BOTANY. PLANT MORPHOLOGY - O. A. Shevchuk - 2014

INFLORESCENCES AND THEIR BIOLOGICAL ROLE

An inflorescence is a SHOOT or a system of shoots that bears groups of flowers in the axils of apical leaves (bracts). Inflorescences with undeveloped, scale-like bracts are called bracteate (such as lilac, lily of the valley, etc.). A cluster of apical leaves surrounding an inflorescence is called an involucre.

The biological advantages of inflorescences over solitary flowers are indisputable. Firstly, they increase the likelihood of flower pollination, and secondly, they reduce the risk of flower damage under adverse environmental conditions due to their gradual opening within the inflorescence.

Classification of Inflorescences

Depending on the branching pattern, inflorescences are divided into two groups: monopodial, botryose, or indefinite, indeterminate, racemose; and sympodial, cymose, or definite, determinate, determinate-flowered.

Among monopodial inflorescences, a distinction is made between simple and complex. Simple botryose inflorescences do not branch; the flowers are located directly on the main axis of the inflorescence. In complex ones, the main axis bears lateral branches with flowers.

Botryose inflorescences are characterized by monopodial branching, meaning they have a well-defined main axis that grows indefinitely. Furthermore, in botryose inflorescences, flowers open in an upward direction—from the base to the apex (acropetal succession), with the apical flower opening last, or toward the center of the inflorescence if the flowers are arranged in a single plane. The number of lateral branches is indefinite, which is why these inflorescences are often called indeterminate. The raceme is considered the most primitive inflorescence in this group.

As a result of the shortening of flower stalks, the raceme transformed into a spike, which, in turn, through the thickening of the main axis, turned into a spadix. In another case, the main axis of the spike became thin and flexible, leading to The formation of a catkin. The branching of the lateral branches of a raceme resulted in a panicle, or compound raceme. Alternatively, a corymb was formed from a raceme through the elongation of the lowermost flower stalks on the axis. From the corymb, through the shortening of the main axis, an umbel arose. Finally, through the shortening of the flower stalks and the thickening and expansion of the main axis, an umbel transforms into a HEAD (capitulum).

All these evolutionary changes in inflorescence architecture occurred in parallel with the evolution of pollination agents and had an adaptive significance.

In cymose inflorescences, unlike botryose ones, branching is not monopodial but sympodial or pseudodichotomous; therefore, the main axis of the inflorescence is either indistinct or false, and the growth of the inflorescence is determinate. Finally, flower opening in cymose inflorescences proceeds from the apex to the base (basipetal succession) or from the center to the periphery (centrifugal, as in spurge). In addition, these inflorescences are determinate because the number of lateral branches is fixed and characteristic of the genus or species.

Simple Monopodial (Indeterminate, Botryose) Inflorescences

Simple Monopodial (Indeterminate, Botryose) Inflorescences with an Elongated Axis

Raceme — individual flowers are arranged at some distance from one another on an elongated main axis of the inflorescence, on pedicels of nearly equal length (bird cherry, hyacinth, robinia, lily of the valley). The pedicels may emerge from the axils of bracts (lupine), or bracts may be absent (mustards, barberry). If the flowers are located on only one side of the axis, a secund (one-sided) raceme is formed (certain legume species: bird's-FOOT trefoil, tufted vetch, lathyrus).

Spike — sessile flowers lacking pedicels are arranged along an elongated main axis (plantain, vervain). Spadix or fleshy spike — an inflorescence with a thickened, fleshy axis densely covered with flowers (female corn inflorescence, cattail). Catkin — a pendulous spike, i.e., a spike with a soft axis; the inflorescence usually falls off after flowering (walnut, poplar).

Simple Monopodial (Indeterminate, Botryose) Inflorescences with a Shortened Axis

Head (Capitulum) — an inflorescence with a shortened, thickened axis and densely crowded sessile or nearly sessile flowers (clover). Corymb — a raceme in which the lower pedicels are longer than the upper ones, and all flowers are positioned at approximately the same level (pear, meadowsweet). Head / Anthodium (Basket) — the main axis of the inflorescence is greatly expanded to form a so-called common receptacle, upon which a varying number of flowers sit. The margins of the common receptacle bear small leaves forming an involucre (sunflower, dandelion, asters, marigolds). Umbel — the main axis of the inflorescence is so shortened that the nearly equal pedicels of all flowers appear to originate from a single point, forming the rays of the umbel (primrose, onion, cherry). At the Base of the umbel rays, there are several bracts forming an involucre.

An inflorescence that resembles a single flower in shape and appearance is called an anthodium (head, basket).

Complex Monopodial (Indeterminate, Botryose) Inflorescences

Compound Spike — instead of individual flowers, simple spikelets are arranged along the elongated main axis of the inflorescence (rye, wheat, ryegrass, couch grass). Panicle, or compound raceme — a complex inflorescence whose lateral branches bear simple or branched inflorescences of the spike type (cereals) or raceme type (male corn inflorescence, millet, lilac). A distinction is made between a spicate panicle (contracted panicle), in which the lateral axes are pressed against the main axis (timothy grass, foxtail); and an open panicle, in which the lateral axes diverge in various directions from the main axis (oats, millet, male corn inflorescence). Compound Corymb — a corymb in which individual flowers are replaced by small corymbose inflorescences (yarrow, rowan). Compound Umbel — differs from a simple umbel in that each lateral branch does not end in a single flower, but instead forms a simple umbel itself, commonly called a umbellet (Apiaceae). Often, leaves at the base of second-order axes form a general involucre (Allioideae), and those at the base of the pedicels form involucels.

Sympodial (Determinate, Cymose) Inflorescences

Monochasium — the main axis terminates in a flower, and a second-order axis develops beneath it, which also terminates in a flower; branching is sympodial, and the axis of each order produces only a single branch. Among monochasia, one distinguishes: a cincinnus (or scorpioid cyme), in which the axis forms a series of flowers in one direction (Boraginaceae) and a bostryx (or helicoid cyme), in which flowers develop on alternating sides of the branches (gladiolus, sundew). Dichasium (forked cyme) — the main axis terminates in a flower, and two opposite axes develop beneath it, each of which also terminates in a flower; branching is pseudodichotomous (Caryophyllaceae). Pleiochasium — several lateral axes depart from the main axis (which bears a single terminal flower), forming a whorl of monochasia or dichasia (sedum, potato, spurge). Flowers open from the center to the periphery. A distinction is made between a simple pleiochasium — in which only second-order axes depart from the main axis (Ranunculaceae) — and a compound one, in which several third-order axes depart from each second-order axis, and so on (elderberry, guelder rose, spurge).

In addition to the aforementioned inflorescences, there are complex inflorescences known as aggregate inflorescences. They are formed by various types of inflorescences. For instance, flower heads may be gathered into a corymbose panicle (yarrow), simple spikelets into a panicle (oats), and a dichasium into a catkin (birch, alder).

Thyrsus — the main axis bears the aforementioned inflorescences; they have a pyramidal shape (silene).

Types and Methods of Flower Pollination

Pollination is The transfer of pollen from the anthers to the stigma of a flower in angiosperms, and from microstrobili to megastrobilus scales in gymnosperms. Two main types are distinguished: self-pollination and cross-pollination. Self-pollination (autogamy) is the transfer of pollen from the anthers to the stigma of the same flower in angiosperms. Obligate self-pollination refers to Fertilization of the stigma exclusively by pollen from the exact same flower. Neighboring pollination, or geitonogamy, is the pollination of a flower by pollen from another flower on the same plant. Autofertility is the ability of plants to produce seeds via self-pollination (e.g., wheat, rye, oats, tomatoes, violets, etc.).

Cross-pollination is the transfer of pollen from the stamen of one flower to the stigma of another flower on the same or a different plant in angiosperms. Bisexual flowers in which the stigma and stamens mature simultaneously are called homogamous flowers. These are predominantly self-pollinating flowers.

The asynchronous maturation of stamens and pistils within flowers is known as dichogamy. Dchogamy is an adaptation that favors cross-Pollination and Fertilization. In protandry, the anthers mature earlier (found in representatives of families such as Asteraceae, Apiaceae, Malvaceae, Caryophyllaceae, and Rosaceae—such as raspberries and strawberries). In protogyny, the stigma matures earlier (found in Brassicaceae, Solanaceae, Plantaginaceae, Poaceae, and Rosaceae—such as apples, pears, and plums).

Legitimate pollination occurs in flowers characterized by heterostyly. In this case, pollen from flowers with long styles is deposited onto the stigmas of flowers with short styles (e.g., buckwheat, lungwort). Cross-pollination is also facilitated by dicliny (unisexual flowers) and autosterility.

There are several modes of cross-pollination: zoophily, which is the pollination of plants mediated by animals. This is a key mechanism of cross-pollination and encompasses several types: entomophily (entomogamy), where cross-pollination is carried out by insects. It occurs in 90% of all plants that rely on cross-pollination (such as apple, pear, linden, and raspberry). Entomophilous plants possess specialized Tissues or glands that secrete nectar—a sugary fluid containing CARBOHYDRATES, nitrogenous compounds, aromatic substances, organic acids, mineral salts, Enzymes, and Essential Oils. Nectaries may form on various PARTS OF THE flower (floral nectaries) or outside the flower (extranpturial or extrafloral nectaries). In linden, they are located on the inner side of the sepals; in beans, inside the spur; in buttercups, on the petals; and in buckwheat, near the base of the Ovary. The cavity on floral structures containing nectar is called a nectar fossa or honey pit. It may be covered by a small scale known as a nectary scale (Ranunculaceae). Honey plants are species from which bees gather nectar and pollen (e.g., buckwheat, linden, robinia). Certain plant species have adapted to pollination by specific insect groups. For instance, some members of Lamiaceae and Scrophulariaceae feature flowers with deeply seated nectaries accessible only to long-tongued insects (hemitropous flowers).

Cantharophily is beetle-mediated pollination, characteristic of cycads. Ornithophily is a type of zoophily where cross-pollination is performed by small birds (such as members of the Orchidaceae family). Malacophily is snail- or slug-mediated pollination. Hydrophily (hydrogamy) is a mode of cross-pollination in aquatic flowering plants facilitated by Water, which can be epihydrophilous (e.g., Vallisneria) or hypohydrophilous (e.g., Zannichellia). Wind-mediated pollination is called anemophily (found in Poaceae, Cyperaceae, poplar, birch, oak, hazel, etc.).

Artificial pollination is pollination performed by humans, typically utilized in breeding new varieties and in Hybridization.

Inbreeding (incubation/selfing) is forced self-pollination, where pollen is applied to the stigma of the same flower, or the crossing of closely related forms to produce uniform progeny.

Pollen grains and palynology

Pollen is the aggregate of pollen grains in seed plants. A pollen grain is essentially the male gametophyte of angiosperms. The microspore wall develops into the pollen grain wall following the completion of divisions, though it only achieves full maturity at the pollen grain stage rather than the microspore stage. Pollen dimensions range in diameter from a few micrometers (in some Boraginaceae) to 240 μm (e.g., in certain Malvaceae). Generally, primitive families tend to have larger pollen grains, although exceptionally large pollen also occurs in advanced families such as Cucurbitaceae. There is a certain correlation between pollen size and flower size; however, the most critical factor determining pollen size is the distance the pollen tube must travel to reach the embryo sac, i.e., the length of the pistil style.

Certain plants (such as orchids and milkweeds) produce pollinia—groups of pollen grains aggregated into a coherent mass. This is related to their specific pollination syndrome.

The shape of pollen grains is highly diverse. They can be spherical (buttercup), ellipsoidal (magnolia, flowering rush), triangular (peony), and so on. The pollen grain wall (sporoderm) consists of two primary layers: an inner layer, the intine, and an outer layer, the exine. The intine is a thin, delicate wall composed mainly of pectic substances, whereas the exine is comparatively thick, layered, and cutinized, containing extremely resistant carbohydrates known as sporopollenins that are insoluble in acids and alkalis. The exine, in turn, comprises two strata: the outer sexine (the sculpted portion of the exine) and the inner nexine (the unculpted portion). The Structure of the sexine is exceptionally diverse yet constant within taxonomic groups, lending it significant systematic value. The surface of the sexine bears various bumps, crests, and spinules, for which a rather complex terminology has been developed. The exine typically features thinned areas or perforation sites that serve as exits for the pollen tube. Any such area or aperture is termed an aperture. The arrangement and Morphology of apertures vary widely. Based on their position, apertures may be polar, zonal (located at the equator or on lines parallel to it), or global (evenly distributed across the surface). Structurally, apertures are classified into colpi (furrows) and pori (pores). Monoporate pollen grains possess only a single pore in the exine (e.g., certain palm species).

Monosulcate pollen grains are the most primitive (found in magnolia and flowering rush) and are the only type present in gymnosperms. Among angiosperms, monosulcate grains occur primarily in primitive families, notably within Magnoliaceae, which concentrates the highest number of primitive traits. Other aperture types evolved from monosulcate grains through an increase in aperture number and modification of shape, following the evolutionary sequence: monosulcate — monoporate — tricolpate — triporate — multicolpate — multiporate.

The majority of dicots are characterized by tricolpate pollen, whereas monocots predominantly feature the monoporate type.

As noted above, the structural Diversity of the sporoderm, combined with its constancy and durability, led to The Emergence of a specialized botanical discipline: palynology. Fossil pollen is exceptionally well-preserved, and pollen analysis of peat deposits allows researchers to reconstruct the taxonomic composition of past floras and detect climatic shifts in cases where other evidence is entirely lacking.

Detailed examination of the pollen grain wall is conducted using light Cell/15.html">Microscopy and, in some instances, Electron microscopy. Pollen analysis is preceded by various chemical and physical preparation methods. The wall ultrastructure is depicted in palynograms, which illustrate the equatorial view, optical cross-section, and fine structural details.

The Study of the developmental patterns of spore and pollen walls serves as a reliable marker for delimiting major taxa, such as plant classes and divisions, while mature wall architecture provides critical diagnostic information for distinguishing lower taxonomic categories, including genera and species.