LEAVES--FORM AND POSITION
Leaves may be studied from four points of view,--with reference (1) to
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 WILLOW.]
[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.]
[Illustration: FIG. 96.--LOBED LEAF OF SUGAR MAPLE.]
Leaves that are not divided as far as to the midrib are said to be:
=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;
=parted=, if the sinuses reach two thirds or more to the midrib (Fig.
97);
=divided=, if 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=.
[Illustration: FIG. 97.--DIGITATELY PARTED LEAVES OF BEGONIA.]
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 leaflike
and perform the function of foliage, as in the pea and 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=, 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=, 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]
=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=, 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=, a narrow leaf that is broadest toward the apex. The top is
usually rounded.
[Illustration: FIG. 104.--ELLIPTIC LEAF OF PURPLE BEECH.]
[Illustration: FIG. 105.--OVATE SERRATE LEAF OF HIBISCUS.]
[Illustration: FIG. 106.--LEAF OF APPLE, showing blade, petiole, and
small narrow stipules.]
[Illustration]
=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=, 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=, kidney-shaped. This form is sometimes seen in wild plants,
particularly in root-leaves. Leaves of wild ginger are nearly reniform.
[Illustration]
=Orbicular=, circular in general outline. Very few leaves are perfectly
circular, but there are many that are nearer circular than 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 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.
Leaves often differ greatly in form on the same plant. 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
and Kentucky coffee tree. Remarkable differences in forms are seen by
comparing seed-leaves with mature leaves of any plant (Fig. 30).
[Illustration: FIG. 109.--DIFFERENT FORMS OF LEAVES FROM ONE PLANT OF
AMPELOPSIS.]
=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 the other 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.
_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 ¹⁄₂, _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. 110.--PHYLLOTAXY OF BOX ELDER, ELM, APPLE.]
_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 parts.
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?
[Illustration: FIG. 111.--PHYLLOTAXY OF THE POTATO TUBER. Work it out
on a fresh long tuber.]
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.
[Illustration: FIG. 112.--COWPEA. Describe the leaves. For what is the
plant used?]
SUGGESTIONS.--=55.= The pupil should match leaves to determine
whether any two are alike. Why? Compare leaves from the same plant
in size, shape, color, 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 foregoing 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 colors or shades? =73.= How many lengths of petioles?
=74.= Bring in all the shapes of leaves that you can find.