LEAVES—FORM AND POSITION
Leaves may be studied from four points of view,—with reference to (1)
their _kinds_ and _shapes_; (2) their _position_, or _arrangement_ on
the plant; (3) their _anatomy_, or _structure_; (4) their _function_,
or the work they perform. This chapter is concerned with the first
two categories.
[Illustration: FIG. 88.—A SIMPLE NETTED-VEINED LEAF.]
[Illustration: FIG. 89.—A SIMPLE PARALLEL-VEINED LEAF.]
[Illustration: FIG. 90.—COMPOUND OR BRANCHED LEAF OF BRAKE (a common
fern).]
=Kinds.=—Leaves are =simple= or unbranched (Figs. 88, 89), and
=compound= or branched (Fig. 90). The method of compounding or
branching follows the mode of veining. The veining, or =venation=,
is of two general kinds. In some plants the main veins diverge, and
there is a conspicuous network of smaller veins; such leaves are
=netted-veined=. They are characteristic of the dicotyledons. In
other plants the main veins are parallel, or nearly so, and there is
no conspicuous network; these are =parallel-veined= leaves (Figs.
89, 102). These leaves are the rule in monocotyledonous plants.
The venation of netted-veined leaves is =pinnate= or feather-like
when the veins arise from the side of a continuous midrib (Fig.
91); =palmate= or =digitate= (hand-like) when the veins arise from
the apex of the petiole (Figs. 88, 92). If leaves were divided
between the main veins, the former would be pinnately and the latter
digitately compound.
[Illustration: FIG. 91.—COMPLETE LEAVES OF WILLLOW.]
[Illustration: FIG. 92.—DIGITATE-VEINED PELTATE LEAF OF NASTURTIUM.]
[Illustration: FIG. 93.—PINNATELY COMPOUND LEAF OF ASH.]
It is customary to speak of a leaf as compound only when the parts or
branches are completely separate blades, as when the division extends
to the midrib (Figs. 90, 93, 94, 95). The parts or branches are known
as =leaflets=. Sometimes the leaflets themselves are compound, and
the whole leaf is then said to be =bi-compound= or =twice-compound=
(Fig. 90). Some leaves are three-compound, four-compound, or
five-compound. =Decompound= is a general term to express any degree
of compounding beyond twice-compound.
[Illustration: FIG. 94.—DIGITATELY COMPOUND LEAF OF RASPBERRY.]
[Illustration: FIG. 95.—POISON IVY. LEAF AND FRUIT.]
Leaves that are not divided as far as to the midrib are said to be:
[Illustration: FIG. 96.—LOBED LEAF OF SUGAR MAPLE.]
=lobed=, if the openings or sinuses are not more than half the depth
of the blade (Fig. 96);
=cleft=, if the sinuses are deeper than the middle;
[Illustration: FIG. 97.—DIGITATELY PARTED LEAVES OF BEGONIA.]
=parted=, if the sinuses reach two thirds or more to the midrib (Fig.
97);
=divided=, if the sinuses reach nearly or quite to the midrib.
The parts are called =lobes=, =divisions=, or =segments=, rather than
leaflets. The leaf may be pinnately or digitately lobed, parted,
cleft, or divided. A pinnately parted or cleft leaf is sometimes said
to be =pinnatifid=.
Leaves may have one or all of three parts—=blade=, or expanded
part; =petiole=, or stalk; =stipules=, or appendages at the base
of the petiole. A leaf that has all three of these parts is said
to be =complete= (Figs. 91, 106). The stipules are often green and
leaf-like and perform the function of foliage as in the pea and the
Japanese quince (the latter common in yards).
[Illustration: FIG. 98.—OBLONG-OVATE SESSILE LEAVES OF TEA.]
Leaves and leaflets that have no stalks are said to be =sessile=
(Figs. 98, 103), _i.e._ sitting. Find several examples. The same is
said of flowers and fruits. The blade of a sessile leaf may partly or
wholly surround the stem, when it is said to be =clasping=. Examples:
aster (Fig. 99), corn. In some cases the leaf runs down the stem,
forming a wing; such leaves are said to be =decurrent= (Fig. 100).
When opposite sessile leaves are joined by their bases, they are said
to be =connate= (Fig. 101).
[Illustration: FIG. 99.—CLASPING LEAF OF A WILD ASTER.]
Leaflets may have one or all of these three parts, but the stalks
of leaflets are called =petiolules= and the stipules of leaflets
are called =stipels=. The leaf of the garden bean has leaflets,
petiolules, and stipels.
[Illustration: FIG. 100.—DECURRENT LEAVES OF MULLEIN.]
The blade is usually attached to the petiole by _its lower edge_.
In pinnate-veined leaves, the petiole seems to continue through the
leaf as a =midrib= (Fig. 91). In some plants, however, the petiole
joins the blade inside or beyond the margin (Fig. 92). Such leaves
are said to be =peltate= or shield-shaped. This mode of attachment
is particularly common in floating leaves (_e.g._ the water lilies).
Peltate leaves are usually digitate-veined.
[Illustration: FIG. 101.—TWO PAIRS OF CONNATE LEAVES OF HONEYSUCKLE.]
=How to Tell a Leaf.=—It is often difficult to distinguish compound
leaves from leafy branches, and leaflets from leaves. As a rule
leaves can be distinguished by the following tests: (1) Leaves are
_temporary structures_, sooner or later falling. (2) Usually _buds
are borne in their axils_. (3) Leaves are usually _borne at joints or
nodes_. (4) They arise on wood of the _current year’s growth_. (5)
They have a more or less _definite arrangement_. When leaves fall,
the twig that bore them remains; when leaflets fall, the main petiole
or stalk that bore them also falls.
[Illustration: FIG. 102.—LINEAR-ACUMINATE LEAF OF GRASS.]
[Illustration: FIG. 103.—SHORT-OBLONG LEAVES OF BOX.]
=Shapes.=—Leaves and leaflets are infinitely variable in shape. Names
have been given to some of the more definite or regular shapes. These
names are a part of the language of botany. The names represent ideal
or typical shapes; there are no two leaves alike and very few that
perfectly conform to the definitions. The shapes are likened to those
of familiar objects or of geometrical figures. Some of the commoner
shapes are as follows (name original examples in each class):
[Illustration: (Linear leaf)]
=Linear=, several times longer than broad, with the sides nearly or
quite parallel. Spruces and most grasses are examples (Fig. 102). In
linear leaves, the main veins are usually parallel to the midrib.
[Illustration: (Oblong leaf)]
=Oblong=, twice or thrice as long as broad, with the sides parallel
for most of their length. Fig. 103 shows the short-oblong leaves of
the box, a plant that is used for permanent edgings in gardens.
[Illustration: FIG. 104.—ELLIPTIC LEAF OF PURPLE BEECH.]
[Illustration: (Elliptic leaf)]
=Elliptic= differs from the oblong in having the sides gradually
tapering to either end from the middle. The European beech (Fig. 104)
has elliptic leaves. (This tree is often planted in this country.)
[Illustration: (Lanceolate leaf)]
=Lanceolate=, four to six times longer than broad, widest below
the middle, and tapering to either end. Some of the narrow-leaved
willows are examples. Most of the willows and the peach have
oblong-lanceolate leaves.
[Illustration: (Spatulate leaf)]
=Spatulate=, a narrow leaf that is broadest toward the apex. The top
is usually rounded.
[Illustration: FIG. 105.—OVATE SERRATE LEAF OF HIBISCUS.]
[Illustration: FIG. 106.—LEAF OF APPLE, showing blade, petiole, and
small narrow stipules.]
[Illustration: (Ovate leaf)]
=Ovate=, shaped somewhat like the longitudinal section of an egg:
about twice as long as broad, tapering from near the base to the
apex. This is one of the commonest leaf forms (Figs. 105, 106).
[Illustration: (Obovate leaf)]
=Obovate=, ovate inverted,—the wide part towards the apex. Leaves of
mullein and leaflets of horse-chestnut and false indigo are obovate.
This form is commonest in leaflets of digitate leaves: why?
[Illustration: (Reniform leaf)]
=Reniform=, kidney-shaped. This form is sometimes seen in wild
plants, particularly in root-leaves. Leaves of wild ginger are nearly
reniform.
[Illustration: (Orbicular leaf)]
=Orbicular=, circular in general outline. Very few leaves are
perfectly circular, but there are many that are nearer circular than
of any other shape. (Fig. 107).
[Illustration: FIG. 107.—ORBICULAR LOBED LEAVES.]
[Illustration: FIG. 108.—TRUNCATE LEAF OF TULIP TREE.]
The shape of many leaves is described in combinations of these terms:
as =ovate-lanceolate=, =lanceolate-oblong=.
The shape of the base and the apex of the leaf or leaflet is often
characteristic. The base may be =rounded= (Fig. 104), =tapering=
(Fig. 93), =cordate= or heart-shaped (Fig. 105), =truncate= or
squared as if cut off. The apex may be blunt or =obtuse=, =acute=
or sharp, =acuminate= or long-pointed, =truncate= (Fig. 108). Name
examples.
The shape of the margin is also characteristic of each kind of leaf.
The margin is =entire= when it is not indented or cut in any way
(Figs. 99, 103). When not entire, it may be =undulate= or wavy (Fig.
92), =serrate= or saw-toothed (Fig. 105), =dentate= or more coarsely
notched (Fig. 95), =crenate= or round-toothed, =lobed=, and the like.
Give examples.
[Illustration: FIG. 109.—DIFFERENT FORMS OF LEAVES FROM ONE PLANT OF
AMPELOPSIS.]
Leaves on the same plant often differ greatly in form. Observe the
different shapes of leaves on the young growths of mulberries (Fig.
2) and wild grapes; also on vigorous squash and pumpkin vines. In
some cases there may be simple and compound leaves on the same plant.
This is marked in the so-called Boston ivy or ampelopsis (Fig. 109),
a vine that is used to cover brick and stone buildings. Different
degrees of compounding, even in the same leaf, may often be found in
honey locust. Remarkable differences in forms are seen by comparing
seed-leaves with mature leaves of any plant (Fig. 30).
=The Leaf and its Environment.=—The form and shape of the leaf
often have direct relation to the _place in which the leaf grows_.
_Floating leaves are usually expanded and flat_, and the petiole
varies in length with the depth of the water. _Submerged leaves
are usually linear or thread-like_, or are cut into very narrow
divisions: thereby more surface is exposed, and possibly the leaves
are less injured by moving water. Compare the sizes of the leaves on
the ends of branches with those at the base of the branches or in
the interior of the tree top. In dense foliage masses, the petioles
of the lowermost or undermost leaves _tend to elongate_—to push the
leaf to the light.
On the approach of winter the leaf usually ceases to work, and dies.
It may drop, when it is said to be =deciduous=; or it may remain on
the plant, when it is said to be =persistent=. If persistent leaves
remain green during the winter, the plant is said to be =evergreen=.
Give examples in each class. Most leaves fall by breaking off at the
lower end of the petiole with a _distinct joint or articulation_.
There are many leaves, however, that wither and hang on the plant
until torn off by the wind; of such are the leaves of grasses,
sedges, lilies, orchids, and other plants of the monocotyledons. Most
leaves of this character are parallel-veined.
_Leaves also die and fall from lack of light._ Observe the yellow and
weak leaves in a dense tree top or in any thicket. Why do the lower
leaves die on house plants? Note the carpet of needles under the
pines. All evergreens shed their leaves after a time. Counting back
from the tip of a pine or spruce shoot, determine how many years the
leaves persist. In some spruces a few leaves may be found on branches
ten or more years old.
=Arrangement of Leaves.=—Most leaves have a regular _position or
arrangement_ on the stem. _This position or direction is determined
largely by exposure to sunlight._ In temperate climates they usually
hang in such a way that they receive the greatest amount of light.
One leaf shades another to the least possible degree. If the plant
were placed in a new position with reference to light, the leaves
would make an effort to turn their blades.
_When leaves are_ =opposite= _the pairs usually alternate._ That is,
if one pair stands north and south, the next pair stands east and
west. See the box elder shoot, on the left in Fig. 110. _One pair
does not shade the pair beneath._ The leaves are in four vertical
ranks.
[Illustration: FIG. 110.—PHYLLOTAXY OF BOX ELDER, ELM, APPLE.]
_There are several kinds of_ =alternate= _arrangement._ In the elm
shoot, in Fig. 110, the third bud is vertically above the first. This
is true no matter which bud is taken as the starting point. Draw a
thread around the stem until the two buds are joined. Set a pin at
each bud. Observe that two buds are passed (not counting the last)
and that the thread makes one circuit of the stem. Representing the
number of buds by a denominator, and the number of circuits by a
numerator, we have the fraction ½, _which expresses the part of the
circle that lies between any two buds_. That is, the buds are one
half of 360 degrees apart, or 180 degrees. Looking endwise at the
stem, the leaves are seen to be 2-ranked. Note that in the apple
shoot (Fig. 110, right) the thread makes two circuits and five buds
are passed: _two fifths represents the divergence between the buds_.
The leaves are 5-ranked.
[Illustration: FIG. 111.—PHYLLOTAXY OF THE POTATO TUBER. Work it out
on a fresh long tuber.]
_Every plant has its own arrangement of leaves._ For opposite leaves,
see maple, box elder, ash, lilac, honeysuckle, mint, fuchsia. For
2-ranked arrangement, see all grasses, Indian corn, basswood, elm.
For 3-ranked arrangement, see all sedges. For 5-ranked (which is
one of the commonest), see apple, cherry, pear, peach, plum, poplar,
willow. For 8-ranked, see holly, osage orange, some willows. More
complicated arrangements occur in bulbs, house leeks, and other
condensed plants. The buds or “eyes” on a potato tuber, which is
an underground stem (why?), show a spiral arrangement (Fig. 111).
_The arrangement of leaves on the stem is known as_ =phyllotaxy=
(literally, “leaf arrangement”). Make out the phyllotaxy on six
different plants nearest the schoolhouse door.
In some plants, several leaves occur at one level, being arranged in
a circle around the stem. Such leaves are said to be =verticillate=,
or =whorled=. Leaves arranged in this way are usually narrow: why?
Although a definite arrangement of leaves is the rule in most
plants, _it is subject to modification_. On shoots that receive the
light only from one side or that grow in difficult positions, the
arrangement may not be definite. Examine shoots that grow on the
under side of dense tree tops or in other partially lighted positions.
SUGGESTIONS.—=55.= The pupil should match leaves to determine
whether any two are alike. Why? Compare leaves from the same
plant in size, shape, colour, form of margin, length of petiole,
venation, texture (as to thickness or thinness), stage of maturity,
smoothness or hairiness. =56.= Let the pupil take an average leaf
from each of the first ten different kinds of plants that he meets
and compare them as to the above points (in Exercise 55), and
also name the shapes. Determine how the various leaves resemble
and differ. =57.= Describe the stipules of rose, apple, fig,
willow, violet, pea, or others. =58.= In what part of the world
are parallel-veined leaves the more common? =59.= Do you know of
parallel-veined leaves that have lobed or dentate margins? =60.=
What becomes of dead leaves? =61.= Why is there no grass or other
undergrowth under pine and spruce trees? =62.= Name several leaves
that are useful for decorations. Why are they useful? =63.= What
trees in your vicinity are most esteemed as shade trees? What is
the character of their foliage? =64.= Why are the internodes so
long in water-sprouts and suckers? =65.= How do foliage characters
in corn or sorghum differ when the plants are grown in rows or
broadcast? Why? =66.= Why may removal of half the plants increase
the yield of cotton or sugar-beets or lettuce? =67.= How do leaves
curl when they wither? Do different leaves behave differently in
this respect? =68.= What kinds of leaves do you know to be eaten by
insects? By cattle? By horses? What kinds are used for human food?
=69.= How would you describe the shape of leaf of peach? apple?
elm? hackberry? maple? sweet-gum? corn? wheat? cotton? hickory?
cowpea? strawberry? chrysanthemum? rose? carnation? =70.= Are any
of the fore-going leaves compound? How do you describe the shape of
a compound leaf? =71.= How many sizes of leaves do you find on the
bush or tree nearest the schoolroom door? =72.= How many colours or
shades? =73.= How many lengths of petioles? =74.= Bring in all the
shapes of leaves that you can find.
[Illustration: FIG. 112.—COWPEA. Describe the leaves. For what is the
plant used?]