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Plant morphology or phytomorphology is the study of the physical form and external structure of plants.Raven, P. H., R. F. Evert, & S. E. Eichhorn. Biology of Plants, 7th ed., page 9. (New York: W. H. Freeman, 2005). . This is usually considered distinct from plant anatomy, which is the study of the internal structure of plants, especially at the microscopic level.Evert, Ray Franklin and Esau, Katherine (2006) Esau's Plant anatomy: meristems, cells, and tissues of the plant body - their structure, function and development Wiley, Hoboken, New Jersey, page xv, Plant morphology is useful in the visual identification of plants. Recent studies in molecular biology started to investigate the molecular processes involved in determining the conservation and diversification of plant morphologies. In these studies, transcriptome conservation patterns were found to mark crucial ontogenetic transitions during the plant life cycle which may result in evolutionary constraints limiting diversification.
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 genetics 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 ( somatic) structures of plants, as well as the reproductive structures. The vegetative structures of 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 sorus, 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 Cell biology, 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.
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 phylogenetics of plant relationships that may lead to new morphological insights.
The vegetative ( somatic) structures of vascular plants include two major organ systems: (1) a shoot system, composed of stems and leaves, and (2) 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 ; sori are only found in ferns; and conifer cone are only found in and other . Reproductive characters are therefore regarded as more useful for the classification of plants than vegetative characters.
When characters are used in descriptions or for identification they are called diagnostic or key characters which can be either qualitative and quantitative.
Both kinds of characters can be very useful for the identification of plants.
Plant pigments include a variety of different kinds of molecule, including , , and . 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.
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 (or hatches from its egg), 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 Review. located at the tips of organs, or between mature tissues. Thus, a living plant always has embryonic tissues.
The properties of organisation seen in a plant are emergence 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."Leopold, A. C. Plant Growth and Development, page 183. (New York: McGraw-Hill, 1964). 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.
Once the embryo germinates from its seed or parent plant, it begins to produce additional organs (leaves, stems, and roots) 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. Review. Branching occurs when small clumps of cells left behind by the meristem, and which have not yet undergone cellular differentiation to form a specialised 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 (phototropism), gravity (gravitropism), water, (hydrotropism), and physical contact (thigmotropism).
Plant growth and development are mediated by specific and plant growth regulators (PGRs) (Ross et al. 1983).Ross, S.D.; Pharis, R.P.; Binder, W.D. 1983. Growth regulators and conifers: their physiology and potential uses in forestry. p. 35–78 in Nickell, L.G. (Ed.), Plant growth regulating chemicals. Vol. 2, CRC Press, Boca Raton FL. Endogenous hormone levels are influenced by plant age, cold hardiness, dormancy, and other metabolic conditions; photoperiod, drought, temperature, and other external environmental conditions; and exogenous sources of PGRs, e.g., externally applied and of rhizospheric origin.
When water freezes in plants, the consequences for the plant depend very much on whether the freezing occurs intracellularly (within cells) or outside cells in intercellular (extracellular) spaces.Glerum, C. 1985. Frost hardiness of coniferous seedlings: principles and applications. p. 107–123 in Duryea, M.L. (Ed.). Proceedings: Evaluating seedling quality: principles, procedures, and predictive abilities of major tests. Workshop, October 1984, Oregon State Univ., For. Res. Lab., Corvallis OR. Intracellular freezing usually kills the cell regardless of the hardiness of the plant and its tissues.Lyons, J.M.; Raison, J.K.; Steponkus, P.L. 1979. The plant membrane in response to low temperature: an overview. p. 1–24 in Lyons, J.M.; Graham, D.; Raison, J.K. (Eds.). Low Temperature Stress in Crop Plants. Academic Press, New York NY. Intracellular freezing seldom occurs in nature, but moderate rates of decrease in temperature, e.g., 1 °C to 6 °C/hour, cause intercellular ice to form, and this "extraorgan ice"Sakai, A.; Larcher, W. (Eds.) 1987. Frost Survival of Plants. Springer-Verlag. may or may not be lethal, depending on the hardiness of the tissue.
At freezing temperatures, water in the intercellular spaces of plant tissues freezes first, though the water may remain unfrozen until temperatures fall below 7 °C. After the initial formation of ice intercellularly, the cells shrink as water is lost to the segregated ice. The cells undergo freeze-drying, the dehydration being the basic cause of freezing injury.
The rate of cooling has been shown to influence the frost resistance of tissues,Sakai, A. 1979a. Freezing avoidance mechanism of primordial shoots of conifer buds. Plant Cell Physiol. 20:1381–1390. but the actual rate of freezing will depend not only on the cooling rate, but also on the degree of supercooling and the properties of the tissue.Levitt, J. 1980. Responses of Plants to Environmental Stresses. Volume 1. Chilling, Freezing, and High Temperature Stresses, 2nd ed. Academic Press, New York NY. 497 p. Sakai (1979a) demonstrated ice segregation in shoot primordia of Alaskan white and black spruces when cooled slowly to 30 °C to -40 °C. These freeze-dehydrated buds survived immersion in liquid nitrogen when slowly rewarmed. Floral primordia responded similarly. Extraorgan freezing in the primordia accounts for the ability of the hardiest of the boreal conifers to survive winters in regions when air temperatures often fall to -50 °C or lower. The hardiness of the winter buds of such conifers is enhanced by the smallness of the buds, by the evolution of faster translocation of water, and an ability to tolerate intensive freeze dehydration. In boreal species of Picea and Pinus, the frost resistance of 1-year-old seedlings is on a par with mature plants,Sakai, A.; Okada, S. 1971. Freezing resistance of conifers. Silvae Genet. 20(3):91–97. given similar states of dormancy.
The transition from early to late growth forms is referred to as 'vegetative phase change', but there is some disagreement about terminology.
Honoring Agnes Arber, author of the partial-shoot theory of the leaf, Rutishauser and Isler called the continuum approach Fuzzy Arberian Morphology (FAM). "Fuzzy" refers to fuzzy logic, "Arberian" to Agnes Arber. Rutishauser and Isler emphasised 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 (2009) concluded that "it is now widely accepted that... radiality characteristic and dorsiventrality characteristic are but extremes of a continuous spectrum. In fact, it is simply the timing of the KNOX gene expression!." Eckardt and Baum (2010) concluded that "it is now generally accepted that compound leaves express both leaf and shoot properties."
Process morphology describes and analyses the dynamic continuum of plant form. According to this approach, structures do not have process(es), they are process(es).Sattler, R. 2019. Structural and dynamic approaches to the development and evolution of plant form. In: Fusco, G. (ed) Perspectives on Evolutionary and Developmental Biology. Essays for Alessandro Minelli. Chapter 6, pp. 57–70 [2] Thus, the structure/process dichotomy is overcome by "an enlargement of our concept of 'structure' so as to include and recognise 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".Woodger, J.H. 1967. Biological Principles. London: Routledge & Kegoan Paul (reissued with a new Introduction).
For Jeune, Barabé and Lacroix, classical morphology (that is, mainstream morphology, based on a qualitative homology concept implying mutually exclusive categories) and continuum morphology are sub-classes of the more encompassing process morphology (dynamic morphology).
Classical morphology, continuum morphology, and process morphology are highly relevant to plant evolution, especially the field of plant evolutionary biology (plant evo-devo) that tries to integrate plant morphology and plant molecular genetics.Minelli, A. 2018. Plant Evolutionary Developmental Biology. The Evolvability of the Phenotype. New York: Cambridge University Press. In a detailed case study on unusual morphologies, Rutishauser (2016) illustrated and discussed various topics of plant evo-devo such as the fuzziness (continuity) of morphological concepts, the lack of a one-to-one correspondence between structural categories and gene expression, the notion of morphospace, the adaptive value of bauplan features versus patio ludens, physiological adaptations, hopeful monsters and saltational evolution, the significance and limits of developmental robustness, etc. Rutishauser (2020) discussed the past and future of plant evo-devo. Our conception of the gynoecium and the search for a fossil ancestor of Angiosperms changes fundamentally from the perspective of evo-devo.
Whether we like it or not, morphological research is influenced by philosophical assumptions such as either/or logic, fuzzy logic, structure/process dualism or its transcendence. And empirical findings may influence the philosophical assumptions. Thus there are interactions between philosophy and empirical findings. These interactions are the subject of what has been referred to as philosophy of plant morphology.(for an expanded version of this article see [3])
One important and unique event in plant morphology of the 21st century was the publication of Principles of Plant Morphology by Donald R. Kaplan, edited by Chelsea D. Specht (2022)Kaplan, D. R. , edited by Specht, C. D. 2022. Kaplan's Principles of Plant Morphology. New York: CRC Press. It is a well illustrated volume of 1305 pages in a very large format that presents a wealth of morphological data. Unfortunately, all of these data are only interpreted in terms of classical morphology and the qualitative homology concept, disregarding modern conceptional innovations. Including continuum and process morphology as well as molecular genetics would provide an enlarged scope.
An even more important event was the publication of a book by Regine Claβen-Bockhoff (2024) Die Pflanze: Morphologie, Entwicklung und Evolution von Vielfalt (The Plant: Morphology, Development and Evolution of Diversity).Classen-Bockhoff,R. 2024. Die Pflanze: Morphologie, Entwicklung und Evolution von Vielfalt. Berlin: Springer Spektrum.Rolf Sattler (2025) Plant Evo-Devo Morphology: An Innovative New Book by Regine Claβen-Bockhoff. The Botanical Review, Dec. 8, 2025. https://doi.org/10.1007/s12229-025-09322-x Like Kaplan's book, this book is very comprehensive (over a thousand pages) and beautifully illustrated (she worked with two illustrators), but unlike Kaplan's book, her book presents major conceptual innovations. Although, for the vegetative region, she accepts the categories of classical morphology, contrary to Kaplan, she recognizes that not all structures can be pressed into these categories. For flowers, she abandoned the classical framework altogether. Instead of interpreting the flower as a modified short shoot (as posited by classical morphology), she proposed that flowers are sporangia bearing units so that stamens and carpels are sporangiophores, which are considered 'de novo' structures not necessarily homologous with vegetative leaves.Rolf Sattler (2025) Plant Evo-Devo Morphology: An Innovative New Book by Regine Claβen-Bockhoff. The Botanical Review, Dec. 8, 2025. https://doi.org/10.1007/s12229-025-09322-x
Rolf Sattler proposed an Articulation Morphology. It is based on the open growth of plants, which occurs through ramification that leads to articulation - the formation of articles between successive ramifications or after a single ramification. Thus, the plant is seen as an articulated whole, consisting of articles. In articulation morphology, the central and most basic concept is no longer morphological homology but transformation: transformation of ramification and articulation. This changes the most basic questions we ask. Instead of asking questions about morphological homology, we ask how ramification and articulation have changed during development and evolution. For this reason, the new approach of articulation morphology may be considered a new paradigm of plant morphology. It changes fundamentally our way of thinking about morphology and morphological investigation.
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