any idea of the enormous quantities of water given off by leaves. It has been calculated that a healthy oak may have as many as 700,000 leaves, and that 111,225 kilograms of water—equal to about 244,700 pounds—may pass from its surface in the five active months from June to October. At this rate 226 times its own weight may pass through it in a year, and it would transpire water enough during that time to cover the ground shaded by it to a depth of 20 feet![2] Lawn grass gives off water at such a rate that a vacant lot of 150 × 50 feet, if well turfed, would be capable of transpiring over a ton of water a day. Compare these figures with the average yearly rainfall in our Gulf States—53 inches, approximately—and you can form some estimate of the injury done to a growing crop from this cause alone. The moisture is drawn from the surface by shallow rooted weeds (81) and dissipated through the leaves. In the case of forest trees the effect is different. Their roots, striking deep into the soil, draw up water from the lower strata and distribute it to the thirsty air in summer.

As the water given off by transpiration is in the form of vapor, it must draw from the plant the amount of heat necessary for its vaporization, and thus has the effect of making the leaves and the air in contact with them cooler than the surrounding medium. At the same time the coolness and moisture of the air tend to check the loss by evaporation from the surface soil. It is partly to this cause, and not alone to their shade, that the coolness of forests is due. Measurements at various weather bureau stations in the United States show that in summer the temperature of oak woods is 4° C. lower during the day than in the open, and as much higher at night. In a beech wood in Germany the difference between the forest and the general temperature amounted to as much as 7° C.

Practical Questions

1. Is there any foundation in fact for the accounts of “weeping
trees” and “rain trees” that we sometimes read about in the papers?
(180; Exp. 48.)

2. Can you explain the fact, sometimes noticed by farmers, that in
wooded districts, springs which have failed or run low during a dry
spell sometimes begin to flow again in autumn when the trees drop
their leaves, even though there has been no rain? (180; Exp. 63.)

3. Other things being equal, which would have the cooler,
pleasanter atmosphere in summer, a well-wooded region or a treeless
one? (180.)

4. Could you keep a bouquet fresh by giving it plenty of fresh air?
(Exp. 62.)

5. Why does a withered leaf become soft and flabby, and a dried one
hard and brittle? (7; Exp. 62.)

6. Why do large-leaved plants, as a general thing, wither more
quickly than those with small leaves? (Exp. 63.)

7. Is the amount of water absorbed always a correct indication of
the amount transpired? Explain. (179.)

8. Explain the difference between the withering caused by excessive
transpiration and the shrinkage of cells due to plasmolysis. Are
both of these physiological processes?

9. Why is it best to trim a tree close when it is transplanted?
(179, 180.)

10. Why should transplanting be done in winter or very early
spring, before the leaves appear? (180.)

IV. ANATOMY OF THE LEAF

MATERIAL.—For study of the epidermis, leaves of the white garden
lily (_Lilium album_) are best, as the stomata can be seen on
their lower surface with the naked eye. Wandering Jew, Spanish
bayonet (_Yucca aloifolia_), anemone, narcissus, iris, canna, show
them under a hand lens, but less distinctly. For sections, beet,
mustard, and beech leaves may be used, or ready-mounted specimens
obtained of a dealer.

A compound microscope is needed for a minute study of the leaf
structure.

=181. Stomata.=—It was shown in Exp. 64 that the water of transpiration escapes most rapidly, as a general thing, from the under surface of leaves. To find out why this is so, a careful study of the epidermis will be necessary. For this purpose procure, if possible, the leaf of a white garden lily (_Lilium album_), wandering Jew, Spanish bayonet (_Yucca aloifolia_), anemone, narcissus, iris, or canna. The first-named is preferable, as the transpiration pores can be seen on it with the naked eye. Examine the under surface with a hand lens, and you will see that it is covered with small eye-shaped dots like those shown in Figs. 218 and 219. Strip off a portion of the epidermis, hold it up to the light on a piece of moistened glass, and they can be seen quite clearly with a lens. These dots are the pores through which the water vapor escapes in transpiration, and through which air finds its way into the tissues of the leaf. They are called _stomata_ (sing., _stoma_), from a Greek word meaning “a mouth.” Look for stomata on the upper epidermis; do you find any, and if so, are there as many as on the under surface? Do you see any relation between this fact and the results obtained from Exp. 64? Can you see any good reasons why the stomata should be placed on the under side in preference to the upper? Are they as much exposed to excessive light and heat, or as liable to be choked by dust, rain, and dew here as on the upper side?

=182. Distribution of stomata.=—While stomata are generally more abundant on the under side of leaves, this is not always the case. In vertical leaves, like those of the iris, which have both sides equally exposed to the sun, they are distributed equally on both sides. In plants like the water lily, where the under surface lies upon the water, they occur only on the upper side. Succulent leaves, as a general thing, have very few, because they need to conserve all their moisture. Submerged leaves have none at all; why?

=183. Minute study of a leaf epidermis.=—Place a bit of the lower epidermis of a leaf under the microscope, and examine with a high power. It will appear, if a monocotyl, to be composed of long, flat, rectangular spaces (Fig. 221); if the leaf of a dicotyl is used, they will be more or less irregular (Fig. 220), with the outlines fitting into each other like the tiling of a floor or the blocks of a Chinese puzzle. These spaces are the cells of the epidermis, and the lines are the cell walls. Can you find any of the cell contents? The cell sap is not often visible; do you see the nuclei? Can you give a reason why the epidermal cells are so thin and flat? Between some of the cells you will see two kidney-shaped bodies placed with their concave sides together so as to leave a lenticular opening between them. This is a _stoma_, and the kidney-shaped bodies (Figs. 218, 219) are _guard cells_. They are given this name because they open or close the mouth of the stoma. If you will imagine a toy balloon made in the form of a hollow ring, like the tire of a bicycle, you can easily see, from Figs. 218, 219, that when the ring is strongly inflated, it will expand, and in enlarging its own circumference, will at the same time increase the diameter of the opening in the center. When the expansive force is removed, it collapses, thus closing, or greatly reducing, the aperture.

In the same way the guard cells, when there is abundance of water in them, expand, thus opening the stoma so that the water vapor passes out more readily. But when there is a dearth of moisture, or when, by reason of chemical action in the soil, the roots fail to supply it, the leaves wilt, the guard cells, losing their water, collapse, closing the pore, and transpiration is thus prevented or greatly retarded. (Fig. 222.)

Sketch a portion of the epidermis as it appears under the microscope, labeling the parts. If stomata can be found in both conditions, make sketches showing them both open and closed.

=184. Internal structure of a leaf.=—Roll a leaf blade, or fold it tightly to facilitate cutting, and with a scalpel, or a very sharp razor, cut the thinnest possible slice through the roll. This will give a section at right angles to the epidermis. It should be so thin as to appear almost transparent. Put a small bit of a section in a drop of water on a slide, place under the microscope, using a high power, and look for the parts shown in Fig. 223. Notice the horizontally flattened cells of the upper epidermis, _e_, and of the lower epidermis, _e′_; also the vertically elongated palisade cells, _p_, filled with particles of green coloring matter. These particles are the chlorophyll bodies, to which the green color of the leaf is due. They are the active agents in the manufacture of plant food, and in a leaf removed from the plant during the day time and viewed under a high power, the chlorophyll bodies, on treatment with iodine, will be seen to contain granules of starch which they are in the act of elaborating. The collecting cells, _t_, receive the assimilated product from the palisade cells and pass it on through the spongy parenchyma, _sch_, to the fibrovascular bundles. Notice how much more abundant the green matter is in the upper part of the leaf than in the lower; has this anything to do with the deeper color of the upper surfaces of leaves? Notice the opening, _st_, lower epidermis; do you recognize it? (See Fig. 222.) It is a stoma, seen in vertical section. Notice the intercellular air spaces, _i_, _i_, in the spongy parenchyma, and the much larger one, _a_, just behind the stoma. Why is this last so much larger?

Sketch the section of your specimen as it appears under the microscope. It will perhaps differ in some details from the one shown in the figure, but you can recognize and label the corresponding parts. Be sure that your drawing represents accurately the relative size and shapes of the different kinds of cells.

It is in the upper surface, where the chlorophyll particles abound, that the manufacture of food goes on most actively, and from the under surface, where the stomata are situated, that transpiration takes place and air and other gases pass to and from the interior. These facts have important bearings on the growth and external characters of leaves.

Practical Questions

1. Explain why a plant cannot thrive if its stomata are clogged
with foreign matter. (179; Exp. 64; 184.)

2. Mention some of the ways in which this might happen. (181.)

3. Why must the leaves of house plants be washed occasionally to
keep them healthy? (179, 181.)

4. Why is it so hard for trees and hedges to remain healthy in a
large manufacturing town?

V. FOOD MAKING

MATERIAL.—A sprig of pondweed, mare’s-tail (_Hippuris_), hornwort
(_Ceratophyllum_), marsh St.-John’s-wort (_Elodea_), or other green
aquatic plant; bean or tropæolum, or other green leaves gathered
from plants growing in the sunshine; a healthy potted plant; a
small, fresh cutting.

APPLIANCES.—A shallow dish of water and two glass tumblers or
wide-mouthed jars; a bent glass or rubber tube; a piece of black
cloth or paper; a half pint of alcohol; iodine solution; a glass
funnel or a long-necked bottle from which the bottom has been
removed.

EXPERIMENT 65. IS THERE ANY RELATION BETWEEN SUNLIGHT AND THE GREEN
COLOR OF LEAVES?—Place a seedling of oats, or other rapidly growing
shoot, in the dark for a few days, and note its loss of color.
Leave it in the dark indefinitely, and it will lose all color and
die. Hence we may conclude that there is some intimate connection
between the action of light and the green coloring matter of leaves.

EXPERIMENT 66. DO LEAVES GIVE OFF ANYTHING ELSE BESIDES
WATER?—Submerge a green water plant, with the cut end uppermost,
in a glass vessel full of water, and invert over it a glass
funnel, or a long-necked bottle from which the bottom has been
removed as directed in Exp. 53. Expel the air from the neck of the
funnel—or bottle—by submerging and corking under water so as to
make it air-tight. Place in the sunlight and notice the bubbles
that begin to rise from the cut end of the plant. When they have
partly filled the neck of the funnel, remove the stopper and thrust
in a glowing splinter. If it bursts into flame, or glows more
brightly, what is the gas that was given off? (Exp. 22.)

As oxygen is not a product of respiration, some other process must
be at work here, during which oxygen is set free, and some other
substance used up. (Exps. 24 and 25.)

EXPERIMENT 67. WHAT IS THE SUBSTANCE TAKEN IN WHEN OXYGEN IS GIVEN
OFF?—Fill two glass jars, or two tumblers, with water, to expel the
air, and invert in a shallow dish of water, having first introduced
a freshly cut sprig of some healthy green plant into one of them.
Then, by means of a bent tube, blow into the mouth of each tumbler
till all the water is expelled by the impure air from the lungs.
Set the dish in the sunshine and leave it, taking care that the
end of the cutting is in the water of the dish. After forty-eight
hours remove the tumblers by running under the mouth of each,
before lifting from the dish, a piece of glass well coated with
vaseline (lard will answer), and pressing it down tight so that no
air can enter. Place the tumblers in an upright position, keeping
them securely covered. Fasten a lighted taper or match to the end
of a wire, plunge it quickly first into one tumbler, then into the
other, and note the result. What was the gas blown from your lungs
into the jars? (Exps. 23, 24.) Why did the taper not go out in the
second jar? What had become of the carbon dioxide?

EXPERIMENT 68. TO SHOW THAT LIGHT IS NECESSARY FOR A PLANT TO
ABSORB CARBON DIOXIDE AND GIVE OFF OXYGEN.—Repeat Exp. 66, keeping
the plant in a dark or shady place; do you see any bubbles? Test
with a glowing match; is any oxygen formed in the tube of the
funnel? Move back into the sunlight and leave for a few hours; what
happens when you thrust a glowing splinter into the tube?

EXPERIMENT 69. IS ANY FOOD PRODUCT FOUND IN LEAVES?—Crush a few
leaves of bean, sunflower, or tropæolum, and soak in alcohol until
all the chlorophyll is dissolved out. Rinse them in water, and soak
the leaves thus treated in a weak solution of iodine for a few
minutes, then wash them and hold them up to the light. If there
are any blue spots on the leaves, what are you to conclude? If a
test for sugar is to be made, use sap pressed from fresh leaves;
for oils and fats, leaves should be dried without being placed in
alcohol.

EXPERIMENT 70. HAS THE PRESENCE OR ABSENCE OF LIGHT ANYTHING TO DO
WITH THE OCCURRENCE OF STARCH IN LEAVES?—Exclude the light from
parts of healthy leaves on a growing plant of tropæolum, bean,
etc., by placing patches of black cloth or paper over them. Leave
in a bright window, or preferably out of doors, for several hours,
and then test for starch as in the last experiment; do you find any
in the shaded spots?

EXPERIMENT 71. IS THE PRESENCE OF AIR NECESSARY FOR THE PRODUCTION
OF STARCH?—Cover the blades and the petioles of several leaves with
vaseline or other oily substance so as to exclude the air, and
after a day or two test as before.

=185. Influence of plants on the atmosphere.=—These experiments show that leaves cannot do their work without light and air. The particular element of the atmosphere used by them in the process of food making is carbon dioxide. Their action in absorbing this gas and giving off oxygen tends to counterbalance the opposite action of respiration, decomposition, and combustion of all kinds, by which the proportion of it in the atmosphere tends to be constantly increased. In this way they help to regulate the quantity of it present and have a beneficial effect in ridding the air of one source of impurity.

=186. Photosynthesis.=—In our examination of the internal structure of the leaf, the chlorophyll bodies (184) were found to contain small granules of starch which the chlorophyll, under the stimulus of light, had elaborated as a nutriment for the plant tissues. Hence, the leaf may be regarded as a factory in which vegetable food, mainly starch, is manufactured out of the water brought up from the soil, and the carbon dioxide derived through the stomata from the atmosphere. In this process carbon dioxide (CO_{2}) is combined with water (H_{2}O) in such proportions that part of the oxygen is returned to the surrounding air. This is a fundamental food-forming process characteristic of green plants, and can take place only in the light. For this reason it has been named _Photosynthesis_, a word which means “building up by means of light,” just as _photography_ means “drawing or engraving by means of light.”

In carrying on the operation of photosynthesis, sunshine is the power, the chlorophyll bodies the working machinery, carbon dioxide and water the raw materials, and starch or oil the finished product, while oxygen and the water of transpiration represent the waste or by-products.

=187. How the new combination is effected.=—It may seem strange that a gas and a liquid should combine to make something so different from either as starch, but their chemical constituents are the same in different proportions. Water is made up of 2 parts hydrogen and 1 part oxygen; carbon dioxide, of 1 part carbon and 2 parts oxygen, while starch contains carbon, hydrogen, and oxygen, in the ratios of 6, 10, and 5, respectively. Hence, by taking sufficient quantities of water and carbon dioxide and combining them in the proper proportions, the leaf factory can turn them into starch. If we use the letters C, H, and O, to represent Carbon, Hydrogen, and Oxygen, respectively, the new combination of materials can be expressed by an equation; thus:—

_water_ _carbon dioxide_ _starch_ _by-products_
5(H_{2}O) + 6(CO_{2}) = (C_{6}H_{10}O_{5}) + 6(O_{2}) = 12(O).

The water not used up in the process is given off as a waste product in transpiration, while the oxygen is returned to the air, as shown by Exp. 66. This equation is not to be understood as representing the chemical changes that actually take place in the leaf. These are too complicated, and at present too imperfectly known, to be considered here. It will serve, however, to give a fair idea of the final result from the process of photosynthesis, however brought about.

Simple as the operation appears, the chemist has not, as yet, been able to imitate it. He can analyze starch into its original constituents, but while he has the ingredients at hand in abundance, and knows the exact proportions of their combination, it is beyond his power, in the present state of our knowledge, to put them together. Hence, both man and the lower animals are dependent on plants for this most important food element. The so-called factories that supply the starch of commerce do not _make_ starch any more than the miller makes wheat, but merely separate and render available for use that already elaborated by plants.

=188. Proteins.=—Foods of this class are mainly instrumental in furnishing material for the growth and repair of the tissues out of which the bodies of both plants and animals are built up. They embrace a great variety of substances, but their chemical nature is very complex and very imperfectly understood. Nitrogen is an important element in their composition, whence they are commonly distinguished as “nitrogenous foods.” Besides nitrogen, there are present carbon, hydrogen, oxygen, and sulphur, and traces of the mineral salts absorbed from the soil are found in varying quantities in the ash of different proteins. The percentages in which these ingredients are combined and the processes concerned in their formation are at present a matter of pure hypothesis. Botanists are not agreed even as to whether they are made in the leaf or in some other part or parts of the plant, though the weight of opinion inclines to the view that their construction takes place in the leaf.

=189. The activities of leaves.=—As there are only 4 parts of CO_{2} to every 10,000 parts of ordinary free air, it has been estimated that in order to supply the leaf factory with the raw material it needs, an active leaf surface of one square meter—a little over one square yard—uses up, during every hour of sunshine, the CO_{2} contained in 1000 liters (1000 quarts, approximately) of air. Suppose an oak tree to bear 500,000 leaves, each having a surface of 16 sq. cm., or 4 sq. in., and working 12 hours a day for 6 months in the year; you will then have some idea of the enormous quantity of air that passes each season through its leaf system. Add to this the almost incredible volume of water transpired in the same time (180), and we may well stand amazed at the tremendous activities of these silent workers that we are in the habit of regarding as mere passive elements in the general landscape.

=190. The economic value of leaves.=—Besides their importance as sanitary and food-making agencies, leaves have a direct commercial value as food products in the hay and fodder they supply for our domestic animals, the tea and salads with which they provide our tables, the aromatic flavors and seasonings contained in them, and the drugs, medicines, and dyes of various kinds for which they furnish the ingredients.

Practical Questions

1. Why do gardeners “bank” celery? (Exp. 65.)

2. Why are the buds that sprout on potatoes in the cellar, white?
(Exp. 65.)

3. Why does young cotton look pale and sickly in long-continued wet
or cloudy weather? (Exp. 65.)

4. Why do parasitic plants generally have either no leaves or very
small, scalelike ones? (85, 186, 187.)

5. The mistletoe is an exception to this; explain why, in the light
of your answer to question 4.

6. Could an ordinary nonparasitic plant live without green leaves?
(186, 187.)

7. Are abundance and color of foliage any indication of the health
of a plant? (186, 187; Exp. 65.)

8. Is the practice of lopping and pruning very closely, as in the
process called “pollarding,” beneficial to a tree under ordinary
conditions? (186, 189; Exp. 63.)

9. Name some plants of your neighborhood that grow well in the
shade.

10. Compare in this respect Bermuda grass and Kentucky blue
grass; cotton and maize; horse nettle (_Solanun Carolinense_) and
dandelion; beech, oak, red maple, dogwood, pine, cedar, holly,
magnolia, etc.

11. Name all the aromatic leaves you can think of; all that are
used as food, beverages, drugs, and dyes.

12. What is the use of aromatic and medicinal leaves to the plant
itself? (Suggestion: Why does the housewife put lavender or tobacco
leaves in her woolen chest?)

13. Which would be richer in nourishment, hay cut in the evening or
in the morning, and why? (54, 186; Exp. 70.)

14. Mention three important sanitary services that are rendered by
a tree like that shown in plate 6 or 8. (180, 185, 189.)

15. Name some of the plants employed in the manufacture of starch.

VI. THE LEAF AN ORGAN OF RESPIRATION

MATERIAL.—A number of vigorous, freshly cut green leaves; a liter
or two (one or two quarts) of expanding flower or leaf buds.

APPLIANCES.—Some wide-mouthed jars of one or two liters’ capacity;
two small open vials of limewater.

EXPERIMENT 72. DO LEAVES GIVE OFF CARBON DIOXIDE?—Cover the
bottoms of two wide-mouthed jars with water about two centimeters
(1 inch) deep. Place in one a number of healthy green leaves with
their stalks in the water, and insert among them a small open
vial containing limewater. In the other jar place only a vial of
limewater in the clear water at the bottom, this last being merely
to make the conditions in both vessels the same. Seal both tight
and keep together in the dark for about 48 hours, and then examine.
In which jar does the limewater indicate the greater accumulation
of CO_{2}? (It may show a slight milkiness in the other vessel
due to gas derived from the inclosed air and water.) From this
experiment, what process would you conclude has been going on among
the leaves in jar No. 1? (Exp. 25.)

EXPERIMENT 73. IS THE EXHALATION OF CARBON DIOXIDE ACCOMPANIED
BY ANY OTHER CONCOMITANT OF RESPIRATION?—In Exps. 24, 25, it was
shown that respiration is accompanied by heat; hence, if the
production of carbon dioxide by the leaf is due to this cause, it
should be attended by the evolution of heat. To find out whether
this is the case, partly fill a glass jar of two liters’ capacity
with unfolding leaf buds arranged in layers alternating with damp
cotton batting or blotting paper (Fig. 228); close the jar tightly
and leave from 12 to 24 hours in the dark to prevent the action
of photosynthesis. Then insert a thermometer and note the rise in
temperature. If a lighted taper is plunged in, it will quickly be
extinguished, showing that respiration has been going on.

=191. Respiration in leaves.=—We see from experiments like the foregoing that the leaf, besides carrying on the functions of digestion, photosynthesis, and transpiration, is also an active agent in the work of respiration. In this function oxygen is used up and carbon dioxide given off, just as in the respiration of animals; but the process is so slow in plants that it is much more difficult to detect than the contrary action in photosynthesis, and is, in fact, not perceptible at all while the latter is going on, though it does not cease even then.

But while the leaf is the principal organ of respiration, the process is carried on in other parts of the plant as well, else it could not survive during the leafless months of winter. It _appears_ to be most active at night, but this is only because it is not obscured then, as during the day, by the more active function of photosynthesis. Indeed, it was for a long time supposed that plants “breathed” only at night, and it was thought to be unwholesome to keep them in a bedroom. It is now known, however, that respiration goes on at all times and in all living parts of the plant, but the quantity of oxygen taken in is so small from a hygienic point of view that it may be disregarded.

=192. Distinctions between respiration and photosynthesis.=—While these two functions are contrasting and antipodal, so to speak, in their action, they are mutually complementary and interdependent, the one manufacturing food and the other using it up, or rather marking the activity of those life processes by which it is used up. The difference between them will be made clear by a comparison of the two processes as summarized in the following statement:

PHOTOSYNTHESIS RESPIRATION

Goes on only in sunlight and in Goes on at all times and in all
the green parts of plants. parts of the plant.

Produces starch and sugar. Releases energy (heat and working
power).

Gives off, as by-product, oxygen. Gives off, as by-products, CO₂
and water.

A constructive process, in which A destructive, or consumptive
energy is used up to make food. process, in which food is used up
in expending energy.

=193. Metabolism.=—The total of all the life processes of plants, including growth, waste, repair, etc., is summed up under the general term _metabolism_. It is a _constructive_ or building-up process when it results in the making of new tissues out of food material absorbed from the earth and air, and the consequent increase of the plant in size or numbers. But, as in the case of animals, so with plants, not all the food provided is converted into new tissue, part being used as a source of energy, and part decomposed and excreted as waste. In this sense, metabolism is said to be _destructive_. The waste in healthy growing plants is always, of course, less than the gain, and a portion of the food material is laid by as a reserve store. For this reason, photosynthesis, being a constructive process, is usually more energetic than respiration, which is the measure of the destructive change of materials that attends all life processes.

It is evident also, from what has been said, that growth and repair of tissues can take place only so long as the plant has sufficient oxygen for respiration, since the energy liberated by it is necessary for the assimilation of nourishment by the tissues.

Thus we see that plants are dependent on air not only for respiration, but for nutrition, and none of their life processes can be carried on without it.

Practical Questions

1. Can a plant be suffocated, and if so, in what ways? (87, 193;
Exps. 26, 27.)

2. The roots on the palm shown in plate 3 are not drawing any sap
from it as parasites; why does their continued growth bring about
the death of the tree? (87, 193.)

3. Is it unwholesome to keep flowering plants in a bedroom? Leafy
ones? Why, in each case? (191.)

4. Would there be any more reason for objecting to the presence of
flowers by night than by day? Explain. (191.)

5. Why is respiration much less marked in plants than in animals?
(30, 31.)

VII. THE ADJUSTMENT OF LEAVES TO EXTERNAL RELATIONS

MATERIAL.—A potted plant of oxalis, spotted medick, white clover,
or other sensitive species. The subject is better suited for
outdoor observation than for laboratory work.

EXPERIMENT 74. TO SHOW THAT LEAVES ADJUST THEMSELVES TO CHANGES IN
INTENSITY OF LIGHT.—Keep a healthy potted plant of oxalis, white
clover, or spotted medick in your room for observation. Note the
daily changes of position the leaves undergo. Sketch one as it
appears at night and in the morning.

In order to determine whether these changes are due to want of
light or of warmth, put your plant in a dark closet in the middle
of the day, without change of temperature. After several hours note
results. Transfer to a refrigerator, or in winter place outside a
window where it will be exposed to a temperature of about 5° C.
(40° F.) for several hours, and see if any change takes place. Next
put it at night in a well-lighted room and note the effect. If
practicable, keep a specimen for several weeks in some place where
electric lights are burning continuously all night, and watch the
results.

EXPERIMENT 75. TO SHOW THAT THE FALL OF THE LEAF MAY RESULT FROM
OTHER CAUSES THAN COLD OR FROST.—Wrap some leaves of ailanthus,
Kentucky coffee tree, ash, walnut, or hickory in a damp towel and
keep them in the dark for several days; the leaflets will fall
away, leaving a clear scar like those on winter twigs.

EXPERIMENT 76. TO SHOW THAT ADJUSTMENTS TO TEMPERATURE MAY BE MADE
BY CHEMICAL MEANS.—Place a small twig of oleander, laurestinus,
or other broad-leaved evergreen in a 5 to 10 per cent solution of
sugar, and transfer it at the end of a few days to a temperature of
6° to 8° below freezing. On comparison with a similar twig that has
stood for the same length of time in pure water, it will be found
to possess a greater power of resistance to cold.