This manual is written with
the assumption that most readers have either taken a botany course or had
practical experience working with plants or botanical materials, so that they
have some familiarity with the basic structure of a plant. Space limitations
require that a review of plant morphology be kept to the barest minimum. If a
more detailed review is needed, any textbook of botany or plant biology will
suffice (Raven, Evert & Eichhorn’s Biology
of Plants is highly recommended). Technical vocabulary is kept to a minimum
in this text; that which cannot be avoided is defined in the glossary. A more
complete reference is Harris & Harris’ Plant
Identification Terminology. Within those limitations, let us take a few
paragraphs to review the structure of typical flowering plants.
Flowering plants, like other
“advanced” vascular plants, have complex forms that are constructed from just a
few basic types of plant organ. An average plant has a root or roots, a stem,
leaves, and flowers, which produce fruits and seeds. There are of course
variations; for example, plants such as dandelions have all leaves borne at
ground level, with no stem, while others, such as many cacti, have no leaves. In
some plants, the stems become thick and woody trunks, branches or twigs.
These organs are constructed
in turn from a limited number of basic tissue types. Many of the soft tissues
of a plant, such as the photosynthetic inner parts of a leaf, the ground tissue
of a stem, or the fleshy cortex of a root, are made of unspecialized cells
called parenchyma. There are also specialized tissues that transport nutrients,
including phloem, which transports sugars from the leaves to the roots, and
xylem, which transports water from the roots to the leaves. Each of these is a
complex tissue that contains different types of cells adapted for specific
purposes, including rays of unspecialized parenchyma. Xylem often includes
hollow pipe-like cells of large diameter, which have a distinctive appearance,
whereas phloem, except at high magnification, does not appear much different
from parenchyma. These vascular tissues are found in small strands or bundles
in herbaceous stems, in roots, and in the veins of leaves, petals, etc. In
woody stems and roots, secondary xylem and phloem are produced in larger
quantities; the secondary xylem and associated tissues make up the wood. There
are also many different specialized cell types that may occur within
parenchymatous tissues. These may play a support function, like collenchyma
(the strings in celery) and sclerenchyma (thick-walled elongated fibers, or
rounded stone cells such as the grit in pears), or they may have the function
of secreting or storing special compounds, such as resins, oils or mineral
crystals. The outer surface of a plant is covered with a protective epidermis
or, in woody stems and roots, with a cork layer. The epidermis may produce
hairs (trichomes) or glands.
Roots may be taproots, which
are thick and single or few in number (like the carrot), or they may be fibrous
roots, which are thin and numerous (like grasses). In some plants, the taproots
have well-developed secondary wood, and the outer surface has a thickened bark.
The anatomy of a root is best seen in cross-section. A typical small root, such
as a fibrous root or a very young taproot, contains, moving from the outside
toward the center, a single-layered epidermis, a ring of parenchyma called the
cortex, a single-layered hypodermis, and a central vascular cylinder, which
contains a small amount of outer parenchyma (pericycle), small bundles of
phloem, and a variously shaped central xylem. Occasionally (especially in
monocots) vascular tissues are found in a ring around a central area of
parenchyma (pith) without vascular tissue, but usually roots have no pith. Such
very small roots are seldom harvested (except incidentally when attached to
stems, rhizomes, etc.) and provide few identifying characters.
In many larger taproots,
secondary xylem develops, making a conspicuous central cylinder of wood that may
occupy most of the root’s volume. (This “wood” need not be woody in the same
sense as an oak tree! For example, the paler area in the middle of a carrot is
secondary xylem, and therefore technically wood.) Secondary xylem is formed by
the vascular cambium, a single ring of cells that produce layers of xylem
toward the inside (thus moving outward as the thickness of the wood increases)
and secondary phloem toward the outside. The xylem and phloem may be in
continuous rings or in groups; a common morphology is for strands or wedges of
wood and phloem to be separated by parenchyma rays extending radially from the
center of the root toward the surface, giving the root a spoked appearance in
cross-section. The expansion of the wood often ruptures the original cortex and
epidermis, which dies and peels off; a corky bark develops as the outermost
layer. Between the cork and the secondary phloem, there may be a ring of
parenchyma that resembles a cortex, and may be referred to as a secondary
cortex, but that actually developed from the pericycle. “Bark” (as explained
below) refers technically to everything from cork through secondary phloem.
The vascular tissue of most
herbaceous dicot stems is not at the center, but spaced in a ring of vascular
bundles around a central pith; vascular bundles of monocots are scattered
randomly through the parenchyma of the stem. When secondary growth has begun in
a dicot stem, from a single-layered cambium ring as in the root, there is an
outer ring of cortical tissue, a ring of secondary phloem, a cambial layer, and
a ring of secondary xylem that surrounds a pith. The presence of this pith
usually differentiates a stem from a root. Stem anatomy is not usually very
useful for plant identification and is seldom important, as there are few
plants for which stems alone are used. An exception is when rhizomes are used. Some
plants have roots arising from a short rootstock or rhizome that is actually
not the uppermost part of the root but the lowest part of the stem. The
rootstock’s anatomy is often similar to that of the root, except that there is
a central pith in which there is no wood. Other plants have horizontal
underground stems (rhizomes), which may be very long and the means by which the
plant spreads. These are often called stolons if they are above ground level. In
some plants (e.g. black cohosh) the major underground structure is a rhizome
and the roots are small.
Bark is often harvested from
older woody stems. Bark is actually a very complicated tissue, including
several specialized cell layers that cannot be seen without the use of the
light microscope. Bark includes all the tissues between the outermost layer and
the secondary vascular cambium; in other words, everything outside the
secondary xylem (wood). The outer portion includes multiple layers of
protective cork, which is dead at maturity. The epidermis is destroyed when the
cork is produced, and in older stems the production of lower layers of cork
inside the secondary phloem causes the original cortex to rupture and peel off
as part of the outer layers of bark. Since the cork is airtight, the outer bark
has small pores (lenticels) that permit gas exchange; these are sometimes
conspicuous and have a characteristic shape. The inner bark includes the
cork-producing cells, any remaining cortical parenchyma, and a layer or layers
of secondary phloem, which may contain visible parenchyma rays. When bark is
harvested, it is the living inner bark rather than the dead outer bark that is
valuable, so bark is removed down to the wood. In some botanicals, the outer
bark is traditionally stripped off at the time of harvest.
Fortunately, the characters
that describe leaves, flowers, etc. depend more on visible morphology than on
internal anatomy! A typical leaf has a blade (the flattened photosynthetic
portion) and a petiole (the leafstalk). Some also have a pair of small leaflike
structures (stipules) at the point where the petiole attaches to the stem. Leaves
are categorized by their arrangement on the stem (alternate, opposite, or whorled),
whether they have a petiole or are sessile (stalkless), and whether they are
simple (undivided) or compound (including more than one leaflet). If compound,
they may be palmately or pinnately compound, and the leaflets may be be sessile
or stalked. A full description of a leaf includes the size and shape of the
blade, the shape of the base, apex and margins, the pubescence and appearance
of both surfaces, and the pattern of venation. An enormous vocabulary can be
used to describe these characters; some of the most basic terms are defined in
the glossary. Certain characteristics are usually treated as “default” values,
which are assumed to be the case unless otherwise noted. For example, leaves
are presumed to be simple, to have a petiole, to have entire (untoothed)
margins, and to be glabrous (hairless), unless they are specified to be
compound, sessile, toothed, hairy, etc.
Flowers are either borne on
individual stalks (pedicels) or sessile, and may be borne singly or in clusters
called inflorescences. The position, size, and shape of the inflorescence may
be of taxonomic use. They may be terminal (at the ends of stems) or lateral
(borne on short side branches or individually in leaf axils). The stalk
subtending the whole inflorescence is called the peduncle; within the
inflorescence, it is technically called the rachis. Flowers are sometimes
subtended by leaves or by small, often leaflike structures called bracts. Types
of inflorescence are distinguished based on which flowers develop first,
whether the rachis branches and how, and whether the flowers are stalked or
sessile. (The glossary defines several common types.) There are also
specialized or unusual inflorescences, such as the head or capitulum of the
daisy family and the compound umbel of the carrot family, that are found only
in certain plant groups.
A common garden flower
typically includes four whorls of parts arising from a common receptacle: the
usually greenish sepals (collectively called the calyx), the usually colorful
petals (collectively called the corolla), the male stamens (collectively called
the androecium), and the female carpels (collectively called the gynoecium). Sepals
and petals together are called the perianth. In some flowers, the sepals and
petals are identical, or reduced to only one whorl; when the perianth parts
cannot be divided into two groups, they are referred to as tepals. A typical
stamen consists of an anther (which produces the pollen) and a filament (the
stalk on which the anther is raised). A typical carpel includes an ovary
(containing one or more ovules), a stigma (where the pollen lands), and a style
(the stalk on which the stigma is raised above the ovary). Flowers may be
bisexual (having both stamens and carpels) or unisexual, then bearing male and
female flowers separately on the same plant (monoecious) or on different
individuals (dioecious).
Individual sepals, petals and
filaments may be free (not attached to one another), or they may be more or
less fused to one another. They may also be partly fused to parts from other
whorls; for example, in many flowers the bases of the filaments are fused to
the corolla, giving the appearance that the stamens grow from the corolla. It
is very common for all the carpels in a flower to be fused, forming a single
compound ovary; the styles and stigmas may remain separate or may also be
fused. A compound gynoecium is often called a pistil (and a single solitary
carpel is sometimes called a simple pistil), but if there are several free
carpels they are not called pistils. A less common form of fusion is for the
basal parts of the sepals, petals, and filaments all to be fused into a long or
short tube (hypanthium). If the hypanthium is in turn fused to the sides of the
pistil, so that the free parts of the sepals, petals and filaments appear to
arise at the top of the ovary and the ovary appears to be embedded within the
receptacle, the ovary is called inferior (otherwise, it is superior). Descriptions
of flowers may include the number, fusion, symmetry, shape, size, color, and pubescence
of the parts in each whorl. There is, again, a great deal of terminology to
describe their shape; some common shapes are defined in the glossary. As with
leaves, certain character states are commonly treated as the norm, and presumed
to be present unless specified otherwise. The “normal” or default flower is
bisexual and radially symmetrical, without fusion of perianth parts or
filaments, and with a superior ovary.
A fruit is a mature ovary,
sometimes with attached tissues such as hypanthial tissue, and seeds are mature
fertilized ovules. Fruits may be dry or fleshy, dehiscent (splitting open at
maturity) or indehiscent, and one-seeded to many-seeded. If formed from a
single carpel, a fruit will have only one locule (the space inside the ovary
that holds the seeds). A compound ovary may have a single large locule, or may
have as many locules as carpels, divided by septa, as in the tomato (although
it is rarely so easy to observe). Common fruit types are defined in the
glossary.
The specialized descriptive
vocabulary of size, shape, texture, and so forth can seem overwhelming to the
student, and it has been minimized as much as possible in this volume. The
botanical descriptions provided are simplified from descriptions in taxonomic
literature, and the authors of such treatments would probably feel that sloppy
language was being used. Still, the lay reader may feel that the terminology is
excessive and even unnecessary: why, for example, would one use a special Latin
word like “orbicular” to describe the shape of a leaf when the familiar word
“round” is available? The primary reason is that common English adjectives may
have unclear meanings when applied to specialized structures. A flat leaf with
a circular shape could be described as round, but so could a fleshy or
needle-like cylindrical leaf that is round in cross-section. Having two
different terms for these conditions (orbicular vs. terete) makes it
immediately clear, once the vocabulary becomes familiar, which state is meant.