_Leaves._
Leaves are the lungs of plants. The food taken in by the roots has to
pass through the stem to the leaves to be acted upon by the air, before
it becomes sap and is fit to be used for the growth of the plant. No
portion of a plant is more varied in parts, forms, surface, and duration
than the leaf.
No one can become familiar with leaves, and appreciate their beauty and
variety, who does not study them upon the plants themselves. This
chapter therefore will be devoted mainly to the words needed for leaf
description, together with their application.
THE LEAF.--In the axil of the whole leaf the bud forms for the growth of
a new branch. So by noting the position of the buds, all the parts
included in a single leaf can be determined. As a general thing the leaf
has but one blade, as in the Chestnut, Apple, Elm, etc.; yet the
Horse-chestnut has 7 blades, the Common Locust often has 21, and a
single leaf of the Honey-locust occasionally has as many as 300. Figs.
17-58 (Chapter VII.) are all illustrations of single leaves, except Fig.
43, where there are two leaves on a twig. A number of them show the bud
by which the fact is determined (Figs. 25, 26, 31, 33, 34, 36, 40,
etc.); others show branches which grew from the axillary buds, many of
them fruiting branches (Figs. 37, 42, 43, 50, and 54), one (Fig. 51) a
thorny branch.
The cone-bearing plants (Figs. 59-67) have only simple leaves. Each
piece, no matter how small and scale-like, may have a branch growing
from its axil, and so may form a whole leaf. A study of these figures,
together with the observation of trees, will soon teach the student
what constitutes a leaf.
ARRANGEMENT.--There are several different ways in which leaves are
arranged on trees; the most common plan is the _alternate_;
[Illustration] in this only one leaf occurs at a joint or node on the
stem. The next in frequency is the _opposite_, [Illustration] where two
leaves opposite each other are found at the node. A very rare
arrangement among trees, though common in other plants, is the
_whorled_, [Illustration] where more than two leaves, regularly arranged
around the stem, are found at the node. When a number of leaves are
bundled together,--a plan not rare among evergreens,--they are said to
be _fasciculated_ or in _fascicles_. [Illustration] The term _scattered_
is used where alternate leaves are crowded on the stem. This plan is
also common among evergreens.
CAUTION.--In some plants the leaves on the side shoots or spurs of a
twig are so close together, the internodes being so short, that at first
sight they seem opposite. In such cases, the leaf-scars of the preceding
years, or the arrangement of the branches, is a better test of the true
arrangement of the leaves. The twig of Birch shown in Fig. 5 has
alternate leaves.
[Illustration: Fig. 5.]
[Illustration: Fig. 6.]
There is one variety of alternation, called _two-ranked_, which is quite
characteristic of certain trees; that is, the leaves are so flattened
out as to be in one plane on the opposite sides of the twig (Fig. 6).
The Elm-trees form good examples of two-ranked alternate leaves, while
the Apple leaves are alternate without being two-ranked. Most leaves
spread from the stem, but some are _appressed_, as in the Arbor-vitae
(Fig. 7). In this species the _branches_ are _two-ranked_.
PARTS OF LEAVES.--A _complete leaf_ [Illustration] consists of three
parts: the _blade_, the thin expanded portion; the _petiole_, the
leafstalk; and the _stipules_, a pair of small blades at the base of the
petiole. The petiole is often very short and sometimes wanting. The
stipules are often absent, and, even when present, they frequently fall
off as soon as the leaves expand; sometimes they are conspicuous. Most
Willows show the stipules on the young luxuriant growths.
[Illustration: Fig. 7.]
VEINING.--The leaves of most trees have a distinct framework, the
central line of which is called a _midrib_; sometimes the leaf has
several other lines about as thick as the midrib, which are called
_ribs_; the lines next in size, including all that are especially
distinct, are called _veins_, the most minute ones being called
_veinlets_ (Fig. 8).
[Illustration: Fig. 8.]
KINDS.--Leaves are _simple_ when they have but one blade; [Illustration]
_compound_ when they have more than one. Compound leaves are _palmate_
when all the blades come from one point, as in the Horse-chestnut;
[Illustration] _pinnate_ when they are arranged along the sides, as in
the Hickory. Pinnate leaves are of two kinds: _odd-pinnate_,
[Illustration] when there is an odd leaflet at the end, as in the Ash,
and _abruptly pinnate_ [Illustration] when there is no end leaflet.
Many trees have the leaves _twice pinnate_; they are either _twice
odd-pinnate_ [Illustration] or _twice abruptly pinnate_. [Illustration]
The separate blades of a compound leaf are called _leaflets_. Leaves or
leaflets are _sessile_ when they have no stems, and _petiolate_ when
they have stems.
When there are several ribs starting together from the base of a blade,
it is said to be _radiate-veined_ or _palmate-veined_.[Illustration]
When the great veins all branch from the midrib, the leaf is
_feather-veined_ or _pinnate-veined_. [Illustration] If these veins are
straight, distinct, and regularly placed, the leaf is said to be
_straight-veined_. The Chestnut is [Illustration] a good example. Leaves
having veinlets joining each other like a net are said to be
_netted-veined_. All the trees with broad leaves in the northern United
States, with one exception, have netted-veined foliage. A leaf having
its veinlets parallel to one another is said to be _parallel-veined_ or
_-nerved_. [Illustration] The Ginkgo-tree, the Indian Corn, and the
Calla Lily have parallel-veined leaves. The narrow leaves of the
cone-bearing trees are also parallel-veined.
FORMS.--Leaves can readily be divided into the three following groups
with regard to their general outline:
1. _Broadest at the middle._ _Orbicular_, [Illustration] about as broad
as long and rounded. _Oval_, [Illustration] about twice as long as wide,
and regularly curved. _Elliptical_, [Illustration] more than twice as
long as wide, and evenly curved. _Oblong_, [Illustration] two or three
times as long as wide, with the sides parallel. _Linear_, [Illustration]
elongated oblong, more than three times as long as wide. _Acerose_,
[Illustration] needle-shaped, like the leaf of the Pine-tree.
2. _Broadest near the base._ _Deltoid_, [Illustration] broad and
triangular. _Ovate_, [Illustration] evenly curved, with a broad, rounded
base. _Heart-shaped_ or _cordate_, [Illustration] similar to ovate, but
with a notch at the base. _Lanceolate_, [Illustration] shaped like the
head of a lance. _Awl-shaped_, [Illustration] shaped like the
shoemaker's curved awl. _Scale-shaped_, [Illustration] short, rounded,
and appressed to the stem. The Arbor-vitae has both awl-shaped and
scale-shaped leaves.
3. _Broadest near the apex._ _Obovate_, [Illustration] same as ovate,
but with the stem at the narrow end. _Obcordate_, [Illustration] a
reversed heart-shape. _Oblanceolate_, [Illustration] a reversed
lanceolate. _Wedge-shaped_ or _cuneate_, [Illustration] having a
somewhat square end and straight sides like a wedge.
These words are often united to form compound ones when the form of the
leaf is somewhat intermediate. The term which most nearly suits the
general form is placed at the end; thus _lance-ovate_ indicates a leaf
between lanceolate and ovate, but nearer ovate than lanceolate; while
_ovate-lanceolate_ indicates one nearer lanceolate.
BASES.--Oftentimes leaves are of some general form, but have a peculiar
base, one that would not be expected from the statement of shape. An
ovate leaf which should have a rounded base might have a tapering one;
it would then be described as ovate with a _tapering base_.
[Illustration] A lanceolate leaf should naturally have a tapering base,
but might have an _abrupt_ one. [Illustration] Many leaves, no matter
what their general form may be, have more or less notched bases; such
bases are called _cordate_, [Illustration] _deeply_ or _slightly_, as
the case may be; and if the lobes at base are elongated, _auriculate_.
[Illustration] If the basal lobes project outward, the term
_halberd-shaped_ [Illustration] is used. Any form of leaf may have a
base more or less _oblique_. [Illustration]
POINTS.--The points as well as the bases of leaves are often peculiar,
and need to be described by appropriate terms. _Truncate_ [Illustration]
indicates an end that is square; _retuse_, [Illustration] one with a
slight notch; _emarginate_, one with a decided notch; _obcordate_, with
a still deeper notch; _obtuse_, [Illustration] angular but abrupt;
_acute_, [Illustration] somewhat sharpened; _acuminate_, [Illustration]
decidedly sharp-pointed; _bristle-pointed_ and _awned_, [Illustration]
with a bristle-like tip; _spiny-pointed_, with the point sharp and stiff
(Holly); _mucronate_, [Illustration] with a short, abrupt point.
MARGINS.--_Entire_, [Illustration] edge without notches; _repand_,
[Illustration] slightly wavy; _sinuate_, [Illustration] decidedly wavy;
_dentate_, [Illustration] with tooth-like notches; _serrate_,
[Illustration] with notches like those of a saw; _crenate_,
[Illustration] with the teeth rounded; _twice serrate_, [Illustration]
when there are coarse serrations finely serrated, as on most Birch
leaves; _serrulate_, with minute serrations; _crenulate_, with minute
crenations. Leaves can be _twice crenate_ or _sinuate-crenate_.
_Revolute_ indicates that the edges are rolled over.
When a leaf has a few great teeth, the projecting parts are called
_lobes_, and the general form of the leaf is what it would be with the
notches filled in. In the description of such leaves, certain terms are
needed in describing the plan of the notches, and their depth and form.
Leaves with palmate veining are _palmately lobed_ [Illustration] or
_notched_; those with pinnate veining are _pinnately lobed_
[Illustration] or _notched_. While the term _lobe_ is applied to all
great teeth of a leaf, whether rounded or pointed, long or short, still
there are four terms sometimes used having special signification with
reference to the depth of the notches. _Lobed_ indicates that the
notches extend about one fourth the distance to the base or midrib;
_cleft_, that they extend one half the way; _parted_, about three
fourths of the way; and _divided_, that the notches are nearly deep
enough to make a compound leaf of separate leaflets.
So leaves may be palmately lobed, cleft, parted or divided, and
pinnately lobed, cleft, parted or divided. The term _pinnatifid_
[Illustration] is often applied to pinnately cleft leaves. The terms
_entire_, _serrate_, _crenate_, _acute-pointed_, etc., are applied to
the lobes as well as to the general margins of leaves.
SURFACE.--The following terms are needed in describing the surface of
leaves and fruit.
_Glabrous_, smooth; _glaucous_, covered with a whitish bloom which can
be rubbed off (Plum); _rugous_, wrinkled; _canescent_, so covered with
minute hairs as to appear silvery; _pubescent_, covered with fine, soft,
plainly seen hairs; _tomentose_, densely covered with matted hairs;
_hairy_, having longer hairs; _scabrous_, covered with stiff, scratching
points; _spiny_, having stiff, sharp spines; _glandular-hairy_, having
the hairs ending in glands (usually needing a magnifying glass to be
seen).
TEXTURE.--_Succulent_, fleshy; _scarious_, dry and chaffy; _punctate_,
having translucent glands, so that the leaf appears, when held toward
the light, as though full of holes; _membranous_, thin, soft, and rather
translucent; _thick_, _thin_, etc.
DURATION.--_Evergreen_, hanging on the tree from year to year. By
noticing the color of the different leaves and their position on the
twigs, all evergreen foliage can readily be determined at any time
during the year. _Deciduous_, falling off at the end of the season.
_Fugacious_, falling early, as the stipules of many leaves.