Plant morphology or phytomorphology is the study of the physical form and external structure
of plants. This is usually considered distinct from plant anatomy, which is the study of
the internal structure of plants, especially at the microscopic level. Plant morphology
is useful in the visual identification of plants.
Scope
Plant morphology "represents a study of the development, form, and structure of plants,
and, by implication, an attempt to interpret these on the basis of similarity of plan and
origin." There are four major areas of investigation in plant morphology, and each overlaps with
another field of the biological sciences. First of all, morphology is comparative, meaning
that the morphologist examines structures in many different plants of the same or different
species, then draws comparisons and formulates ideas about similarities. When structures
in different species are believed to exist and develop as a result of common, inherited
genetic pathways, those structures are termed homologous. For example, the leaves of pine,
oak, and cabbage all look very different, but share certain basic structures and arrangement
of parts. The homology of leaves is an easy conclusion to make. The plant morphologist
goes further, and discovers that the spines of cactus also share the same basic structure
and development as leaves in other plants, and therefore cactus spines are homologous
to leaves as well. This aspect of plant morphology overlaps with the study of plant evolution
and paleobotany. Secondly, plant morphology observes both the
vegetative structures of plants, as well as the reproductive structures. The vegetative
structures of vascular plants includes the study of the shoot system, composed of stems
and leaves, as well as the root system. The reproductive structures are more varied, and
are usually specific to a particular group of plants, such as flowers and seeds, fern
sori, and moss capsules. The detailed study of reproductive structures in plants led to
the discovery of the alternation of generations found in all plants and most algae. This area
of plant morphology overlaps with the study of biodiversity and plant systematics.
Thirdly, plant morphology studies plant structure at a range of scales. At the smallest scales
are ultrastructure, the general structural features of cells visible only with the aid
of an electron microscope, and cytology, the study of cells using optical microscopy. At
this scale, plant morphology overlaps with plant anatomy as a field of study. At the
largest scale is the study of plant growth habit, the overall architecture of a plant.
The pattern of branching in a tree will vary from species to species, as will the appearance
of a plant as a tree, herb, or grass. Fourthly, plant morphology examines the pattern
of development, the process by which structures originate and mature as a plant grows. While
animals produce all the body parts they will ever have from early in their life, plants
constantly produce new tissues and structures throughout their life. A living plant always
has embryonic tissues. The way in which new structures mature as they are produced may
be affected by the point in the plant's life when they begin to develop, as well as by
the environment to which the structures are exposed. A morphologist studies this process,
the causes, and its result. This area of plant morphology overlaps with plant physiology
and ecology. A comparative science
A plant morphologist makes comparisons between structures in many different plants of the
same or different species. Making such comparisons between similar structures in different plants
tackles the question of why the structures are similar. It is quite likely that similar
underlying causes of genetics, physiology, or response to the environment have led to
this similarity in appearance. The result of scientific investigation into these causes
can lead to one of two insights into the underlying biology:
Homology - the structure is similar between the two species because of shared ancestry
and common genetics. Convergence - the structure is similar between
the two species because of independent adaptation to common environmental pressures.
Understanding which characteristics and structures belong to each type is an important part of
understanding plant evolution. The evolutionary biologist relies on the plant morphologist
to interpret structures, and in turn provides phylogenies of plant relationships that may
lead to new morphological insights. Homology
When structures in different species are believed to exist and develop as a result of common,
inherited genetic pathways, those structures are termed homologous. For example, the leaves
of pine, oak, and cabbage all look very different, but share certain basic structures and arrangement
of parts. The homology of leaves is an easy conclusion to make. The plant morphologist
goes further, and discovers that the spines of cactus also share the same basic structure
and development as leaves in other plants, and therefore cactus spines are homologous
to leaves as well. Convergence
When structures in different species are believed to exist and develop as a result of common
adaptive responses to environmental pressure, those structures are termed convergent. For
example, the fronds of Bryopsis plumosa and stems of Asparagus setaceus both have the
same feathery branching appearance, even though one is an alga and one is a flowering plant.
The similarity in overall structure occurs independently as a result of convergence.
The growth form of many cacti and species of Euphorbia is very similar, even though
they belong to widely distant families. The similarity results from common solutions to
the problem of surviving in a hot, dry environment.
Vegetative and reproductive characteristics Plant morphology treats both the vegetative
structures of plants, as well as the reproductive structures.
The vegetative structures of vascular plants include two major organ systems: a shoot system,
composed of stems and leaves, and a root system. These two systems are common to nearly all
vascular plants, and provide a unifying theme for the study of plant morphology.
By contrast, the reproductive structures are varied, and are usually specific to a particular
group of plants. Structures such as flowers and fruits are only found in the angiosperms;
sori are only found in ferns; and seed cones are only found in conifers and other gymnosperms.
Reproductive characters are therefore regarded as more useful for the classification of plants
than vegetative characters. Use in identification
Plant biologists use morphological characters of plants which can be compared, measured
counted and described to assess the differences or similarities in plant taxa and use these
characters for plant identification, classification and descriptions.
When characters are used in descriptions or for identification they are called diagnostic
or key characters which can be either qualitative and quantitative.
Quantitative characters are morphological features that can be counted or measured for
example a plant species has flower petals 10–12 mm wide.
Qualitative characters are morphological features such as leaf shape, flower color or pubescence.
Both kinds of characters can be very useful for the identification of plants.
Alternation of generations
The detailed study of reproductive structures in plants led to the discovery of the alternation
of generations, found in all plants and most algae, by the German botanist Wilhelm Hofmeister.
This discovery is one of the most important made in all of plant morphology, since it
provides a common basis for understanding the life cycle of all plants.
Pigmentation in plants
The primary function of pigments in plants is photosynthesis, which uses the green pigment
chlorophyll along with several red and yellow pigments that help to capture as much light
energy as possible. Pigments are also an important factor in attracting insects to flowers to
encourage pollination. Plant pigments include a variety of different
kinds of molecule, including porphyrins, carotenoids, anthocyanins and betalains. All biological
pigments selectively absorb certain wavelengths of light while reflecting others. The light
that is absorbed may be used by the plant to power chemical reactions, while the reflected
wavelengths of light determine the color the pigment will appear to the eye.
Development Plant development is the process by which
structures originate and mature as a plant grows. It is a subject studies in plant anatomy
and plant physiology as well as plant morphology. The process of development in plants is fundamentally
different from that seen in vertebrate animals. When an animal embryo begins to develop, it
will very early produce all of the body parts that it will ever have in its life. When the
animal is born, it has all its body parts and from that point will only grow larger
and more mature. By contrast, plants constantly produce new tissues and structures throughout
their life from meristems located at the tips of organs, or between mature tissues. Thus,
a living plant always has embryonic tissues. The properties of organization seen in a plant
are emergent properties which are more than the sum of the individual parts. "The assembly
of these tissues and functions into an integrated multicellular organism yields not only the
characteristics of the separate parts and processes but also quite a new set of characteristics
which would not have been predictable on the basis of examination of the separate parts."
In other words, knowing everything about the molecules in a plant are not enough to predict
characteristics of the cells; and knowing all the properties of the cells will not predict
all the properties of a plant's structure. Growth
A vascular plant begins from a single celled zygote, formed by fertilisation of an egg
cell by a sperm cell. From that point, it begins to divide to form a plant embryo through
the process of embryogenesis. As this happens, the resulting cells will organize so that
one end becomes the first root, while the other end forms the tip of the shoot. In seed
plants, the embryo will develop one or more "seed leaves". By the end of embryogenesis,
the young plant will have all the parts necessary to begin in its life.
Once the embryo germinates from its seed or parent plant, it begins to produce additional
organs through the process of organogenesis. New roots grow from root meristems located
at the tip of the root, and new stems and leaves grow from shoot meristems located at
the tip of the shoot. Branching occurs when small clumps of cells left behind by the meristem,
and which have not yet undergone cellular differentiation to form a specialized tissue,
begin to grow as the tip of a new root or shoot. Growth from any such meristem at the
tip of a root or shoot is termed primary growth and results in the lengthening of that root
or shoot. Secondary growth results in widening of a root or shoot from divisions of cells
in a cambium. In addition to growth by cell division, a
plant may grow through cell elongation. This occurs when individual cells or groups of
cells grow longer. Not all plant cells will grow to the same length. When cells on one
side of a stem grow longer and faster than cells on the other side, the stem will bend
to the side of the slower growing cells as a result. This directional growth can occur
via a plant's response to a particular stimulus, such as light, gravity, water,, and physical
contact. Morphological variation
Plants exhibit natural variation in their form and structure. While all organisms vary
from individual to individual, plants exhibit an additional type of variation. Within a
single individual, parts are repeated which may differ in form and structure from other
similar parts. This variation is most easily seen in the leaves of a plant, though other
organs such as stems and flowers may show similar variation. There are three primary
causes of this variation: positional effects, environmental effects, and juvenility.
Positional effects
Although plants produce numerous copies of the same organ during their lives, not all
copies of a particular organ will be identical. There is variation among the parts of a mature
plant resulting from the relative position where the organ is produced. For example,
along a new branch the leaves may vary in a consistent pattern along the branch. The
form of leaves produced near the base of the branch will differ from leaves produced at
the tip of the plant, and this difference is consistent from branch to branch on a given
plant and in a given species. This difference persists after the leaves at both ends of
the branch have matured, and is not the result of some leaves being younger than others.
Environmental effects The way in which new structures mature as
they are produced may be affected by the point in the plants life when they begin to develop,
as well as by the environment to which the structures are exposed. This can be seen in
aquatic plants and emergent plants. Juvenility
The organs and tissues produced by a young plant, such as a seedling, are often different
from those that are produced by the same plant when it is older. This phenomenon is known
as juvenility. For example, young trees will produce longer, leaner branches that grow
upwards more than the branches they will produce as a fully grown tree. In addition, leaves
produced during early growth tend to be larger, thinner, and more irregular than leaves on
the adult plant. Specimens of juvenile plants may look so completely different from adult
plants of the same species that egg-laying insects do not recognize the plant as food
for their young. Differences are seen in rootability and flowering and can be seen in the same
mature tree. Juvenile cuttings taken from the base of a tree will form roots much more
readily than cuttings originating from the mid to upper crown. Flowering close to the
base of a tree is absent of less profuse than flowering in the higher branches especially
when a young tree first reaches flowering age.
Some recent developments Rolf Sattler has revised fundamental concepts
of comparative morphology such as the concept of homology. He emphasized that homology should
also include partial homology and quantitative homology. This leads to a continuum morphology
that demonstrates a continuum between the morphological categories of root, shoot, stem,
leaf, and hair. How intermediates between the categories are best described has been
discussed by Bruce K. Kirchoff et al. Honoring Agnes Arber, author of the partial-shoot
theory of the leaf, Rutishauser and Isler called the continuum approach Fuzzy Arberian
Morphology. “Fuzzy” refers to fuzzy logic, “Arberian” to Agnes Arber. Rutishauser
and Isler emphasized that this approach is not only supported by many morphological data
but also by evidence from molecular genetics. More recent evidence from molecular genetics
provides further support for continuum morphology. James concluded that "it is now widely accepted
that... radiality [characteristic of most shoots] and dorsiventrality [characteristic
of leaves] are but extremes of a continuous spectrum. In fact, it is simply the timing
of the KNOX gene expression!." Eckardt and Baum concluded that "it is now generally accepted
that compound leaves express both leaf and shoot properties.”.
Process morphology describes and analyzes the dynamic continuum of plant form. According
to this approach, structures do not have process(es), they are process(es). Thus, the structure/process
dichotomy is overcome by "an enlargement of our concept of 'structure' so as to include
and recognize that in the living organism it is not merely a question of spatial structure
with an 'activity' as something over or against it, but that the concrete organism is a spatio-temporal
structure and that this spatio-temporal structure is the activity itself."
For Jeune, Barabé and Lacroix, classical morphology and continuum morphology are sub-classes
of the more encompassing process morphology. See also
List of plant morphology terms Portal:plants
Plant anatomy Plant identification
Plant physiology Plant evolutionary developmental biology
Taxonomy Simulated growth of plants
References
External links Botanical Visual Glossary
Plant morphology: continuum and process morphology