[Illustration: FIG. 18.--Palmately compound leaf of the Horse-chestnut
(_Aesculus Hippocastanum_).]
[Illustration: FIG. 19.--Imparipinnate (unequal pinnate) leaf of
Robinia. There are nine pairs of shortly-stalked leaflets (foliola,
pinnae), and an odd one at the extremity. At the base of the leaf the
spiny stipules are seen.]
Petiole.
The _petiole_ or leaf-stalk is the part which unites the limb or blade
of the leaf to the stem. It is absent in _sessile_ leaves, and this is
also frequently the case when a sheath is present, as in grasses (fig.
5). It consists of the fibro-vascular bundles with a varying amount of
cellular tissue. When the vascular bundles reach the base of the
lamina they separate and spread out in various ways, as already
described under venation. The lower part of the petiole is often
swollen (fig. 20, _p_), forming the _pulvinus_, formed of cellular
tissue, the cells of which exhibit the phenomenon of irritability. In
_Mimosa pudica_ (fig. 20) a sensitiveness is located in the pulvinus
which upon irritation induces a depression of the whole bipinnate
leaf, a similar property exists in the pulvini at the base of the
leaflets which fold upwards. The petiole varies in length, being
usually shorter than the lamina, but sometimes much longer. In some
palms it is 15 or 20 ft. long, and is so firm as to be used for poles
or walking-sticks. In general, the petiole is more or less rounded in
its form, the upper surface being flattened or grooved. Sometimes it
is compressed laterally, as in the aspen, and to this peculiarity the
trembling of the leaves of this tree is due. In aquatic plants the
leaf-stalk is sometimes distended with air, as in _Pontederia_ and
_Trapa_, so as to float the leaf. At other times it is _winged_, and
is either leafy, as in the orange (fig. 21, p), lemon and _Dionaea_,
or pitcher-like, as in _Sarracenia_ (fig. 22). In some Australian
acacias, and in some species of _Oxalis_ and _Bupleurum_, the petiole
is flattened in a vertical direction, the vascular bundles separating
immediately after quitting the stem and running nearly parallel from
base to apex. This kind of petiole (fig. 23, p) has been called a
_phyllode_. In these plants the laminae or blades of the leaves are
pinnate or bipinnate, and are produced at the extremities of the
phyllodes in a horizontal direction; but in many instances they are
not developed, and the phyllode serves the purpose of a leaf. These
phyllodes, by their vertical position and their peculiar form, give a
remarkable aspect to vegetation. On the same acacia there occur leaves
with the petiole and lamina perfect; others having the petiole
slightly expanded or winged, and the lamina imperfectly developed; and
others in which there is no lamina, and the petiole becomes large and
broad. Some petioles are long, slender and sensitive to contact, and
function as tendrils by means of which the plant climbs; as in the
nasturtiums (_Tropaeolum_), clematis and others; and in compound
leaves the midrib and some of the leaflets may similarly be
transformed into tendrils, as in the pea and vetch.
[Illustration: FIG. 20.--Branch and leaves of the Sensitive plant
(_Mimosa pudica_), showing the petiole in its erect state, a, and in
its depressed state, b; also the leaflets closed, c, and the leaflets
expanded, d. Irritability resides in the pulvinus, p.]
Leaf base.
The leaf base is often developed as a _sheath_ (_vagina_), which
embraces the whole or part of the circumference of the stem (fig. 5).
This sheath is comparatively rare in dicotyledons, but is seen in
umbelliferous plants. It is much more common amongst monocotyledons.
In sedges the sheath forms a complete investment of the stem, whilst
in grasses it is split on one side. In the latter plants there is also
a membranous outgrowth, the _ligule_, at right angles to the median
plane of the leaf from the point where the sheath passes into the
lamina, there being no petiole (fig. 5, _l_).
[Illustration: FIG. 21.--Leaf of Orange (_Citrus Aurantium_), showing
a winged leafy petiole p, which is articulated to the lamina l.]
[Illustration: FIG. 22.--Pitcher (_ascidium_) of a species of
Side-saddle plant (_Sarracenia purpurea_). The pitcher is formed from
the petiole, which is prolonged.]
In leaves in which no sheath is produced we not infrequently find
small foliar organs, _stipules_, at the base of the petiole (fig. 24,
s). The stipules are generally two in number, and they are important
as supplying characters in certain natural orders. Thus they occur in
the pea and bean family, in rosaceous plants and the family Rubiaceae.
They are not common in dicotyledons with opposite leaves. Plants
having stipules are called _stipulate_; those having none are
_exstipulate_. Stipules may be large or small, entire or divided,
deciduous or persistent. They are not usually of the same form as the
ordinary foliage leaves of the plant, from which they are
distinguished by their lateral position at the base of the petiole. In
the pansy (fig. 24) the true leaves are stalked and crenate, while the
stipules s are large, sessile and pinnatifid. In _Lathyrus Aphaca_ and
some other plants the true pinnate leaves are abortive, the petiole
forms a tendril, and the stipules alone are developed, performing the
office of leaves. When stipulate leaves are opposite to each other, at
the same height on the stem, it occasionally happens that the stipules
on the two sides unite wholly or partially, so as to form an
_interpetiolary_ or _interfoliar_ stipule, as in members of the family
Rubiaceae. In the case of alternate leaves, the stipules at the base
of each leaf are sometimes united to the petiole and to each other, so
as to form an _adnate_, _adherent_ or _petiolary_ stipule, as in the
rose, or an _axillary_ stipule, as in _Houttuynia cordata_. In other
instances the stipules unite together on the side of the stem opposite
the leaf forming an _ocrea_, as in the dock family (fig. 25).
[Illustration: FIG. 23.--Leaf of an Acacia (_Acacia heterophylla_),
showing a flattened leaf-like petiole p, called a phyllode, with
straight venation, and a bipinnate lamina.]
In the development of the leaf the stipules frequently play a most
important part. They begin to be formed after the origin of the
leaves, but grow much more rapidly than the leaves, and in this way
they arch over the young leaves and form protective chambers wherein
the parts of the leaf may develop. In the figs, magnolia and pondweeds
they are very large and completely envelop the young leaf-bud. The
stipules are sometimes so minute as to be scarcely distinguishable
without the aid of a lens, and so fugacious as to be visible only in
the very young state of the leaf. They may assume a hard and spiny
character, as in _Robinia Pseudacacia_ (fig. 19), or may be cirrose,
as in _Smilax_, where each stipule is represented by a tendril. At the
base of the leaflets of a compound leaf, small stipules (_stipels_)
are occasionally produced.
[Illustration: FIG. 24.--Leaf of Pansy. s, Stipules.]
[Illustration: FIG. 25.--Leaf of Polygonum, with part of stem. o,
Ocrea.]
Modifications.
Variations in the structure and forms of leaves and leafstalks are
produced by the increased development of cellular tissue, by the
abortion or degeneration of parts, by the multiplication or repetition
of parts and by adhesion. When cellular tissue is developed to a great
extent, leaves become succulent and occasionally assume a crisp or
curled appearance. Such changes take place naturally, but they are
often increased by the art of the gardener, and the object of many
horticultural operations is to increase the bulk and succulence of
leaves. It is in this way that cabbages and savoys are rendered more
delicate and nutritious. By a deficiency in development of parenchyma
and an increase in the mechanical tissue, leaves are liable to become
hardened and spinescent. The leaves of barberry and of some species of
_Astragalus_, and the stipules of the false acacia (_Robinia_) are
spiny. To the same cause is due the spiny margin of the holly-leaf.
When two lobes at the base of a leaf are prolonged beyond the stem and
unite (fig. 26), the leaf is _perfoliate_, the stem appearing to pass
through it, as in _Bupleurum perfoliatum_ and _Chlora perfoliata_;
when two leaves unite by their bases they become _connate_ (fig. 27),
as in _Lonicera Caprifolium_; and when leaves adhere to the stem,
forming a sort of winged or leafy appendage, they are _decurrent_, as
in thistles. The formation of peltate leaves has been traced to the
union of the lobes of a cleft leaf. In the leaf of the _Victoria
regia_ the transformation may be traced during germination. The first
leaves produced by the young plant are linear, the second are
sagittate and hastate, the third are rounded-cordate and the next are
orbicular. The cleft indicating the union of the lobes remains in the
large leaves. The parts of the leaf are frequently transformed into
_tendrils_, with the view of enabling the plants to twine round others
for support. In Leguminous plants (the pea tribe) the pinnae are
frequently modified to form tendrils, as in _Lathyrus Aphaca_, in
which the stipules perform the function of true leaves. In
_Flagellaria indica_, _Gloriosa superba_ and others, the midrib of the
leaf ends in a tendril. In _Smilax_ there are two stipulary tendrils.
[Illustration: FIG. 26.--Perfoliate leaf of a species of Hare's-ear
(_Bupleurum rotundifolium_). The two lobes at the base of the leaf are
united, so that the stalk appears to come through the leaf.]
[Illustration: FIG. 27.--Connate leaves of a species of Honeysuckle
(_Lonicera Caprifolium_). Two leaves are united by their bases.]
The vascular bundles and cellular tissue are sometimes developed in
such a way as to form a circle, with a hollow in the centre, and thus
give rise to what are called _fistular_ or hollow leaves, as in the
onion, and to _ascidia_ or _pitchers_. Pitchers are formed either by
petioles or by laminae, and they are composed of one or more leaves.
In _Sarracenia_ (fig. 22) and _Heliamphora_ the pitcher is composed of
the petiole of the leaf. In the pitcher plant, _Nepenthes_, the
pitcher is a modification of the lamina, the petiole often plays the
part of a tendril, while the leaf base is flat and leaf-like (fig.
28).
[Illustration: FIG. 28.--Pitcher of a species of pitcher-plant
(_Nepenthes distillatoria_).]
In _Utricularia_ bladder-like sacs are formed by a modification of
leaflets on the submerged leaves.
In some cases the leaves are reduced to mere _scales_--_cataphyllary_
leaves; they are produced abundantly upon underground shoots. In
parasites (_Lathraea_, _Orobanche_) and in plants growing on decaying
vegetable matter (_saprophytes_), in which no chlorophyll is formed,
these scales are the only leaves produced. In _Pinus_ the only leaves
produced on the main stem and the lateral shoots are scales, the
acicular leaves of the tree growing from axillary shoots. In _Cycas_
whorls of scales alternate with large pinnate leaves. In many plants,
as already noticed, phyllodia or stipules perform the function of
leaves. The production of leaf-buds from leaves sometimes occurs as
in _Bryophyllum_, and many plants of the order Gesneraceae. The leaf
of Venus's fly-trap (_Dionaea muscipula_) when cut off and placed in
damp moss, with a pan of water underneath and a bell-glass for a
cover, has produced buds from which young plants were obtained. Some
species of saxifrage and of ferns also produce buds on their leaves
and fronds. In _Nymphaea micrantha_ buds appear at the upper part of
the petiole.
Phyllotaxis.
Leaves occupy various positions on the stem and branches, and have
received different names according to their situation. Thus leaves
arising from the crown of the root, as in the primrose, are called
_radical_; those on the stem are _cauline_; on flower-stalks, _floral_
leaves (see FLOWER). The first leaves developed are known as seed leaves
or _cotyledons_. The arrangement of the leaves on the axis and its
appendages is called _phyllotaxis_.
[Illustration: FIG. 29.--A stem with opposite leaves. The pairs are
placed at right angles alternately, or in what is called a decussate
manner. In the lowest pair one leaf is in front and the other at the
back; in the second pair the leaves are placed laterally, and so on.]
[Illustration: FIG. 30.--A stem with alternate leaves, arranged in a
pentastichous or quincuncial manner. The sixth leaf is directly above
the first, and commences the second cycle. The fraction of the
circumference of the stem expressing the divergence of the leaves is
two-fifths.]
In their arrangement leaves follow a definite order. The points on the
stem at which leaves appear are called nodes; the part of the stem
between the nodes is the _internode_. When two leaves are produced at
the same node, one on each side of the stem or axis, and at the same
level, they are _opposite_ (fig. 29); when more than two are produced
they are _verticillate_, and the circle of leaves is then called a
_verticil_ or _whorl_. When leaves are opposite, each successive pair
may be placed at right angles to the pair immediately preceding. They
are then said to _decussate_, following thus a law of alternation
(fig. 29). The same occurs in the verticillate arrangement, the leaves
of each whorl rarely being _superposed_ on those of the whorl next it,
but usually alternating so that each leaf in a whorl occupies the
space between two leaves of the whorl next to it. There are
considerable irregularities, however, in this respect, and the number
of leaves in different whorls is not always uniform, as may be seen in
_Lysimachia vulgaris_. When a single leaf is produced at a node, and
the nodes are separated so that each leaf is placed at a different
height on the stem, the leaves are _alternate_ (fig. 30). A plane
passing through the point of insertion of the leaf in the node,
dividing the leaf into similar halves, is the median plane of the
leaf; and when the leaves are arranged alternately on an axis so that
their median planes coincide they form a straight row or
_orthostichy_. On every axis there are usually two or more
orthostichies. In fig. 31, leaf 1 arises from a node n; leaf 2 is
separated from it by an internode m, and is placed to the right or
left; while leaf 3 is situated directly above leaf 1. In this case,
then, there are two orthostichies, and the arrangement is said to be
_distichous_. When the fourth leaf is directly above the first, the
arrangement is _tristichous_. The same arrangement continues
throughout the branch, so that in the latter case the 7th leaf is
above the 4th, the 10th above the 7th; also the 5th above the 2nd, the
6th above the 3rd and so on. The size of the angle between the median
planes of two consecutive leaves in an alternate arrangement is their
_divergence_; and it is expressed in fractions of the circumference of
the axis which is supposed to be a circle. In a regularly-formed
straight branch covered with leaves, if a thread is passed from one to
the other, turning always in the same direction, a spiral is
described, and a certain number of leaves and of complete turns occur
before reaching the leaf directly above that from which the
enumeration commenced. If this arrangement is expressed by a fraction,
the numerator of which indicates the number of turns, and the
denominator the number of internodes in the spiral cycle, the fraction
will be found to represent the angle of divergence of the consecutive
leaves on the axis. Thus, in fig. 32, a, b, the cycle consists of
five leaves, the 6th leaf being placed vertically over the 1st, the
7th over the 2nd and so on; while the number of turns between the 1st
and 6th leaf is two; hence this arrangement is indicated by the
fraction 2/5. In other words, the distance or divergence between the
first and second leaf, expressed in parts of a circle, is 2/5 of a
circle or 360° × 2/5 = 144°. In fig. 31, a, b, the spiral is ½, i.e.
one turn and two leaves; the third leaf being placed vertically over
the first, and the divergence between the first and second leaf being
one-half the circumference of a circle, 360° × ½ = 180°. Again, in a
tristichous arrangement the number is 1/3, or one turn and three
leaves, the angular divergence being 120°.
[Illustration: FIG. 31.--Portion of a branch of a Lime tree, with four
leaves arranged in a distichous manner, or in two rows. a, The branch
with the leaves numbered in their order, n being the node and m the
internode; b is a magnified representation of the branch, showing the
points of insertion of the leaves and their spiral arrangement, which
is expressed by the fraction ½, or one turn of the spiral for two
internodes.]
By this means we have a convenient mode of expressing on paper the
exact position of the leaves upon an axis. And in many cases such a
mode of expression is of excellent service in enabling us readily to
understand the relations of the leaves. The divergences may also be
represented diagrammatically on a horizontal projection of the
vertical axis, as in fig. 33. Here the outermost circle represents a
section of that portion of the axis bearing the lowest leaf, the
innermost represents the highest. The broad dark lines represent the
leaves, and they are numbered according to their age and position. It
will be seen at once that the leaves are arranged in orthostichies
marked I.-V., and that these divide the circumference into five equal
portions. But the divergence between leaf 1 and leaf 2 is equal to
(2/5)ths of the circumference, and the same is the case between 2 and
3, 3 and 4, &c. The divergence, then, is 2/5, and from this we learn
that, starting from any leaf on the axis, we must pass twice round the
stem in a spiral through five leaves before reaching one directly over
that with which we started. The line which, winding round an axis
either to the right or to the left, passes through the points of
insertion of all the leaves on the axis is termed the _genetic_ or
_generating spiral_; and that margin of each leaf which is towards the
direction from which the spiral proceeds is the _kathodic_ side, the
other margin facing the point whither the spiral passes being the
_anodic_ side.
[Illustration: FIG. 32.--Part of a branch of a Cherry with six leaves,
the sixth being placed vertically over the first, after two turns of
the spiral. This is expressed by two-fifths. a, The branch, with the
leaves numbered in order; b, a magnified representation of the
branch, showing the points of insertion of the leaves and their spiral
arrangement.]
In cases where the internodes are very short and the leaves are
closely applied to each other, as in the house-leek, it is difficult
to trace the _generating spiral_. Thus, in fig. 34 there are thirteen
leaves which are numbered in their order, and five turns of the spiral
marked by circles in the centre (5/13 indicating the arrangement); but
this could not be detected at once. So also in fir cones (fig. 35),
which are composed of scales or modified leaves, the generating spiral
cannot be determined easily. But in such cases a series of _secondary
spirals_ or _parastichies_ are seen running parallel with each other
both right and left, which to a certain extent conceal the genetic
spiral.
The spiral is not always constant throughout the whole length of an
axis. The angle of divergence may alter either abruptly or gradually,
and the phyllotaxis thus becomes very complicated. This change may be
brought about by arrest of development, by increased development of
parts or by a torsion of the axis. The former are exemplified in many
Crassulaceae and aloes. The latter is seen well in the screw-pine
(_Pandanus_). In the bud of the screw-pine the leaves are arranged in
three orthostichies with the phyllotaxis 1/3, but by torsion the
developed leaves become arranged in three strong spiral rows running
round the stem. These causes of change in phyllotaxis are also well
exemplified in the alteration of an opposite or verticillate
arrangement to an alternate, and vice versa; thus the effect of
interruption of growth, in causing alternate leaves to become opposite
and verticillate, can be distinctly shown in _Rhododendron ponticum_.
The primitive or generating spiral may pass either from right to left
or from left to right. It sometimes follows a different direction in
the branches from that pursued in the stem. When it follows the same
course in the stem and branches, they are _homodromous_; when the
direction differs, they are _heterodromous_. In different species of
the same genus the phyllotaxis frequently varies.
[Illustration: FIG. 33.--Diagram of a phyllotaxis represented by the
fraction 2/5.]
All modifications of leaves follow the same laws of arrangement as
true leaves--a fact which is of importance in a morphological point of
view. In dicotyledonous plants the first leaves produced (the
cotyledons) are opposite. This arrangement often continues during the
life of the plant, but at other times it changes, passing into
distichous and spiral forms. Some tribes of plants are distinguished
by their opposite or verticillate, others by their alternate, leaves.
Labiate plants have decussate leaves, while Boraginaceae have
alternate leaves, and Tiliaceae usually have distichous leaves;
Rubiaceae have opposite leaves. Such arrangements as 2/5, 3/8, 5/13
and 8/21 are common in Dicotyledons. The first of these, called a
_quincunx_, is met with in the apple, pear and cherry (fig. 32); the
second, in the bay, holly, _Plantago media_; the third, in the cones
of _Picea alba_ (fig. 35); and the fourth in those of the silver fir.
In monocotyledonous plants there is only one seed-leaf or cotyledon,
and hence the arrangement is at first alternate; and it generally
continues so more or less, rarely being verticillate. Such
arrangements as ½, 1/3 and 2/3 are common in Monocotyledons, as in
grasses, sedges and lilies. It has been found in general that, while
the number 5 occurs in the phyllotaxis of Dicotyledons, 3 is common in
that of Monocotyledons.
[Illustration: FIG. 34.--Cycle of thirteen leaves placed closely
together so as to form a rosette, as in _Sempervivum_. A is the very
short axis to which the leaves are attached. The leaves are numbered
in their order, from below upwards. The circles in the centre indicate
the five turns of the spiral, and show the insertion of each of the
leaves. The divergence is expressed by the fraction (5/13)ths._]
[Illustration: FIG. 35.--Cone of _Picea alba_ with the scales or
modified leaves numbered in the order of their arrangement on the axis
of the cone. The lines indicate a rectilinear series of scales and two
lateral secondary spirals, one turning from left to right, the other
from right to left.]
In the axil of previously formed leaves leaf-buds arise. These
leaf-buds contain the rudiments of a shoot, and consist of leaves
covering a growing point. The buds of trees of temperate climates,
which lie dormant during the winter, are protected by scale leaves.
These scales or protective appendages of the bud consist either of the
altered laminae or of the enlarged petiolary sheath, or of stipules,
as in the fig and magnolia, or of one or two of these parts combined.
These are often of a coarse nature, serving a temporary purpose, and
then falling off when the leaf is expanded. They are frequently
covered with a resinous matter, as in balsam-poplar and
horse-chestnut, or by a thick downy covering as in the willow. In
plants of warm climates the buds have often no protective appendages,
and are then said to be _naked_.
[Illustration: FIG. 36.--Circinate vernation.
FIG. 37.--Transverse section of a conduplicate leaf.
FIG. 38.--Transverse section of a plicate or plaited leaf.
FIG. 39.--Transverse section of a convolute leaf.
FIG. 40.--Transverse section of an involute leaf.