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.
[Illustration: FIG. 177.—A typical leaf and its parts: _b_, blade;
_p_, petiole; _s_, _s_, stipules.]
[Illustration: FIG. 178.—Spiny stipules of clotbur.]
=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.)
[Illustration: FIG. 179.—Adnate stipules of clover. FIG. 180.—Leaves
of smilax, showing stipular tendrils. FIG. 181.—Leafy stipules of
Japan quince.]
=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.
[Illustration: FIGS. 182-187.—Petioles, and leaf attachment: 182,
petioles of jasmine nightshade (_Solanum jasminoides_) acting as
tendrils; 183, acacia, showing petiole transformed to leaf blade;
184, sessile leaves of epilobium; 185, clasping leaf of lactuca; 186,
perfoliate leaves of uvularia; 187, peltate leaf of tropæolum. (182
and 186 _after_ GRAY.)]
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.
[Illustration: FIG. 188.—Whorled leaves of Indian cucumber.]
[Illustration: FIG. 189.—Twig of a hackberry (_Celtis cinerea_),
showing the two-ranked arrangement. Notice how the position of the
stems and branches of the main axis corresponds to that of the
leaves.]
=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.
[Illustration: PLATE 9.—Vegetation of a moist, shady ravine. Notice
the expanded surface of the leaf blades and the long internodes that
separate the individual leaves. (From Rep’t. Mo. Botanical Garden.)]
[Illustration: FIG. 190.—Narrow leaves in crowded vertical rows.]
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.
[Illustration: FIGS. 191-197.—Shapes of leaves: 191, lanceolate; 192,
spatulate; 193, oval; 194, obovate; 195, kidney-shaped; 196, deltoid;
197, lyrate. (191-195 _after_ GRAY.)]
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?
[Illustration: FIGS. 198-202.—Margins of leaves: 198, serrate; 199,
dentate; 200, crenate; 201, undulate; 202, sinuate. (_After_ GRAY.)]
[Illustration: FIGS. 203, 204.—Leaves of paper mulberry tree: 203,
leaf from an old tree; 204, leaf from a two-year-old sprout.]
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.
[Illustration: FIG. 205.—Parallel-veined leaf of lily of the valley
(_After_ GRAY).]
[Illustration: FIG. 206.—Net-veined leaf of a willow.]
[Illustration: FIG. 207.—Pinnately parallel-veined leaf of calla lily
(_After_ GRAY).]
[Illustration: FIG. 208.—Palmately net-veined leaf of wild ginger.]
=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.
[Illustration: FIG. 209.—Ribbed leaf of plantain.]
=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_.
[Illustration: FIG. 210.—Pinnately lobed leaf of horse nettle. FIG.
211.—Palmately lobed leaf of grape.]
[Illustration: FIG. 212.—Palmately parted leaf of a buttercup. FIG.
213.—Pinnately compound leaf of black locust.]
[Illustration: FIG. 214.—Palmately compound leaf of horse-chestnut.
FIG. 215.—Pinnately trifoliolate leaf of a desmodium. FIG.
216.—Palmately trifoliolate leaf of wood sorrel.]
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.
[Illustration: FIG. 217.—A “weeping tree,” showing the effect where
absorption exceeds transpiration. Notice the position of the tree
near the water where the roots have unlimited moisture. (_After_
FRANCÉ.)]
=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.
[Illustration: FIGS. 218, 219.—Stomata of white lily leaf: 218,
closed; 219, open. (_After_ GRAY.)]
=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?
[Illustration: FIG. 220.—A small piece of the under epidermis of
an oak leaf, highly magnified to show the stomata, _g_, and minute
hairs, _h_.]
[Illustration: FIG. 221.—Under epidermis of an oat leaf, showing
stomata.]
=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.
[Illustration: FIG. 222.—Outline of a stoma of hellebore in vertical
section. The darker lines show the shape assumed by the guard cells
when the stoma is open; the lighter lines, when the stoma is closed.
The cavities of the guard cells with the stoma closed are shaded, and
are distinctly smaller than when the stoma is open.]
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?
[Illustration: FIG. 223.—Transverse section through a leaf of beet:
_e_, upper epidermis; _e′_, lower epidermis; _st_, stoma; _a_, air
space; _p_, palisade cells; _t_, collecting cells; _sch_, spongy
parenchyma; _i_, _i_, intercellular air spaces; _Fbv_, section of a
vein (fibrovascular bundle).]
[Illustration: FIG. 224.—Chlorophyll bodies containing starch grains
in the course of formation. Magnified 250 times.]
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.)
[Illustration: FIG. 225.—Experiment showing that green plants give
off oxygen in sunlight.]
[Illustration: FIG. 226.—Experiment for showing that leaves absorb
carbon dioxide from the atmosphere.]
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.
[Illustration: FIG. 227.—Leaf arranged with a piece of tin foil to
exclude light from a portion of the surface.]
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.)
[Illustration: FIG. 228.—Arrangement of apparatus to show that heat
and carbon dioxide are given off by leaf buds.]
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.
[Illustration: FIGS. 229, 230.—Leaves of a peanut plant: 229, in day
position; 230, in night position.]
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.
[Illustration: FIG. 231.—A plant that has been growing near an open
window, showing the leaves all turned toward the light.]
=194. The light relation.=—The principal external conditions to which
leaves have to adjust themselves are light, air, moisture, gravity,
temperature, and the attacks of animals. From the knowledge of their
work and function gained in the preceding sections, it will be clear
that the primary relation of the leaf is a light relation, and to
this, first of all, it must adjust itself.
It was shown in Exps. 56 and 57 how promptly leaves respond to
changes in the direction of light, and a little observation (Exp.
74) will convince us that they are equally sensitive to changes in
intensity and periodicity of illumination.
=195. Phototropism.=—The movement of plants in response to light is
called _phototropism_—a word that means “turning toward or away from
light.” It includes all kinds of light adjustments, and examples of
it are to be met with everywhere in the disposition of leaves with
reference to their light exposure.
=196. Horizontal and vertical adjustment.=—Take two sprigs, one
upright, the other horizontal, from any convenient shrub or tree—and
notice the difference in the position of the leaves. Examine their
points of attachment and see how this is brought about, whether by a
twist of the petiole or of the base of the leaf blades, or by a half
twist of the stem between two consecutive leaves, or by some other
means.
[Illustration: PLATE 10.—A mosaic of moonseed leaves, showing
adjustment for light exposure.
(_From_ Mo. Botanical Garden Rep’t.)]
Observe both branches in their natural position; what part of the
leaf is turned upward, the edge or the surface of the blade? Change
the position of the two sprigs, placing the vertically growing one
horizontal, and the horizontal one vertical. What part of the leaves
is turned upward in each?
[Illustration: FIGS. 232, 233.—Adjustment of leaves to different
positions: 232, upright; 233, procumbent.]
[Illustration: FIG. 234.—Leaf mosaic of elm.]
=197. Leaf mosaics.=—Trees with horizontal or drooping branches,
like the elm and beech, and vines growing along walls or trailing
on the ground, generally display their foliage in flat, spreading
layers, each leaf fitting in between the interstices of the others
like the stones in a mosaic, whence this has been called the _mosaic_
arrangement. (Plate 10.) In plants of more upright or bunchy habit,
the leaves are placed at all angles, giving the appearance of a
rosette when viewed from above, whence this is called the _rosette_
arrangement.
A variety of the same disposition is seen in the pyramidal shape
assumed by plants with large, undivided leaves like the mullein and
burdock (Fig. 237), in which access of light is secured by a mutual
adjustment between the size and position of leaves, the upper ones
becoming successively smaller.
=198. Heliotropism=—“turning with the sun”—is the name given to the
daily movement of plants like the cotton and sunflower in turning
their leaves or their blossoms to face the sun. If you live where
cotton is grown, notice the leaves in a field about ten o’clock on
a bright sunny morning, and again from the same point of view at
about four or five in the afternoon. Do you perceive any difference
in their general disposition? Watch on a cloudy day and see if any
change takes place. Find out by observation whether the “heliotrope”
of the hothouses is really heliotropic.
[Illustration: FIGS. 235, 236.—Horse-chestnut leaves: 235, leaf
rosette seen from above; 236, the same seen sidewise, showing the
formation of rosettes by the lengthening of the lower petioles.]
[Illustration: FIG. 237.—Leaf pyramid of mullein.]
[Illustration: FIGS. 238, 239.—A compass plant, rosinweed (_Silphium
laciniatum_): 238, seen from the east; 239, seen from the south.]
=199. Adjustment against too great intensity of light.=—Plants
frequently have to protect themselves against excess of light and
heat. An interesting example of this kind of adjustment is furnished
by the rosinweed, or compass plant (_Silphium laciniatum_, Figs. 238,
239), which grows in the prairies of Alabama and westward, where it
is exposed to intense sunlight. The leaves not only stand vertical,
but have a tendency to turn their edges north and south so that the
blades are exposed only to the gentler morning and evening rays. The
prickly lettuce manifests the same habit in a less marked degree.
[Illustration: FIGS. 240, 241.—A plant of the guayule (_Parthenium
argentatum_), to the leaves of which indexes have been affixed to
show their day and night position: 240, day position; 241, night
position. (_From_ photographs by Prof. F. E. Lloyd.)]
=200. Night and day adjustments.=—These are movements in response to
changes in the degree of illumination and temperature, as evidenced
by the fact that they become feeble and soon cease altogether if
the plant is kept a sufficient time under uniform conditions as
to these two factors. (Exp. 74.) They are called “nyctitropic” or
sleep movements, because they are most obvious in certain plants
that undergo periodic adjustments to the alternations of day and
night suggestive of an imaginary likeness to the sleep of animals.
Examples are most frequently met with among members of the pea
family (_Leguminosæ_), the spurges (_Euphorbiaceæ_), and the sorrel
(_Oxalis_) family. They are found among other species also, and
indeed are much more general than is usually supposed, most plants
showing signs of them if carefully tested. A simple way of doing
this is by attaching bristles about two inches long to the tips of
two leaves on opposite sides of the stem, as in Figs. 240, 241,
and comparing the divergence of the bristles during the day and at
nightfall. In this way a change of position in the leaves, too
slight to attract attention otherwise, will be made apparent. The
positions assumed vary in different plants, and even in the parts of
the same compound leaf; in the kidney bean, for instance, the common
petiole turns up at night, while the individual leaflets turn down.
One of the common pigweeds (_Amaranthus Palmeri_, Figs. 242-244) is
heliotropic in the day time and nyctitropic at night.
[Illustration: FIGS. 242-244.—Showing the movements of _Amaranthus
Palmeri_: 242, 243, position at sunrise and sunset (heliotropic);
244, night position (nyctitropic) half an hour after sunset. (_From_
photographs by Prof. F. E. Lloyd.)]
The very striking nyctitropic adjustments of the wild senna (_Cassia
tora_) photographed by Professor Francis E. Lloyd of the Alabama
Polytechnic Institute (Figs. 245-250), though obviously influenced by
the sun, are not directed toward it as in those of truly heliotropic
plants.
[Illustration: FIGS. 245-250.—Wild senna (_Cassia tora_), showing the
nyctitropic adjustments of its leaves. The upper figures show their
horizontal arrangement; those below, the vertical: 245, 248, position
of the leaves at 9 A.M.; 246, 249, at 3 P.M.; 247, 250, at 6.30 P.M.
(_From_ photographs by Prof. F. E. Lloyd.)]
These movements are common also among flowers, many of them having
regular hours for opening and closing, as indicated by such names as
“morning-glory” and “four-o’clock.” In these cases, however, other
causes (277, 280) than the light relation must be taken into account.
=201. Irritability= is a general term applied to the power in plants
of receiving and responding by spontaneous movements to impressions
from without. In its widest acceptation, irritability includes,
besides the various forms of adjustment described in this section and
the next, all movements due to geotropism, those of roots seeking
air and moisture, the revolution of twining stems and tendrils,
the circulation of protoplasm in the cell—any movement, in short,
that is made in response to an impression from the environment is a
manifestation of irritability. It may be of various degrees, but is
possessed to some extent by every living vegetable organism.
The term is usually applied, however, more especially to those
obvious and pronounced responses made by plants to their
surroundings, as exemplified in the cases just given. Still more
marked instances are to be found in the movements of the tentacles
of insectivorous plants, and the sensitive leaflets of the mimosa
that close at the slightest touch. The tendrils of the passion flower
are said to appreciate and respond to a pressure that cannot be
distinguished even by the human tongue, and many plants will detect
and respond to the ultra-violet rays of light, which are entirely
invisible to man.
This faculty of irritability among plants corresponds, in an
imperfect, rudimentary way, to what we recognize in animals as
nervous excitability. By this it is not meant to imply that the two
things are identical in their ultimate manifestations, though we
may regard them as fundamentally the same in that they are both to
be referred to the property inherent in protoplasm of responding to
stimuli. There is no indication, however, that irritability in the
vegetable kingdom is accompanied by anything like consciousness or
volition, or that plants possess any power of initiative. While the
movements in response to stimuli are in many cases eminently adapted
to a purpose, we have no evidence of a controlling power behind them.
The movement comes automatically in response to the stimulus, whether
the effect at the moment be advantageous or the reverse.
=202. Adjustments in relation to moisture.=—These adjustments may
be—(1) To guard against excess of moisture; _e.g._ glands for
excreting water and salts; scales, wax, down, etc., on the surface of
leaves. These may serve also for protection against cold, insects,
excess of light and heat. (2) For the conservation of moisture;
_e.g._ the revolute leaf margins of grasses and sand plants growing
along the seashore; the fleshy leaves of stonecrops and purselanes;
the hard epidermis of yuccas and aloes; the scales, scurf, and down,
by which the moisture absorbed from the soil by plants growing in
dry and barren places is prevented from escaping too rapidly through
the stomata; the leaf cups and holders sometimes formed by winged
petioles and clasping leaf bases for retaining dew or rain water.
(3) For leaf drainage, or the conduction of moisture, by means of
grooves, channels, and taper-pointed leaves, which act as natural
gutters and drain pipes.
[Illustration: FIG. 251.—Cross sections of the leaf of sand grass:
_a_, unrolled in its ordinary position; _b_ and _c_, rolled up to
prevent too rapid transpiration.]
[Illustration: FIG. 252.—Winged petiole of _Polymnia_. FIG.
253.—Water cups of _Silphium perfoliatum_.]
[Illustration: FIG. 254.—Fallen leaves. Notice how they cover the
ground with a warm mulch, protecting the soil from denudation, and
the roots and seeds from frost.]
=203. The fall of the leaf.=—This is, in effect, an adjustment to
change of temperature, but that it is not directly due to cold is
shown by Exp. 75, and also by the fact that leaves in the tropics
and those of evergreens, while they do not fall at stated periods
like the bulk of the foliage in the temperate zones, are cut off just
the same and replaced by new ones, whenever, for any reason, they
are unable to perform their function. In cold climates they fall
at the approach of winter, not because the frost loosens them, but
because the roots are not able to absorb enough moisture to supply
them with material for making food. The needles and the scale-leaves
characteristic of evergreens in cold regions are enabled to persist
indefinitely by reason of their contracted surface. This prevents the
dissipation of moisture and affords no lodging for the accumulations
of sleet and snow that would otherwise cumber and perhaps break the
boughs with their weight. Trees and shrubs that shed their leaves
in winter are said to be _deciduous_, from a Latin word meaning “to
fall.” Can you mention some advantages of the deciduous habit to a
plant with broad, expanded leaves, growing in a cold climate?
The mechanical means by which the leaf fall is accomplished is
through the growth of a corky layer of loose cells that forms at
the base of the petiole and cuts it away from the stem, leaving a
smooth, clean scar. Tear some fresh young leaves from a growing
twig and compare the scars with those on a winter bough. Do you see
any difference? This corky layer can be made to form in some plants
artificially, by depriving them of working material. (Exp. 75.)
=204. The protection of wintergreen leaves.=—A great many, perhaps
the majority of broad-leaved evergreens, bear no obvious protection
against cold, while a large proportion, such as chickweed, violet,
fumitory, groundsel (_Senecio_), and dead nettle (_Lamium_), would
seem peculiarly unfitted, by their delicate structure, to withstand
it. But recent investigations by the Swedish botanist, Lidforss,
have shown that all wintergreen leaves, with the exception of those
on submerged water plants, which are sufficiently protected by the
medium in which they live, lose their starch in winter and contain
instead an increased percentage of sugar. The same is true of other
vegetable structures also, where starch is present, such as roots,
stems, tubers, and winter fruits—nuts, haws, persimmons, and the
like, which, as every schoolboy knows, become perceptibly sweeter
after frost.
The presence of certain substances, of which sugar is the most
frequent, enables plants to withstand a greater degree of cold
than they could otherwise endure (Exp. 76). This effect, as shown
by Lidforss’s experiments, is due to the action of sugar in
counteracting, or retarding, the “salting out” of proteins by cold,
as explained in 33.
As sugar is readily reconverted into starch by exposure to a
moderately high temperature for even a few days, we may find here
an explanation of the fact that plants which have survived the
prolonged cold of winter are often killed by a single sharp night
frost following a few warm days in early spring, before the tender
new growth has appeared. The plant suffers, not from the direct
effects of cold, but from the warmth preceding it, which stimulated
the transformation into starch of the sugar that would have prevented
the loss of proteins. On the same principle we may account for the
puzzling fact that the sunny southern side of trees and shrubs
usually suffers more from the effects of sudden frost than the shaded
and colder northern face.
In apparent conflict with this reasoning is the fact that sugar
cane and the sugar beet are peculiarly susceptible to cold. This,
however, does not invalidate the premises established by Lidforss’s
researches, but merely emphasizes the need of further investigation,
which may either reconcile all the facts, or modify their
interpretation.
=205. The colors of autumn leaves.=—These are due to the breaking
up and disappearance of the chlorophyll when the leaf factory has
to “shut down” for want of raw material to work with (203). It is
closely connected with the appearance of frost, since the same
changes of temperature which produce frost cause the cessation of sap
flow that brings about the disorganization of the chlorophyll and the
formation of various pigments derived from it. Besides these, leaves
may contain other coloring matters that are perceptible only when the
chlorophyll disappears; and in the sap there is a reddish pigment
which becomes either a very bright red, or a dark purplish maroon,
from the effect of chemicals that combine with it in the leaves. With
these coloring materials at command it is easy to see how the autumn
woods can assume such splendid hues.
Practical Questions
1. How would you explain the fact that the outer twigs of trees
generally are the most leafy? (99, 194; Exps. 57, 74.)
2. Is the common sunflower a compass plant? Is cotton?
3. Are there any such plants in your neighborhood?
4. Compare the leaves of half a dozen shade-loving plants of your
neighborhood with those of as many sun-loving ones; which, as a
general thing, are the larger and less incised?
5. Give a reason for the difference. (169.)
6. Why do most leaves—notably grasses—curl their edges backward in
withering? (182.)
7. What advantage is gained by doing this? (202.)
8. Observe such of the following plants as are found in your
neighborhood, and report any changes of position that may take
place in their leaves and the causes to which such changes should
be ascribed: wood sorrel, mimosa, honey locust, wild senna,
partridge pea, wild sensitive plant, redbud, bush clover, Japan
clover, Kentucky coffee tree, sensitive brier (_Schrankia_),
peanut, kidney bean.
9. Which of the trees named below shed their leaves from base to
tip of the bough (centripetally), and which in the reverse order:
ash, beech, hazel, hornbeam, lime, willow, poplar, pear, peach,
sweet gum, elm, sycamore, mulberry, China tree, sumac, chinquapin?
10. Account for the fact that evergreen trees and shrubs have
generally thick, hard, and shiny leaves, like those of the holly
and magnolia, or scales and needles, as the cedar and pine. (203.)
11. Why do many plants which are deciduous at the North tend to
become evergreen at the South? (203.)
12. Why are evergreens more abundant in cold than in warm climates?
(203.)
13. There is an apparent inconsistency between questions 11 and 12;
can you reconcile it? (203.)
14. Why is it more important to protect the south side of trees
against exposure to frost than the northern side? (33, 204.)
15. Explain why peach orchards on the tops and northern slopes of
elevated areas are less liable to have their fruit destroyed by
late frost than those in the valleys and on the southern slopes.
(33, 204.)
VIII. MODIFIED LEAVES
MATERIAL.—Get from a florist a potted plant of sundew,
Venus’s-flytrap, sarracenia, or, if possible, one of all three, and
keep in the schoolroom for observation. The subject can be studied
best in a well-stocked greenhouse, if one is accessible.
[Illustration: FIG. 255.—Spearlike leaves of Spanish bayonet.]
=206. Modification and adaptation.=—Modification is structural
adjustment, or adaptation, carried so far as to obscure the original
form of an organ. Its true nature, however, can generally be
determined by some of the tests mentioned in 100.
Examples of the modification of leaves to do the work of other
organs have already been noticed, as also their entire disappearance
in certain cases (97, 101, 149) and replacement by other parts; it is
unnecessary, therefore, to revert to this branch of the subject here.
=207. Protective modifications.=—The most general protective
modifications that leaves undergo are (1) for the conservation
of moisture, as explained in 202, and (2) for protection against
animals. Many of the adaptations for the former purpose serve
incidentally for defense against animals also. Spines, hairs, scales,
sticky exudations, water holders, clasping and perfoliate leaves bar
the way to crawling insects; horny cuticles, as well as offensive
odors, bitter secretions, and poisonous juices warn leaf-eating
cattle and bugs away. These devices are merely protective, however,
and adapted to a passive attitude of self-defense.
[Illustration: FIGS. 256-258.—Protective hairs magnified: 256,
mullein; 257, cinque-foil 258, Shepherdia.]
=208. Insectivorous leaves.=—But sometimes a plant becomes the
aggressor, and instead of standing on the defensive or suffering
itself to be quietly devoured, proceeds to capture and devour small
game on its own account, and in this case, the leaf sometimes becomes
a deadly weapon of destruction.
=209. Pitcher plants.=—The sarracenia, or trumpet leaf, is a
familiar example of this class. The lower part of the leaf blade is
transformed into a hollow vessel for holding water, and the top is
rounded into a broad flap called the _lamina_. Sometimes the lamina
stands erect, as in the common yellow trumpets of our coast regions,
and when this is the case, it is brilliantly colored and attracts
insects (Fig. 259). Sometimes, as in the parrot-beaked and the
spotted trumpet leaf, it is bent over the top of the water vessel
like a lid, and the back of the leaf, near the foot of the lamina, is
dotted with transparent specks that serve to decoy foolish flies away
from the true opening and tempt them to wear themselves out in futile
efforts to escape, as we often see them do against a window pane.
[Illustration: FIG. 259.—Yellow trumpets (_Sarracenia flava_).
(_From_ the Mo. Botanical Garden Rep’t.)]
[Illustration: FIG. 260.—Plant of sundew.]
If the contents of one of these leaves are examined with a lens,
there will generally be found mixed with the water at the bottom the
remains of the bodies of a large number of insects. The hairs on the
outside all point up, toward the rim of the pitcher, while those on
the inside turn down, thus smoothing the way to destruction, but
making return impossible to a small insect when once it is ensnared.
When we remember that these plants are generally found in poor,
barren soil, we can appreciate the value to them of the animal diet
thus obtained.
[Illustration: FIGS. 261-263.—Leaves of sundew magnified: 261, leaf
expanded; 262, leaf closing over captured insect; 263, leaf digesting
a meal.]
=210. Flytraps.=—The most remarkable examples of insect-catching
leaves are the Venus’s-flytrap, found in the seacoast region of
North Carolina, and the sundew (_Drosera rotundifolia_), common
on the margins of sandy bogs and ponds. The latter is a delicate,
innocent-looking little plant, and owes its poetic name to the
dewlike appearance of a shining, sticky fluid exuded from glands on
its leaves, which glitter in the sun like dewdrops. It is, however,
a most voracious carnivorous plant, the sticky leaves acting as so
many bits of fly paper by means of which it catches its prey. When a
fly has been trapped, the tentacles close upon it, the edges of the
leaf curve inward, making a sort of stomach, from the glands of which
an acid juice exudes and digests the meal. After a number of days,
varying according to the digestibility of the diet, the blades slowly
unfold again and are ready for another capture.
[Illustration: FIG. 264.—Bladderwort, showing finely dissected
submerged leaves bearing bladders for capturing animalculæ.]
The bladderwort, common in pools and still waters nearly everywhere,
has its petioles transformed into floats, while the finely dissected,
rootlike blades bear little bladders which, when examined under the
microscope, are found to contain the decomposed remains of captured
animalculæ.
Practical Questions
1. Can you find any kind of leaf that is not preyed upon by
something? If so, how do you account for its immunity?
2. Make a list of some of the most striking of the protected leaves
of your neighborhood.
3. What is the nature of the protective organ in each case?
4. For protection against what does it seem to be specially adapted?
5. Are the plants in your list for the most part useful ones, or
troublesome weeds?
6. Examine the leaves of the worst weeds that you know of and see
if these will help in any way to account for their persistency.
Field Work
(1) In connection with Sections I and II, observe the effect of the
lobing and branching of leaves in letting the sunlight through.
Notice any general differences that may appear as to shape, margin,
and texture in the leaves of sun plants, shade plants, and water
plants, and account for them. Study the arrangement of leaves on
stems of various kinds, with reference to the size and shapes of
leaves and their light relations. Consider the value of the various
kinds of foliage for shade; for ornament; as producers of moisture;
as food; as insect destroyers, etc.
Make a special study of the twelve principal deciduous trees of
your neighborhood. Compare the leaves, bark, and branches of the
same trees so that you will be able to recognize them by any one of
these means alone.
(2) In connection with Sections III and V, consider the effects
upon soil moisture of transpiration from the leaves of forest
trees and from those of shallow-rooted herbs and weeds that draw
their water supply from the surface. Consider the value of forests
in protecting crops from excessive evaporation by acting as wind
breaks. Study the effect of the fall of leaves upon the formation
of soil. In any undisturbed forest tract turn up a few inches of
soil with a garden trowel and see what it is composed of. Notice
what kind of plants grow in it. Note the absence of weeds and
account for it. Compare the appearance of trees scattered along
windy hillsides, where the fallen leaves are constantly blown away,
or in any position where the soil is unrenewed, with those in an
undisturbed forest, and then give an opinion as to the wisdom of
hauling away the leaves every year from a timber lot.
(3) In Section VII, observe, in different kinds of leaf mosaics,
the means by which the adjustment has been brought about and the
purpose it subserves. Make a list of plants illustrating the two
habits. Notice the form and position of petioles of different
leaves, and their effect upon light exposure, drainage, etc., and
the behavior of the different kinds in the wind. Look for compass
plants in your neighborhood, and for other examples of adjustment
to heat and light. Study the position of leaves at different times
of day and in different kinds of weather and note what changes
occur and to what they are due.
Make a list of ten plants that seem to you to have best worked out
the problem of leaf adjustment, giving the reasons for your opinion.
Study the drainage system of different plants and observe whether
there is any general correspondence between the leaf drainage and
the root systems. This will lead to interesting questions in
regard to irrigation and manuring. Where plants are crowded, the
growth of both roots and leaves is complicated with so many other
factors that it is best to select for observations of this sort
specimens growing in more or less isolated situations.
Notice the time of the expansion and shedding of the leaves of
different plants, and whether the early leafers, as a general
thing, shed early or late; in other words, whether there seems to
be any general time relation between the two acts of leaf expansion
and leaf fall.
(4) Under Section VIII, look for instances of modified leaves;
study the nature of the different modifications you find, and try
to understand their meaning and object. Make a collection (_a_)
of all the leaves you can find modified to serve other than their
normal purposes; (_b_) of all the organs of other kinds that
have been modified to serve as leaves; (_c_) of all the modified
parts of leaves—stipules and petioles—that you can find. Keep the
collections separate, labeling each specimen with the name of the
plant it belongs to, what part it is, what use it serves, when and
where found. These collections need not be made individually, but
by the class as a whole and kept for the use of the school.
Observe also (_d_) the differences between young and old leaves of
the same kind, and the leaves of young and old plants or parts of
plants of the same kind; (_e_) resemblances between young leaves
belonging to plants of different species; (_f_) between young
leaves of one species and mature ones of one or more different
species. Make a collection of all the specimens you can find
illustrating the three points mentioned, referring each to its
proper head, and giving the name and relative age—old or young—of
all specimens collected.