Glossary of plant morphology

This page provides a glossary of plant morphology. Botanists and other biologists who study plant morphology use a number of different terms to classify and identify plant organs and parts that can be observed using no more than a handheld magnifying lens. This page provides help in understanding the numerous other pages describing plants by their various taxa. The accompanying page—Plant morphology—provides an overview of the science of the external form of plants. There is also an alphabetical list: Glossary of botanical terms. In contrast, this page deals with botanical terms in a systematic manner, with some illustrations, and organized by plant anatomy and function in plant physiology.[1]

This glossary primarily includes terms that deal with vascular plants (ferns, gymnosperms and angiosperms), particularly flowering plants (angiosperms). Non-vascular plants (bryophytes), with their different evolutionary background, tend to have separate terminology. Although plant morphology (the external form) is integrated with plant anatomy (the internal form), the former became the basis of the taxonomic description of plants that exists today, due to the few tools required to observe.[2][3]

Many of these terms date back to the earliest herbalists and botanists, including Theophrastus. Thus, they usually have Greek or Latin roots. These terms have been modified and added to over the years, and different authorities may not always use them the same way.[2][3]

This page has two parts: The first deals with general plant terms, and the second with specific plant structures or parts.

Plant habit refers to the overall shape of a plant, and it describes a number of components such as stem length and development, branching pattern, and texture. While many plants fit neatly into some main categories, such as grasses, vines, shrubs, or trees, others can be more difficult to categorise. The habit of a plant provides important information about its ecology: that is, how it has adapted to its environment. Each habit indicates a different adaptive strategy. Habit is also associated with the development of the plant. As such, it may change as the plant grows and is more properly called its growth habit. In addition to shape, habit indicates plant structure; for instance, whether the plant is herbaceous or woody.

Each plant commences its growth as a herbaceous plant. Plants that remain herbaceous are shorter and seasonal, dying back at the end of their growth season. Woody plants (such as trees, shrubs and woody vines (lianas) will gradually acquire woody (lignaceous) tissues, which provide strength and protection for the vascular system,[4] and they tend to be tall and relatively long lived. The formation of woody tissue is an example of secondary growth, a change in existing tissues, in contrast to primary growth that creates new tissues, such as the elongating tip of a plant shoot. The process of wood formation (lignification) is commonest in the Spermatophytes (seed bearing plants) and has evolved independently a number of times. The roots may also lignify, aiding in the role of supporting and anchoring tall plants, and may be part of a descriptor of the plant's habit.

Plant habit can also refer to whether the plant possesses any specialised systems for the storage of carbohydrates or water, allowing the plant to renew its growth after an unfavourable period. Where the amount of water stored is relatively high, the plant is referred to as a succulent. Such specialised plant parts may arise from the stems or roots. Examples include plants growing in unfavourable climates, very dry climates where storage is intermittent depending on climatic conditions, and those adapted to surviving fires and regrowing from the soil afterwards.

Alternating generations: Haploid gametophyte (top), diploid sporophyte (bottom)
Angiosperms. Only the sporophyte is visible, the gametophytes being the pollen and ovule

Plant structures or organs fulfil specific functions, and those functions determine the structures that perform them. Among terrestrial (land) plants, the vascular and non-vascular plants (Bryophytes) evolved independently in terms of their adaptation to terrestrial life and are treated separately here (see Bryophytes).[6]

Common structural elements are present in the embryonic part of the life cycle, which is the diploid multicellular phase. The embryo develops into the sporophyte, which at maturity produces haploid spores, which germinate to produce the gametophyte, the haploid multicellular phase. The haploid gametophyte then produces gametes, which may fuse to form a diploid zygote, and finally an embryo. This phenomenon of alternating diploid and haploid multicellular phases is common to the embryophytes (land plants) and is referred to as the alternation of generations. A major difference between vascular and non-vascular plants is that in the latter the haploid gametophyte is the more visible and longer-lived stage. In vascular plants, the diploid sporophyte has evolved as the dominant and visible phase of the life cycle. In seed plants and some other groups of vascular plants the gametophyte phases are strongly reduced in size and contained within the pollen and ovules. The female gametophyte is entirely contained within the sporophyte's tissues, while the male gametophyte in its pollen grain is released and transferred by wind or animal vectors to fertilize the ovules.[1]

Amongst the vascular plants, the structures and functions of the Pteridophyta (ferns), which reproduce seedlessly, are also sufficiently different to justify separate treatment, as here (see Pteridophytes). The remainder of the vascular plant sections address the higher plants (Spermatophytes or Seed Plants, i.e. Gymnosperms and Angiosperms or flowering plants). In the higher plants, the terrestrial sporophyte has evolved specialised parts. In essence, they have a lower, underground component and an upper, aerial component. The underground part develops roots that seek water and nourishment from the soil, while the upper component, or shoot, grows toward the light and develops a plant stem, leaves and specialised reproductive structures (sporangia). In angiosperms, the sporangia are located in the stamen anthers (microsporangia) and ovules (megasporangia). The specialised sporangia bearing stem is the flower. In angiosperms, if the female sporangium is fertilised, it becomes the fruit, a mechanism for dispersing the seeds produced from the embryo. [6]

Thus, the terrestrial sporophyte has two growth centres, the stem growing upwards while the roots grow downwards. New growth occurs at the tips (apices) of both the shoot and roots, where the undifferentiated cells of the meristem divide. Branching occurs to form new apical meristems. Growth of the stem is indeterminate in pattern (not pre-determined to stop at a particular point).[1] The functions of the stem are to raise and support the leaves and reproductive organs above the level of the soil, to facilitate absorption of light for photosynthesis, gas exchange, water exchange (transpiration), pollination, and seed dispersal. The stem also serves as a conduit, from roots to overhead structures, for water and other growth-enhancing substances. These conduits consist of specialised tissues known as vascular bundles, which give the name "vascular plants" to the angiosperms. The point of insertion, on the stem, of leaves or buds is a node, and the space between two successive nodes, an internode.

The leaves, which emerge from the shoot, are specialised structures that carry out photosynthesis, and gas (oxygen and carbon dioxide) and water exchange. They are sheathed by an outer layer or epidermis that is coated with a waxy waterproof protective layer, which is punctuated by specialised pores, known as stomata, which regulate gas and water exchange. The leaves also possess vascular bundles, which are generally visible as veins, whose patterns are called venation. Leaves tend to have a shorter life span than the stems or branches that bear them, and when they fall, an area at the attachment zone, called the abscission zone leaves a scar on the stem.

In the angle (adaxial) between the leaf and the stem, is the axil. Here can be found buds (axillary buds), which are miniature and often dormant branches with their own apical meristem. They are often covered by leaves.

1: Pedicel 2: Receptacle 3: Nectary 4: Sepal 5: Petal 6: Filament 7: Anther 8: Stigma 9: Style 10: Ovary 11: Ovule 12: Bract

The flower, which is one of the defining features of angiosperms, is essentially a stem whose leaf primordia become specialised, following which the apical meristem stops growing: a determinate growth pattern, in contrast to vegetative stems.[1][6] The flower stem is known as a pedicel, and those flowers with such a stem are called pedicellate, while those without are called sessile.[7] In the angiosperms, the flowers are arranged on a flower stem as an inflorescence. Just beneath (subtended) the flower there may be a modified, and usually reduced, leaf, called a bract. A secondary smaller bract is a bracteole (bractlet, prophyll, prophyllum), often on the side of the pedicel, and generally paired. A series of bracts subtending the calyx (see below) is an epicalyx. Angiosperms are dealt with in more detail here; these structures are very different in gymnosperms.[7]

In angiosperms, the specialised leaves that play a part in reproduction are arranged around the stem in an ordered fashion, from the base to the apex of the flower. The floral parts are arranged at the end of a stem without any internodes. The receptacle (also called the floral axis, or thalamus) is generally very small. Some flower parts are solitary, while others may form a tight spiral, or whorl, around the flower stem. First, at the base, are those non-reproductive structures involved in protecting the flower when it is still a bud, the sepals, then are those parts that play a role in attracting pollinators and are typically coloured, the petals, which together with the sepals make up the perianth (perigon, perigonium). If the perianth is differentiated, the outer whorl of sepals forms the calyx, and the inner whorl of petals, the corolla. If the perianth is not differentiated into sepals and petals, they are collectively known as tepals. In some flowers, a tube or cup-like hypanthium (floral tube) is formed above or around the ovary and bears the sepals, petals, and stamens. There may also be a nectary producing nectar. Nectaries may develop on or in the perianth, receptacle, androecium (stamens), or gynoecium. In some flowers nectar may be produced on nectariferous disks. Disks may arise from the receptacle and are doughnut- or disk-shaped. They may also surround the stamens (extrastaminal), be at the stamen bases (staminal), or be inside the stamina (intrastaminal).[8]

Finally, the actual reproductive parts form the innermost layers of the flower. These leaf primordia become specialised as sporophylls, leaves that form areas called sporangia, which produce spores, and cavitate internally. The sporangia on the sporophytes of pteridophytes are visible, but those of gymnosperms and angiosperms are not. In the angiosperms there are two types. Some form male organs (stamens), the male sporangia (microsporangia) producing microspores. Others form female organs (carpels), the female sporangia (megasporangia) producing a single large megaspore.[8] These in turn produce the male gametophytes and female gametophytes

These two components are the androecium and gynoecium, respectively. The Androecium (literally, men's house) is a collective term for the male organs (stamens or microsporophylls). While sometimes leaflike (laminar), more commonly they consist of a long thread-like column, the filament, surmounted by a pollen bearing anther. The anther usually consists of two fused thecae. A theca is two microspoorangia. The gynoecium (women's house) is the collective term for the female organs (carpels). A carpel is a modified megasporophyll consisting of two or more ovules, which develop conduplicatively (folded along the line). The carpels may be single, or collected together, to form an ovary, and contain the ovules. Another term, pistil, refers to the ovary as its expanded base, the style, a column arising from the ovary, and an expanded tip, the stigma.[8]

Within the stamen, the microsporangium forms grains of pollen, surrounded by a protective microspore, which form the male gametophyte. Within the carpel the megasporangium form the ovules, with its protective layers (integument) in the megaspore, and the female gametophyte. Unlike the male gametophyte, which is transported in the pollen, the female gametophyte remains within the ovule.[8]

Most flowers have both male and female organs, and hence are considered bisexual (perfect), which is thought to be the ancestral state. However, others have either one or the other and are therefore unisexual, or imperfect. In which case they may be either male (staminate) or female (pistillate). Plants may bear either all bisexual flowers (hermaphroditic), both male and female flowers (monoecious), or only one sex (dioecious), in which case separate plants are either male or female flower-bearing. Where both bisexual and unisexual flowers exist on the same plant, it is called polygamous. Polygamous plants may have bisexual and staminate flowers (andromonoecious), bisexual and pistillate flowers (gynomonoecious), or both (trimonoecious). Other combinations include the presence of bisexual flowers on some individual plants and staminate on others (androdioecious), or bisexual and pistillate (gynodioecious). Finally, trioecious plants have bisexual, staminate, or pistillate flowers on different individuals. Arrangements other than hermaphroditic help to ensure outcrossing.[9]

The development of the embryo and gametophytes is called embryology. The study of pollens which persist in soil for many years is called palynology. Reproduction occurs when male and female gametophytes interact. This generally requires an external agent such as wind or insects to carry the pollen from the stamen to the vicinity of the ovule. This process is called pollination. In gymnosperms (literally naked seed) pollen comes into direct contact with the exposed ovule. In angiosperms the ovule is enclosed in the carpel, requiring a specialised structure, the stigma, to receive the pollen. On the surface of the stigma, the pollen germinates; that is, the male gametophyte penetrates the pollen wall into the stigma, and a pollen tube, an extension of the pollen grain, extends towards the carpel, carrying with it the sperm cells (male gametes) until they encounter the ovule, where they gain access through a pore in the ovule's integument (micropyle), allowing fertilisation to occur. Once the ovule has been fertilised, a new sporophyte, protected and nurtured by the female gametophyte, develops and becomes an embryo. When development stops, the embryo becomes dormant, as a seed. Within the embryo are the primordial shoot and root.

In angiosperms, as the seed develops after fertilisation, so does the surrounding carpel, its walls thickening or hardening, developing colours or nutrients that attract animals or birds. This new entity with its dormant seeds is the fruit, whose functions are protecting the seed and dispersing it. In some cases, androecium and gynaecium may be fused. The resulting structure is a gynandrium (gynostegium, gynostemium, or column), which is supported by an androgynosphore.[8]

Plants, with regard to identification and classification, are not often characterized by their roots, which are important in determining plant duration. However, in some groups, including the grasses, roots are important for proper identification.

Leaf Parts: – A complete leaf is composed of a blade, petiole, and stipules, but in many plants one or more might be lacking or highly modified.

Fruits are the mature ovary of seed-bearing plants, and they include the contents of the ovary, which can be floral parts like the receptacle, involucre, calyx, and others that are fused to it. Fruits are often used to identify plant taxa, help to place the species in the correct family, or differentiate different groups within the same family.

Fruits are divided into different types, depending on how they form, where or how they open, and what parts they are composed of.