118. STEMS bear leaves, at definite points (nodes, 13); and these are
produced in a great variety of forms, and subserve various uses. The
commonest kind of leaf, which therefore may be taken as the type or
pattern, is an expanded green body, by means of which the plant exposes
to the air and light the matters which it imbibes, exhales certain
portions, and assimilates the residue into vegetable matter for its
nourishment and growth.
119. But the fact is already familiar (10-30) that leaves occur under
other forms and serve for other uses,--for the storage of food already
assimilated, as in thickened seed-leaves and bulb-scales; for covering,
as in bud-scales; and still other uses are to be pointed out. Indeed,
sometimes they are of no service to the plant, being reduced to mere
scales or rudiments, such as those on the rootstocks of Peppermint (Fig.
97) or the tubers of Jerusalem Artichoke (Fig. 101). These may be said
to be of service only to the botanist, in explaining to him the plan
upon which a plant is constructed.
120. Accordingly, just as a rootstock, or a tuber, or a tendril is a
kind of stem, so a bud-scale, or a bulb-scale, or a cotyledon, or a
petal of a flower, is a kind of leaf. Even in respect to ordinary
leaves, it is natural to use the word either in a wider or in a narrower
sense; as when in one sense we say that a leaf consists of blade and
petiole or leaf-stalk, and in another sense say that a leaf is petioled,
or that the leaf of Hepatica is three-lobed. The connection should make
it plain whether by leaf we mean leaf-blade only, or the blade with any
other parts it may have. And the student will readily understand that by
leaf in its largest or _morphological_ sense, the botanist means the
organ which occupies the place of a leaf, whatever be its form or its
function.
Sec. 1. LEAVES AS FOLIAGE.
121. This is tautological; for foliage is simply leaves: but it is very
convenient to speak of typical leaves, or those which serve the plant
for assimilation, as foliage-leaves, or ordinary leaves. These may first
be considered.
122. =The Parts of a Leaf.= The ordinary leaf, complete in its parts,
consists of _blade_, _foot-stalk_, or _petiole_, and a pair of
_stipules_.
123. First the BLADE or LAMINA, which is the essential part of ordinary
leaves, that is, of such as serve the purpose of foliage. In structure
it consists of a softer part, the _green pulp_, called _parenchyma_,
which is traversed and supported by a fibrous frame, the parts of which
are called _ribs_ or _veins_, on account of a certain likeness in
arrangement to the veins of animals. The whole surface is covered by a
transparent skin, the _Epidermis_, not unlike that which covers the
surface of all fresh shoots.
124. Note that the leaf-blade expands horizontally,--that is, normally
presents its faces one to the sky, the other to the ground, or when the
leaf is erect the upper face looks toward the stem that bears it, the
lower face away from it. Whenever this is not the case there is
something to be explained.
125. The framework consists of _wood_,--a fibrous and tough material
which runs from the stem through the leaf-stalk, when there is one, in
the form of parallel threads or bundles of fibres; and in the blade
these spread out in a horizontal direction, to form the _ribs_ and
_veins_ of the leaf. The stout main branches of the framework are called
the _Ribs_. When there is only one, as in Fig. 112, 114, or a middle one
decidedly larger than the rest, it is called the _Midrib_. The smaller
divisions are termed _Veins_; and their still smaller subdivisions,
_Veinlets_. The latter subdivide again and again, until they become so
fine that they are invisible to the naked eye. The fibres of which they
are composed are hollow; forming tubes by which the sap is brought into
the leaves and carried to every part.
[Illustration: Fig. 112. Leaf of the Quince: _b_, blade; _p_, petiole;
_st_, stipules.]
126. =Venation= is the name of the mode of veining, that is, of the way
in which the veins are distributed in the blade. This is of two
principal kinds; namely, the _parallel-veined_, and the _netted-veined_.
127. In _Netted-veined_ (also called _Reticulated_) leaves, the veins
branch off from the main rib or ribs, divide into finer and finer
veinlets, and the branches unite with each other to form meshes of
network. That is, they _anastomose_, as anatomists say of the veins and
arteries of the body. The Quince-leaf, in Fig. 112, shows this kind of
veining in a leaf with a single rib. The Maple, Basswood, Plane or
Buttonwood (Fig. 74) show it in leaves of several ribs.
128. In _parallel-veined_ leaves, the whole framework consists of
slender ribs or veins, which run parallel with each other, or nearly so,
from the base to the point of the leaf,--not dividing and subdividing,
nor forming meshes, except by minute cross-veinlets. The leaf of any
grass, or that of the Lily of the Valley (Fig. 113) will furnish a good
illustration. Such parallel veins Linnaeus called _Nerves_, and
parallel-veined leaves are still commonly called _nerved_ leaves, while
those of the other kind are said to be _veined_,--terms which it is
convenient to use, although these "nerves" and "veins" are all the same
thing, and have no likeness to the _nerves_ and little to the veins of
animals.
129. _Netted-veined_ leaves belong to plants which have a pair of
seed-leaves or cotyledons, such as the Maple (Fig. 20, 24), Beech (Fig.
33), and the like; while _parallel-veined_ or _nerved_ leaves belong to
plants with one cotyledon or true seed-leaf; such as the Iris (Fig. 59),
and Indian Corn (Fig. 70). So that a mere glance at the leaves generally
tells what the structure of the embryo is, and refers the plant to one
or the other of these two grand classes,--which is a great convenience.
For when plants differ from each other in some one important respect,
they usually differ correspondingly in other respects also.
[Illustration: Fig. 113. A (parallel-veined) leaf of the Lily of the
Valley. 114. One of the Calla Lily.]
130. Parallel-veined leaves are of two sorts,--one kind, and the
commonest, having the ribs or nerves all running from the base to the
point of the leaf, as in the examples already given; while in another
kind they run from a midrib to the margin, as in the common
Pickerel-weed of our ponds, in the Banana, in Calla (Fig. 114), and many
similar plants of warm climates.
131. Netted-veined leaves are also of two sorts, as in the examples
already referred to. In one case the veins all rise from a single rib
(the midrib), as in Fig. 112, 116-127. Such leaves are called
_Feather-veined_ or _Penni-veined_, i. e. _Pinnately-veined_; both terms
meaning the same thing, namely, that the veins are arranged on the sides
of the rib like the plume of a feather on each side of the shaft.
132. In the other case (as in Fig. 74, 129-132), the veins branch off
from three, five, seven, or nine ribs, which spread from the top of the
leaf-stalk, and run through the blade like the toes of a web-footed
bird. Hence these are said to be _Palmately_ or _Digitately_ veined, or
(since the ribs diverge like rays from a centre) _Radiate-veined_.
133. Since the general outline of leaves accords with the framework or
skeleton, it is plain that _feather-veined_ (or _penni-veined_) leaves
will incline to elongated shapes, or at least to be longer than broad;
while in _radiate-veined_ leaves more rounded forms are to be expected.
A glance at the following figures shows this.
[Illustration: Fig. 115-120. A series of shapes of feather-veined
leaves.]
134. =Forms of Leaves as to General Outline.= It is necessary to give
names to the principal shapes, and to define them rather precisely,
since they afford easy marks for distinguishing species. The same terms
are used for all other flattened parts as well, such as petals; so that
they make up a great part of the descriptive language of Botany. It will
be a good exercise for young students to look up leaves answering to
these names and definitions. Beginning with the narrower and proceeding
to the broadest forms, a leaf is said to be
_Linear_ (Fig. 115), when narrow, several times longer than wide, and of
the same breadth throughout.
_Lanceolate_, or _Lance-shaped_, when conspicuously longer than wide,
and tapering upwards (Fig. 116), or both upwards and downwards.
_Oblong_ (Fig. 117), when nearly twice or thrice as long as broad.
_Elliptical_ (Fig. 118) is oblong with a flowing outline, the two ends
alike in width.
_Oval_ is the same as broadly elliptical, or elliptical with the breadth
considerably more than half the length.
_Ovate_ (Fig. 119), when the outline is like a section of a hen's egg
lengthwise, the broader end downward.
_Orbicular_, or _Rotund_ (Fig. 132), circular in outline, or nearly so.
[Illustration: Fig. 121, oblanceolate; 122, spatulate; 123, obovate; and
124, wedge-shaped, feather-veined, leaves.]
135. A leaf which tapers toward the base instead of toward the apex may
be
_Oblanceolate_ (Fig. 121) when of the lance-shaped form, only more
tapering toward the base than in the opposite direction.
_Spatulate_ (Fig. 122) when more rounded above, but tapering thence to a
narrow base, like an old-fashioned spatula.
_Obovate_ (Fig. 123) or inversely ovate, that is, ovate with the
narrower end down.
_Cuneate_ or _Cuneiform_, that is, _Wedge-shaped_ (Fig. 124), broad
above and tapering by nearly straight lines to an acute angle at the
base.
[Illustration: Fig. 125, sagittate; 126, auriculate; and 127,
halberd-shaped or hastate leaves.]
136. =As to the Base=, its shape characterizes several forms, such as
_Cordate_ or _Heart-shaped_ (Fig. 120, 129), when a leaf of an ovate
form, or something like it, has the outline of its rounded base turned
in (forming a notch or _sinus_) where the stalk is attached.
_Reniform_, or _Kidney-shaped_ (Fig. 131), like the last, only rounder
and broader than long.
_Auriculate_, or _Eared_, having a pair of small and blunt projections,
or _ears_, at the base, as in one species of Magnolia (Fig. 126).
_Sagittate_, or _arrow-shaped_, where such ears are acute and turned
downwards, while the main body of the blade tapers upwards to a point,
as in the common Sagittaria or Arrow-head, and in the Arrow-leaved
Polygonum (Fig. 125).
_Hastate_, or _Halberd-shaped_, when such lobes at the base point
outwards, giving the shape of the halberd of the olden time, as in
another Polygonum (Fig. 127).
[Illustration: Fig. 128-132. Various forms of radiate-veined leaves.]
_Peltate_, or _Shield-shaped_ (Fig. 132), is the name applied to a
curious modification of the leaf, commonly of a rounded form, where the
footstalk is attached to the lower surface, instead of the base, and
therefore is naturally likened to a shield borne by the outstretched
arm. The common Watershield, the Nelumbium, and the White Water-lily,
and also the Mandrake, exhibit this sort of leaf. On comparing the
shield-shaped leaf of the common Marsh Pennywort (Fig. 132) with that of
another common species (Fig. 130), it is at once seen that a
shield-shaped leaf is like a kidney-shaped (Fig. 130, 131) or other
rounded leaf, with the margins at the base brought together and united.
137. =As to the Apex=, the following terms express the principal
variations:--
_Acuminate_, _Pointed_, or _Taper-pointed_, when the summit is more or
less prolonged into a narrowed or tapering point; as in Fig. 133.
_Acute_, ending in an acute angle or not prolonged point; Fig. 134.
_Obtuse_, with a blunt or rounded apex; as in Fig. 135, etc.
_Truncate_, with the end as if cut off square; as in Fig. 136.
_Retuse_, with rounded summit slightly indented, forming a very shallow
notch, as in Fig. 137.
_Emarginate_, or _Notched_, indented at the end more decidedly; as in
Fig. 138.
_Obcordate_, that is, inversely heart-shaped, where an obovate leaf is
more deeply notched at the end (Fig. 139), as in White Clover and
Wood-sorrel; so as to resemble a cordate leaf inverted.
_Cuspidate_, tipped with a sharp and rigid point; as in Fig. 140.
_Mucronate_, abruptly tipped with a small and short point, like a mere
projection of the midrib; as in Fig. 141.
_Aristate_, _Awn-pointed_, and _Bristle-pointed_, are terms used when
this mucronate point is extended into a longer bristle-form or slender
appendage.
The first six of these terms can be applied to the lower as well as to
the upper end of a leaf or other organ. The others belong to the apex
only.
[Illustration: Fig. 133-141. Forms of the apex of leaves.]
138. =As to degree and nature of Division=, there is first of all the
difference between
_Simple Leaves_, those in which the blade is of one piece, however much
it may be cut up, and
_Compound Leaves_, those in which the blade consists of two or more
separate pieces, upon a common leaf-stalk or support. Yet between these
two kinds every intermediate gradation is to be met with.
[Illustration: Fig. 142-147. Kinds of margin of leaves.]
139. =As to Particular Outlines of Simple Leaves= (and the same applies
to their separate parts), they are
_Entire_, when their general outline is completely filled out, so that
the margin is an even line, without teeth or notches.
_Serrate_, or _Saw-toothed_, when the margin only is cut into sharp
teeth, like those of a saw, and pointing forwards; as in Fig. 142.
_Dentate_, or _Toothed_, when such teeth point outwards, instead of
forwards; as in Fig. 143.
_Crenate_, or _Scalloped_, when the teeth are broad and rounded; as in
Fig. 144.
_Repand_, _Undulate_, or _Wavy_, when the margin of the leaf forms a
wavy line, bending slightly inwards and outwards in succession; as in
Fig. 145.
_Sinuate_, when the margin is more strongly sinuous or turned inwards
and outwards; as in Fig. 146.
_Incised_, _Cut_, or _Jagged_, when the margin is cut into sharp, deep,
and irregular teeth or incisions; as in Fig. 147.
_Lobed_, when deeply cut. Then the pieces are in a general way called
LOBES. The number of the lobes is briefly expressed by the phrase
_two-lobed_, _three-lobed_, _five-lobed_, _many-lobed_, etc., as the
case may be.
140. When the depth and character of the lobing needs to be more
particularly specified, the following terms are employed, viz.:--
_Lobed_, in a special sense, when the incisions do not extend deeper
than about half-way between the margin and the centre of the blade, if
so far, and are more or less rounded; as in the leaves of the Post-Oak,
Fig. 148, and the Hepatica, Fig. 152.
_Cleft_, when the incisions extend half way down or more, and especially
when they are sharp; as in Fig. 149, 153. And the phrases _two-cleft_,
or, in the Latin form, _bifid_, _three-cleft_ or _trifid_, _four-cleft_
or _quadrifid_, _five-cleft_ or _quinquefid_, etc., or _many-cleft_, in
the Latin form, _multifid_,--express the number of the _Segments_, or
portions.
_Parted_, when the incisions are still deeper, but yet do not quite
reach to the midrib or the base of the blade; as in Fig. 150, 154. And
the terms _two-parted_, _three-parted_, etc., express the number of such
divisions.
_Divided_, when the incisions extend quite to the midrib, as in the
lower part of Fig. 151, or to the leaf-stalk, as in Fig. 155; which
really makes the leaf compound. Here, using the Latin form, the leaf is
said to be _bisected_, _trisected_ (Fig. 155), etc., according to the
number of the divisions.
[Illustration: Fig. 148, pinnately lobed; 149, pinnately cleft; 150,
pinnately parted; 151, pinnately divided, leaves.]
[Illustration: Fig. 152, palmately three-lobed; 153, palmately
three-cleft; 154, palmately three-parted; 155, palmately three-divided
or trisected, leaves.]
141. =The Mode of Lobing or Division= corresponds to that of the
veining, whether _pinnately veined_ or _palmately veined_. In the former
the notches or incisions, or _sinuses_, coming between the principal
veins or ribs are directed toward the midrib: in the latter they are
directed toward the apex of the petiole; as the figures show.
142. So degree and mode of division may be tersely expressed in brief
phrases. Thus, in the four upper figures of pinnately veined leaves, the
first is said to be _pinnately lobed_ (in the special sense), the second
_pinnately cleft_ (or _pinnatifid_ in Latin form), the third _pinnately
parted_, the fourth _pinnately divided_, or _pinnatisected_.
143. Correspondingly in the lower row, of palmately veined leaves, the
first is _palmately lobed_, the second _palmately cleft_, the third
_palmately parted_, the fourth _palmately divided_. Or, in other
language of the same meaning (but now less commonly employed), they are
said to be _digitately lobed_, _cleft_, _parted_, or _divided_.
144. The number of the divisions or lobes may come into the phrase. Thus
in the four last named figures the leaves are respectively _palmately
three-lobed_, _three-cleft_ (or _trifid_), _three-parted_,
_three-divided_, or better (in Latin form), _trisected_. And so for
higher numbers, as _five-lobed_, _five-cleft_, etc., up to
_many-lobed_, _many-cleft_ or _multifid_, etc. The same mode of
expression may be used for pinnately lobed leaves, as _pinnately
7-lobed_, _-cleft_, _-parted_, etc.
145. The divisions, lobes, etc., may themselves be _entire_ (without
teeth or notches), or _serrate_, or otherwise toothed or incised; or
lobed, cleft, parted, etc.: in the latter cases making _twice
pinnatifid_, _twice palmately_ or _pinnately lobed_, _parted_, or
_divided_ leaves, etc. From these illustrations one will perceive how
the botanist, in two or three words, may describe any one of the almost
endlessly diversified shapes of leaves, so as to give a clear and
definite idea of it.
146. =Compound Leaves.= A compound leaf is one which has its blade in
entirely separate parts, each usually with a stalklet of its own; and
the stalklet is often _jointed_ (or _articulated_) with the main
leaf-stalk, just as this is jointed with the stem. When this is the
case, there is no doubt that the leaf is compound. But when the pieces
have no stalklets, and are not jointed with the main leaf-stalk, it may
be considered either as a divided simple leaf, or a compound leaf,
according to the circumstances. This is a matter of names where all
intermediate forms may be expected.
147. While the pieces or projecting parts of a simple leaf-blade are
called _Lobes_, or in deeply cut leaves, etc., _Segments_, or
_Divisions_, the separate pieces or blades of a compound leaf are called
LEAFLETS.
148. Compound leaves are of two principal kinds, namely, the _Pinnate_
and the _Palmate_; answering to the two modes of veining in reticulated
leaves, and to the two sorts of lobed or divided leaves (141).
[Illustration: Fig. 156-158. Pinnate leaves, the first with an odd
leaflet (_odd-pinnate_); the second with a tendril in place of uppermost
leaflets; the third _abruptly pinnate_, or of even pairs.]
149. _Pinnate_ leaves are those in which the leaflets are arranged on
the sides of a main leaf-stalk; as in Fig. 156-158. They answer to the
_feather-veined_ (i. e. _pinnately-veined_) simple leaf; as will be
seen at once on comparing the forms. The _leaflets_ of the former answer
to the _lobes_ or _divisions_ of the latter; and the continuation of the
petiole, along which the leaflets are arranged, answers to the midrib of
the simple leaf.
150. Three sorts of pinnate leaves are here given. Fig. 156 is _pinnate
with an odd_ or _end leaflet_, as in the Common Locust and the Ash. Fig.
157 is _pinnate with a tendril at the end_, in place of the odd leaflet,
as in the Vetches and the Pea. Fig. 158 is evenly or _abruptly pinnate_,
as in the Honey-Locust.
[Illustration: Fig. 159. Palmate (or digitate) leaf of five leaflets, of
the Sweet Buckeye.]
151. _Palmate_ (also named _Digitate_) leaves are those in which the
leaflets are all borne on the tip of the leaf-stalk, as in the Lupine,
the Common Clover, the Virginia Creeper (Fig. 93), and the
Horse-chestnut and Buckeye (Fig. 159). They evidently answer to the
_radiate-veined_ or _palmately-veined_ simple leaf. That is, the
Clover-leaf of three leaflets is the same as a palmately three-ribbed
leaf cut into three separate leaflets. And such a simple five-lobed leaf
as that of the Sugar Maple, if more cut, so as to separate the parts,
would produce a palmate leaf of five leaflets, like that of the
Horse-chestnut or Buckeye.
152. Either sort of compound leaf may have any number of leaflets; yet
palmate leaves cannot well have a great many, since they are all crowded
together on the end of the main leaf-stalk. Some Lupines have nine or
eleven; the Horse-chestnut has seven, the Sweet Buckeye more commonly
five, the Clover three. A pinnate leaf often has only seven or five
leaflets, or only three, as in Beans of the genus Phaseolus, etc.; in
some rarer cases only two; in the Orange and Lemon and also in the
common Barberry there is only one! The joint at the place where the
leaflet is united with the petiole distinguishes this last case from a
simple leaf. In other species of these genera the lateral leaflets also
are present.
153. The leaflets of a compound leaf may be either _entire_ (as in Fig.
126-128), or _serrate_, or lobed, cleft, parted, etc.; in fact, may
present all the variations of simple leaves, and the same terms equally
apply to them.
154. When the division is carried so far as to separate what would be
one leaflet into two, three, or several, the leaf becomes _doubly_ or
_twice compound_, either _pinnately_ or _palmately_, as the case may be.
For example, while the clustered leaves of the Honey-Locust are _simply
pinnate_, that is, _once pinnate_, those on new shoots are _bipinnate_,
or _twice pinnate_, as in Fig. 160. When these leaflets are again
divided in the same way, the leaf becomes _thrice pinnate_, or
_tripinnate_, as in many Acacias. The first divisions are called
_Pinnae_; the others, _Pinnules_; and the last, or little blades
themselves, _Leaflets_.
[Illustration: Fig. 160. A twice-pinnate (abruptly) leaf of the
Honey-Locust.]
155. So the palmate leaf, if again compounded in the same way, becomes
_twice palmate_, or, as we say when the divisions are in threes, _twice
ternate_ (in Latin form _biternate_); if a third time compounded,
_thrice ternate_ or _triternate_. But if the division goes still
further, or if the degree is variable, we simply say that the leaf is
_decompound_; either palmately or pinnately decompound, as the case may
be. Thus, Fig. 161 represents a four times ternately compound (in other
words a _ternately decompound_) leaf of a common Meadow Rue.
[Illustration: Fig. 161. Ternately decompound leaf of Meadow Rue.]
156. When the botanist, in describing leaves, wishes to express the
number of the leaflets, he may use terms like these:--
_Unifoliolate_, for a compound leaf of a single leaflet; from the Latin
_unum_, one, and _foliolum_, leaflet.
_Bifoliolate_, of two leaflets, from the Latin _bis_, twice, and
_foliolum_, leaflet.
_Trifoliolate_ (or _ternate_), of three leaflets, as the Clover; and so
on.
_Palmately bifoliolate_, _trifoliolate_, _quadrifoliolate_,
_plurifoliolate_ (of several leaflets), etc.: or else
_Pinnately bi-_, _tri-_, _quadri-_, or _plurifoliolate_ (that is, of
two, three, four, five, or several leaflets), as the case may be: these
are terse ways of denoting in single phrases both the number of leaflets
and the kind of compounding.
157. Of foliage-leaves having certain peculiarities in structure, the
following may be noted:--
158. =Perfoliate Leaves.= In these the stem that bears them seems to
run through the blade of the leaf, more or less above its base. A common
Bellwort (Uvularia perfoliata, Fig. 162) is a familiar illustration. The
lower and earlier leaves show it distinctly. Later, the plant is apt to
produce some leaves merely clasping the stem by the sessile and
heart-shaped base, and the latest may be merely sessile. So the series
explains the peculiarity: in the formation of the leaf the bases,
meeting around the stem, grow together there.
[Illustration: Fig. 162. A summer branch of Uvularia perfoliata; lower
leaves perfoliate, upper cordate-clasping, uppermost simply sessile.]
[Illustration: Fig. 163. Branch of a Honeysuckle, with
connate-perfoliate leaves.]
159. =Connate-perfoliate.= Such are the upper leaves of true
Honeysuckles. Here (Fig. 163) of the opposite and sessile leaves, some
pairs, especially the uppermost, in the course of their formation unite
around the stem, which thus seems to run through the disk formed by
their union.
[Illustration: Fig. 164. Rootstock and equitant leaves of Iris. 165. A
section across the cluster of leaves at the bottom, showing the
equitation.]
160. =Equitant Leaves.= While ordinary leaves spread horizontally, and
present one face to the sky and the other to the earth, there are some
that present their tip to the sky, and their faces right and left to the
horizon. Among these are the _equitant_ leaves of the Iris or
Flower-de-Luce. Inspection shows that each leaf was formed as if _folded
together lengthwise_, so that what would be the upper surface is
within, and all grown together, except next the bottom, where each leaf
covers the next younger one. It was from their straddling over each
other, like a man on horseback (as is seen in the cross-section, Fig.
165), that Linnaeus, with his lively fancy, called these _Equitant_
leaves.
161. =Leaves with no distinction of Petiole and Blade.= The leaves of
Iris just mentioned show one form of this. The flat but narrow leaves of
Jonquils, Daffodils, and the cylindrical leaf of Onions are other
instances. _Needle-shaped_ leaves, like those of the Pine, Larch, and
Spruce, and the _awl-shaped_ as well as the _scale-shaped_ leaves of
Junipers, Red Cedar, and Arbor-Vitae (Fig. 166), are examples.
[Illustration: Fig. 166. Branch of Arbor-Vitae, with awl-shaped and
scale-shaped leaves.]
162. =Phyllodia.= Sometimes an expanded _petiole_ takes the place of the
blade; as in numerous New Holland Acacias, some of which are now common
in greenhouses. Such counterfeit blades are called _phyllodia_,--meaning
leaf-like bodies. They may be known from true blades by their standing
edgewise, their margins being directed upwards and downwards; while in
true blades the faces look upwards and downwards; excepting in equitant
leaves, as already explained.
163. =Falsely Vertical Leaves.= These are apparent exceptions to the
rule, the blade standing edgewise instead of flatwise to the stem; but
this position comes by a twist of the stalk or the base of the blade.
Such leaves present the two faces about equally to the light. The
Compass-plant (Silphium laciniatum) is an example. So also the leaves of
Boltonia, of Wild Lettuce, and of a vast number of Australian Myrtaceous
shrubs and trees, which much resemble the phyllodia of the Acacias of
the same country. They are familiar in Callistemon, the Bottle-brush
Flower, and in Eucalyptus. But in the latter the leaves of the young
tree have the normal structure and position.
[Illustration: Fig. 167. The ambiguous leaf? (cladophyllum) of
Myrsiphyllum.]
[Illustration: Fig. 168. Same of Ruscus, or Butcher's Broom.]
164. =Cladophylla=, meaning _branch-leaves_. The foliage of Ruscus (the
Butcher's Broom of Europe) and of Myrsiphyllum of South Africa
(cultivated for decoration under the false name of Smilax) is peculiar
and puzzling. If these blades (Fig. 167, 168) are really leaves, they
are most anomalous in occupying the axil of another leaf, reduced to a
little scale. Yet they have an upper and lower face, as leaves should,
although they soon twist, so as to stand more or less edgewise. If they
are branches which have assumed exactly the form and office of leaves,
they are equally extraordinary in not making any further development.
But in Ruscus, flowers are borne on one face, in the axil of a little
scale: and this would seem to settle that they are branches. In
Asparagus just the same things as to position are thread-shaped and
branch-like.
Sec. 2. LEAVES OF SPECIAL CONFORMATION AND USE.
[Illustration: Fig. 169. A young Agave Americana, or Century-plant;
fleshy-leaved.]
165. =Leaves for Storage.= A leaf may at the same time serve both
ordinary and special uses. Thus in those leaves of Lilies, such as the
common White Lily, which spring from the bulb, the upper and green part
serves for foliage and elaborates nourishment, while the thickened
portion or bud-scale beneath serves for the storage of this nourishment.
The thread-shaped leaf of the Onion fulfils the same office, and the
nourishing matter it prepares is deposited in its sheathing base,
forming one of the concentric layers of the onion. When these layers, so
thick and succulent, have given up their store to the growing parts
within, they are left as thin and dry husks. In a Houseleek, an Aloe or
an Agave, the green color of the surface of the fleshy leaf indicates
that it is doing the work of foliage; the deeper-seated white portion
within is the storehouse of the nourishment which the green surface has
elaborated. So, also, the seed-leaves or cotyledons are commonly used
for storage. Some, as in one of the Maples, the Pea, Horse-chestnut,
Oak, etc., are for nothing else. Others, as in Beech and in our common
Beans, give faint indications of service as foliage also, chiefly in
vain. Still others, as in the Pumpkin and Flax, having served for
storage, develop into the first efficient foliage. Compare 11, 22-30,
and the accompanying figures.
[Illustration: Fig. 170. Series of bud-scales and foliage-leaves from a
developing bud of the Low Sweet Buckeye (AEsculus parviflora), showing
nearly complete gradation, from a scale to a compound leaf of five
leaflets; and that the scales answer to reduced petioles.]
166. =Leaves as Bud-Scales= serve to protect the forming parts within.
Having fulfilled this purpose they commonly fall off when the shoot
develops and foliage-leaves appear. Occasionally, as in Fig. 170, there
is a transition of bud-scales to leaves, which reveals the nature of the
former. The Lilac also shows a gradation from bud-scale to simple leaf.
In Cornus florida (the Flowering Dogwood), the four bud-scales which
through the winter protect the head of forming flowers remain until
blossoming, and then the base of each grows out into a large and very
showy petal-like leaf; the original dry scale is apparent in the notch
at the apex.
[Illustration: Fig. 171. Shoot of common Barberry, showing transition of
foliage-leaves to spines.]
167. =Leaves as Spines= occur in several plants. A familiar instance is
that of the common Barberry (Fig. 171). In almost any summer shoot, most
of the gradations may be seen between the ordinary leaves, with sharp
bristly teeth, and leaves which are reduced to a branching spine or
thorn. The fact that the spines of the Barberry produce a leaf-bud in
their axil also proves them to be leaves.
[Illustration: Fig. 172. Leaves of Solanum jasminoides, the petiole
adapted for climbing.]
[Illustration: Fig. 173. Leaf of Lathyrus Aphaca, consisting of a pair
of stipules and a tendril.]
168. =Leaves for Climbing= are various in adaptation. True
foliage-leaves serve this purpose; as in Gloriosa, where the attenuated
tip of a simple leaf (otherwise like that of a Lily) hooks around a
supporting object; or in Solanum jasminoides of the gardens (Fig. 172),
and in Maurandia, etc., where the leaf-stalk coils round and clings to a
support; or in the compound leaves of Clematis and of Adlumia, in which
both the leaflets and their stalks hook or coil around the support.
169. Or in a compound leaf, as in the Pea and most Vetches, and in
Cobaea, while the lower leaflets serve for foliage, some of the uppermost
are developed as tendrils for climbing (Fig. 167). In the common Pea
this is so with all but one or two pairs of leaflets.
170. In one European Vetch, the leaflets are wanting and the whole
petiole is a tendril, while the stipules become the only foliage (Fig.
173).
171. =Leaves as Pitchers=, or hollow tubes, are familiar in the common
Pitcher-plant or Side-saddle Flower (Sarracenia, Fig. 174) of our bogs.
These pitchers are generally half full of water, in which flies and
other insects are drowned, often in such numbers as to make a rich
manure for the plant. More curious are some of the southern species of
Sarracenia, which seem to be specially adapted to the capture and
destruction of flies and other insects.
[Illustration: Fig. 174. Leaf of Sarracenia purpurea, entire, and
another with the upper part cut off.]
172. The leaf of Nepenthes (Fig. 175) combines three structures and
uses. The expanded part below is foliage: this tapers into a tendril for
climbing; and this bears a pitcher with a lid. Insects are caught, and
perhaps digested, in the pitcher.
[Illustration: Fig. 175. Leaf of Nepenthes; foliage, tendril, and
pitcher combined.]
[Illustration: Fig. 176. Leaves of Dionaea; the trap in one of them open,
in the others closed.]
173. =Leaves as Fly-traps.= Insects are caught in another way, and more
expertly, by the most extraordinary of all the plants of this country,
the Dionaea or Venus's Fly-trap, which grows in the sandy bogs around
Wilmington, North Carolina. Here (Fig. 176) each leaf bears at its
summit an appendage which opens and shuts, in shape something like a
steel-trap, and operating much like one. For when open, no sooner does a
fly alight on its surface, and brush against any one of the two or three
bristles that grow there, than the trap suddenly closes, capturing the
intruder. If the fly escapes, the trap soon slowly opens, and is ready
for another capture. When retained, the insect is after a time moistened
by a secretion from minute glands of the inner surface, and is digested.
In the various species of Drosera or Sundew, insects are caught by
sticking fast to very viscid glands at the tip of strong bristles, aided
by adjacent gland-tipped bristles which bend slowly toward the captive.
The use of such adaptations and operations may be explained in another
place.
Sec. 3. STIPULES.
174. A leaf complete in its parts consists of blade, leaf-stalk or
petiole, and a pair of stipules. But most leaves have either fugacious
or minute stipules or none at all; many have no petiole (the blade being
_sessile_ or stalkless); some have no clear distinction of blade and
petiole; and many of these, such as those of the Onion and all phyllodia
(166), consist of petiole only.
175. The base of the petiole is apt to be broadened and flattened,
sometimes into thin margins, sometimes into a sheath which embraces the
stem at the point of attachment.
[Illustration: Fig. 177. Leaf of Red Clover: _st_, stipules, adhering to
the base of _p_, the petiole; _b_, blade of three leaflets.]
[Illustration: Fig. 178. Part of stem and leaf of Prince's-Feather
(Polygonum orientale) with the united sheathing stipules forming a
sheath or _ocrea_.]
[Illustration: Fig. 179. Terminal winter bud of Magnolia Umbrella,
natural size. 180. Outermost bud-scale (pair of stipules) detached.]
176. =Stipules= are such appendages, either wholly or partly separated
from the petiole. When quite separate they are said to be _free_, as in
Fig. 112. When attached to the base of the petiole, as in the Rose and
in Clover (Fig. 177), they are _adnate_. When the two stipules unite
and sheathe the stem above the insertion, as in Polygonum (Fig. 178),
this sheath is called an _Ocrea_ from its likeness to a greave or
leggin.
177. In Grasses, when the sheathing base of the leaf may answer to
petiole, the summit of the sheath commonly projects as a thin and short
membrane, like an ocrea: this is called a LIGULA or LIGULE.
178. When stipules are green and leaf-like they act as so much foliage.
In the Pea they make up no small part of the actual foliage. In a
related plant (Lathyrus Aphaca, Fig. 173), they make the whole of it,
the remainder of the leaf being tendril.
179. In many trees the stipules are the bud-scales, as in the Beech, and
very conspicuously in the Fig-tree, Tulip-tree, and Magnolia (Fig. 179).
These fall off as the leaves unfold.
180. The stipules are spines or prickles in Locust and several other
Leguminous trees and shrubs; they are tendrils in Smilax or Greenbrier.
Sec. 4. THE ARRANGEMENT OF LEAVES.
181. =Phyllotaxy=, meaning leaf-arrangement, is the study of the
position of leaves, or parts answering to leaves, upon the stem.
[Illustration: Fig. 181. Alternate leaves, in Linden, Lime-tree, or
Basswood.]
[Illustration: Fig. 182. Opposite leaves, in Red Maple.]
182. The technical name for the attachment of leaves to the stem is the
_insertion_. Leaves (as already noticed, 54) are _inserted_ in three
modes. They are
_Alternate_ (Fig. 181), that is, one after another, or in other words,
with only a single leaf to each node;
_Opposite_ (Fig. 182), when there is a pair to each node, the two
leaves in this case being always on opposite sides of the stem;
_Whorled_ or _Verticillate_ (Fig. 183) when there are more than two
leaves on a node, in which case they divide the circle equally between
them, forming a _Verticel_ or whorl. When there are three leaves in the
whorl, the leaves are one third of the circumference apart; when four,
one quarter, and so on. So the plan of opposite leaves, which is very
common, is merely that of whorled leaves, with the fewest leaves to the
whorl, namely, two.
[Illustration: Fig. 183. Whorled leaves of Galium.]
183. In both modes and in all their modifications, the arrangement is
such as to distribute the leaves systematically and in a way to give
them a good exposure to the light.
[Illustration: Fig. 184. A piece of stem of Larch with two clusters
(fascicles) of numerous leaves.]
[Illustration: Fig. 185. Piece of a branch of Pitch Pine, with three
leaves in a fascicle or bundle, in the axil of a thin scale which
answers to a primary leaf. The bundle is surrounded at the base by a
short sheath, formed of the delicate scales of the axillary bud.]
184. No two or more leaves ever grow from the same point. The so-called
_Fascicled_ or _Clustered_ leaves are the leaves of a branch the nodes
of which are very close, just as they are in the bud, so keeping the
leaves in a cluster. This is evident in the Larch (Fig. 184), in which
examination shows each cluster to be made up of numerous leaves crowded
on a spur or short axis. In spring there are only such clusters; but in
summer some of them lengthen into ordinary shoots with scattered
alternate leaves. So, likewise, each cluster of two or three
needle-shaped leaves in Pitch Pines (as in Fig. 185), or of five leaves
in White Pine, answers to a similar extremely short branch, springing
from the axil of a thin and slender scale, which represents a leaf of
the main shoot. For Pines produce two kinds of leaves,--1. primary, the
proper leaves of the shoots, not as foliage, but in the shape of
delicate scales in spring, which soon fall away; and 2. secondary, the
_fascicled_ leaves, from buds in the axils of the former, and these form
the actual foliage.
185. =Phyllotaxy of Alternate Leaves.= Alternate leaves are distributed
along the stem in an order which is uniform for each species. The
arrangement in all its modifications is said to be _spiral_, because, if
we draw a line from the _insertion_ (i. e. the point of attachment) of
one leaf to that of the next, and so on, this line will wind spirally
around the stem as it rises, and in the same species will always bear
the same number of leaves for each turn round the stem. That is, any two
successive leaves will always be separated from each other by an equal
portion of the circumference of the stem. The distance in _height_
between any two leaves may vary greatly, even on the same shoot, for
that depends upon the length of the _internodes_, or spaces between the
leaves; but the distance as measured around the circumference (in other
words, the _Angular Divergence_, or angle formed by any two successive
leaves) is uniformly the same.
186. =Two-ranked.= The greatest possible divergence is, of course, where
the second leaf stands on exactly the opposite side of the stem from the
first, the third on the side opposite the second, and therefore over the
first, and the fourth over the second. This brings all the leaves into
two ranks, one on one side of the stem and one on the other, and is
therefore called the _Two-ranked_ arrangement. It occurs in all
Grasses,--in Indian Corn, for instance; also, in the Basswood (Fig.
181). This is the simplest of all arrangements, and the one which most
widely distributes successive leaves, but which therefore gives the
fewest vertical ranks. Next is the
187. =Three-ranked= arrangement,--that of all Sedges, and of White
Hellebore. Here the second leaf is placed one third of the way round the
stem, the third leaf two thirds of the way round, the fourth leaf
accordingly directly over the first, the fifth over the second, and so
on. That is, three leaves occur in each turn round the stem, and they
are separated from each other by one third of the circumference. (Fig.
186, 187.)
[Illustration: Fig. 186. Two-ranked arrangement, shown in a piece of the
stalk of a Sedge, with the leaves cut off above their bases; the leaves
are numbered in order, from 1 to 6. 187. Diagram or cross-section of the
same, in one plane; the leaves similarly numbered; showing two cycles of
three.]
188. =Five-ranked= is the next in the series, and the most common. It is
seen in the Apple (Fig. 188), Cherry, Poplar, and the greater number of
trees and shrubs. In this case the line traced from leaf to leaf will
pass twice round the stem before it reaches a leaf situated directly
over any below (Fig. 189). Here the sixth leaf is over the first; the
leaves stand in five perpendicular ranks, with equal angular distance
from each other; and this distance between any two successive leaves is
just two fifths of the circumference of the stem.
[Illustration: Fig. 188. Shoot with its leaves 5-ranked, the sixth leaf
over the first; as in the Apple-tree.]
[Illustration: Fig. 189. Diagram of this arrangement, with a spiral line
drawn from the attachment of one leaf to the next, and so on; the parts
on the side turned from the eye are fainter.]
[Illustration: Fig. 190. A ground-plan of the same; the section of the
leaves similarly numbered; a dotted line drawn from the edge of one leaf
to that of the next marks out the spiral.]
189. The five-ranked arrangement is expressed by the fraction 2/5. This
fraction denotes the divergence of the successive leaves, i. e. the
angle they form with each other: the numerator also expresses the number
of turns made round the stem by the spiral line in completing one cycle
or set of leaves, namely, two; and the denominator gives the number of
leaves in each cycle, or the number of perpendicular ranks, namely,
five. In the same way the fraction 1/2 stands for the two-ranked mode,
and 1/3 for the three-ranked: and so these different sorts are expressed
by the series of fractions 1/2, 1/3, 2/5. Other cases follow in the same
numerical progression, the next being the
190. =Eight-ranked= arrangement. In this the ninth leaf stands over the
first, and three turns are made around the stem to reach it; so it is
expressed by the fraction 3/8. This is seen in the Holly, and in the
common Plantain. Then comes the
191. =Thirteen-ranked= arrangement, in which the fourteenth leaf is over
the first, after five turns around the stem. The common Houseleek (Fig.
191) is a good example.
192. The series so far, then, is 1/2, 1/3, 2/5, 3/8, 5/13; the numerator
and the denominator of each fraction being those of the two next
preceding ones added together. At this rate the next higher should be
8/21, then 13/34, and so on; and in fact just such cases are met with,
and (commonly) no others. These higher sorts are found in the Pine
Family, both in the leaves and the cones and in many other plants with
small and crowded leaves. But in those the number of the ranks, or of
leaves in each cycle, can only rarely be made out by direct inspection.
They may be indirectly ascertained, however, by studying the _secondary_
spirals, as they are called, which usually become conspicuous, at least
two series of them, one turning to the right and one to the left, as
shown in Fig. 191. For an account of the way in which the character of
the phyllotaxy may be deduced from the secondary spirals, see Structural
Botany, Chapter IV.
[Illustration: Fig. 191. A young plant of the Houseleek, with the leaves
(not yet expanded) numbered, and exhibiting the 13-ranked arrangement;
and showing secondary spirals.]
193. =Phyllotaxy of Opposite and whorled Leaves.= This is simple and
comparatively uniform. The leaves of each pair or whorl are placed over
the intervals between those of the preceding, and therefore under the
intervals of the pair or whorl next above. The whorls or pairs alternate
or cross each other, usually at right angles, that is, they _decussate_.
Opposite leaves, that is, whorls of two leaves only, are far commoner
than whorls of three or four or more members. This arrangement in
successive decussating pairs gives an advantageous distribution on the
stem in four vertical ranks. Whorls of three give six vertical ranks,
and so on. Note that in descriptive botany leaves in whorls of two are
simply called _opposite_ leaves; and that the term _verticillate_ or
_whorled_, is employed only for cases of more than two, unless the
latter number is specified.
[Illustration: Fig. 192. Opposite leaves of Euonymus, or Spindle-tree,
showing the successive pairs crossing each other at right angles.]
194. =Vernation or Praefoliation=, the disposition of the leaf-blades in
the bud, comprises two things; 1st, the way in which each separate leaf
is folded, coiled, or packed up in the bud; and 2d, the arrangement of
the leaves in the bud with respect to one another. The latter of course
depends very much upon the phyllotaxy, i. e. the position and order of
the leaves upon the stem. The same terms are used for it as for the
arrangement of the leaves of the flower in the flower-bud. See,
therefore, "AEstivation, or Praefloration."
195. As to each leaf separately, it is sometimes _straight_ and open in
vernation, but more commonly it is either _bent_, _folded_, or _rolled
up_. When the upper part is bent down upon the lower, as the young blade
in the Tulip-tree is bent upon the leaf-stalk, it is said to be
_Inflexed_ or _Reclined_ in vernation. When folded by the midrib so that
the two halves are placed face to face, it is _Conduplicate_ (Fig. 193),
as in the Magnolia, the Cherry, and the Oak. When folded back and forth
like the plaits of a fan, it is _Plicate_ or _Plaited_ (Fig. 194), as
in the Maple and Currant. If rolled, it may be so either from the tip
downwards, as in Ferns and the Sundew (Fig. 197), when in unrolling it
resembles the head of a crosier, and is said to be _Circinate_; or it
may be rolled up parallel with the axis, either from one edge into a
coil, when it is _Convolute_ (Fig. 195), as in the Apricot and Plum; or
rolled from both edges towards the midrib,--sometimes inwards, when it
is _Involute_ (Fig. 198), as in the Violet and Water-Lily; sometimes
outwards, when it is _Revolute_ (Fig. 196), in the Rosemary and Azalea.
The figures are diagrams, representing sections through the leaf, in the
way they were represented by Linnaeus.
[Illustration: Fig. 193. 194. 195. 196. 197. 198.]