PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 7. THE FLOWER AS A SPECIAL REPRODUCTIVE ORGAN OF ANGIOSPERMS

7.2. Arrangement of Flowers on the Plant. Inflorescences and Their Types

An inflorescence is a flower-bearing SHOOT whose overall structural plan is similar to that of a vegetative shoot; the only essential difference between them is the presence of flowers in the former and their absence in the latter.

Like a vegetative shoot, an inflorescence has a main axis—the peduncle (or rachis)—which is divided into nodes and internodes. Nodes are the points of attachment of floral leaves, or bracts (*bractea*). Bracts are modified upper leaves that perform a protective function during the early Selection/3.html">Stages of development when the flower is initiated. They develop earlier and faster than the flower itself. Later, after the flower opens, they may perform Photosynthesis, attract insect pollinators, or persist as rudimentary structures without any specific function. Internodes, just as in a vegetative shoot, are the distances between adjacent nodes.

The peduncle (axis of the first order) can branch, forming lateral axes of various orders. The ultimate axes bearing flowers are called pedicels. They bear small leaf-like structures—bracteoles (*bracteolae*)—the number of which varies: one, two, or several.

In some plant species, the inflorescence is clearly demarcated from the vegetative part (lily of the valley — *Convallaria majalis*, tufted vetch — *Vicia cracca*), while in others, the vegetative shoot transitions smoothly into the flower-bearing shoot (spring speedwell — *Veronica verna*).

Inflorescences are extremely diverse in their morphological features, which makes it possible to group them; however, there is virtually no universally accepted Classification of inflorescences.

Currently, There are two approaches to the classification of inflorescences: the physiognomic and the structural approach. The physiognomic approach is based on the morphological features of inflorescences and is widely used in classical plant Morphology and systematics. The structural approach is based on studying the patterns of mutual arrangement of flower-bearing axes, as well as their integration into the plant's overall system. The Development of the structural approach was initiated by the works of the German botanist W. Troll (1964, 1969). His views are shared by A number of foreign scientists and some Russian botanists. However, according to Al. A. Fedorov and Z. T. Artyushenko (1979), this classification is inconvenient for Structure/182.html">Practical Application, overloaded with terminology, and hardly suitable for descriptive morphology.

Based on the morphological features of inflorescences, they can be classified According to the following criteria: the branching pattern of the peduncle, the Size and Structure of bracts, the structural Nature of the apex of the main peduncle, and the degree of complexity of the inflorescence.

Depending on their branching characteristics, inflorescences can be divided into two types: botrytic (from Greek *botrys* — cluster), or indeterminate, racemose (from Latin *racemus* — cluster), and cymose (from Latin *cyma* — cyme), or determinate.

Botrytic inflorescences branch monopodially. With this type of branching, the peduncle grows in length relatively indefinitely, and its flowers are initiated acropetally (from the base toward the apex); therefore, their number can vary.

Cymose inflorescences branch sympodially or pseudo-dichotomously. In sympodial branching, the peduncle consists of axes of various orders, and the apex of each axis terminates in a single flower. In pseudo-dichotomous branching, the peduncle axis terminates in a flower,

below which two lateral axes are formed from the axillary buds, each also bearing a single flower. Thus, a strictly determined number of flowers is formed on the axes.

One of the Characteristic Features of inflorescences is the presence of bracts and their structural peculiarities. Bracts may not differ in shape and size from the leaves of the vegetative part of the shoot and, along with them, perform an assimilatory function. Such inflorescences are called frondose (polygonatum — *Polygonatum multiflorum*, creeping loosestrife — *Lysimachia nummularia*). If the bracts are green but smaller than the leaves of the vegetative part of the shoot, the inflorescences are called frondulous (creeping bellflower — *Campanula rapunculoides*, common lilac — *Syringa vulgaris*). In some plants, the upper leaves are strongly modified and can be herbaceous or scarious. Inflorescences with such bracts are called bracteate (lily of the valley — *Convallaria majalis*, heracleum-leaved begonia — *Begonia heracleifolia*).

In Representatives of the Arum family (anthurium — *Anthurium*, spathiphyllum — *Spathiphyllum*), large, brightly colored bracts form a spathe at the Base of the inflorescence. Bracts are absent in representatives of the Brassicaceae family.

The apex of the peduncle may terminate in a flower, which limits its apical growth, or, as the peduncle develops, the apical meristem forms only the lateral PARTS OF THE inflorescence, while the main axis grows indefinitely. If the inflorescence peduncle terminates in a flower, the inflorescence is called closed (barberry — *Barberry vulgaris*, greater celandine — *Chelidonium majus*); if the terminal flower on the main axis of the inflorescence is absent, it is open (lily of the valley — *Convallaria majalis*).

Depending on whether single flowers or lateral (partial) inflorescences are located on the main peduncle, all inflorescences can be divided into simple and complex.

7.2.1. Simple Inflorescences

In simple inflorescences, single flowers develop on the peduncle—either sessile or attached to the peduncle by means of a pedicel.

When characterizing simple and complex inflorescences, the following factors are taken into account: the branching type of the main axis, the thickness of the peduncle, the specific attachment of flowers to it, and the spatial arrangement of the flowers. Simple inflorescences (as already mentioned) are either botrytic (indeterminate) or cymose (determinate).

Simple botrytic inflorescences include the raceme, spike, spadix, umbel, corymb, head, and capitulum (Fig. 143).

Class="center">Fig. 143. Simple botrytic (indeterminate) inflorescences: A — spike; B — raceme; C — spadix; D — head: 1 — sessile flowers; 2 — flowers on pedicels; D — umbel; E — corymb; G — head

The raceme (bird cherry — Prunus padus) and the spike (greater plantain — Plantago major) share a similar peduncle structure, yet they differ in how the flowers are attached. In a raceme, flowers are borne on pedicels, whereas in a spike, they are sessile. Sessile flowers also occur in the spadix (sweet flag — Acorus calamus, bog arum — Calla palustris), but this inflorescence features a significantly thickened peduncle that sets it apart from a spike.

The head (capitulum) inflorescence occupies an intermediate position between the raceme and the spike, as it features a shortened peduncle compared to the latter. This type of inflorescence is typically found in representatives of the clover genus, Trifolium. In some clover species (strawberry clover — Trifolium fragiferum), the flowers are nearly sessile, while in others (lupine clover — Trifolium lupinaster), they are borne on well-developed pedicels.

Simple umbels and simple corymbs show a certain degree of structural similarity. In a simple umbel, all pedicels arise from the apex of the peduncle and are of equal length (cowslip — Primula veris). In a corymb, the pedicel of each flower originates in the axil of its own bract, meaning that the outer pedicels are longer than the inner ones (pear — Pyrus communis, birchleaf spirea — Spiraea chamaedrifolia).

The head (capitulum) differs from all other simple inflorescences in that the apex of the peduncle is expanded to varying degrees, forming a "common receptacle" upon which the flowers are arranged. Externally, the head is surrounded by an involucre formed by uppermost leaves. The flowers in a head are always sessile. The number of flowers per head ranges from a single one (globe thistle — Echinops sphaerocephalus) to an indefinitely large number (common sunflower — Helianthus annuus, dandelion — Taraxacum officinale). In some plants (common chicory — Cichorium intybus), the ligulate corollas are positioned centrifugally, while the styles with their surrounding stamens occupy a central position. This makes the head inflorescence resemble a single flower, which is why such inflorescences are referred to as pseudanthia (from the Greek anthos — flower).

The opening of flowers in racemose (botryose) inflorescences is either acropetal (spike, raceme, spadix, head) or centripetal (umbel, corymb, head).

Simple cymose inflorescences include the monochasium (from the Greek monos — single, chasis — cleft), dichasium (from the Greek dis — double, chasis — cleft), and pleiochasium (from the Greek pleion — more, chasis — cleft) (Fig. 144).

Fig. 144. Simple cymose (determinate) inflorescences: A — monochasium: 1 — cincrinus; 2 — drepanium; B — dichasium; C — pleiochasium

In a monochasium, the peduncle develops via sympodial branching, but the spatial orientation of the flowers can vary. When the flowers are arranged on one side of the peduncle, the monochasium is known as a cincrinus (alkanet — Anchusa officinalis, lungwort — Pulmonaria angustifolia). If the flowers point in alternate directions from the peduncle, a monochasial inflorescence called a drepanium (or cyme) is formed (gladiolus — Gladiolus imbricatus, hybrid freesia — Freesia hybrida).

The peduncles of dichasial and pleiochasial inflorescences branch in a false-dichotomous (dichasial) manner. A simple dichasium contains three flowers, where the two lateral ones outgrow the central one (family Caryophyllaceae) or, due to the reduction of pedicels, lie at the same level (dichasia in birch inflorescences). In a pleiochasium, rather than two, several lateral axes are formed (Mexican stonecrop — Sedum mexicanum).

The opening of flowers in cymose inflorescences proceeds centrifugally (from the center to the periphery).

7.2.2. Complex Inflorescences

Like simple inflorescences, complex inflorescences can branch monopodially, sympodially, or falsely dichotomously; consequently, one can distinguish complex botryose and complex cymose inflorescences.

Complex botryose inflorescences bear partial inflorescences on their main peduncle, which, much like the main axis, branch monopodially. The names of complex botryose inflorescences mirror those of simple ones, with The addition of the word "compound" or "complex": compound spike (rye — Secale cereale, couch grass — Elytrigia repens), compound raceme (lilac — Syringa vulgaris). In grasses, a compound raceme is referred to as a panicle (Kentucky bluegrass — Poa pratensis) (Fig. 145).

Fig. 145. Complex botryose (indeterminate) inflorescences: A — raceme; B — spike; C — corn cob (Zea mays); D — umbel; E — corymb

Complex inflorescences may form from uniform simple inflorescences or from simple inflorescences of a different structure. For instance, compound umbels in parsley (Petroselinum sativum), dill (Anethum graveolens), and other members of the Apiaceae family consist of simple umbels. A compound corymb in rowan (Sorbus aucuparia) is formed by simple corymbs, whereas in yarrow (Achillea millefolium) it consists of capitula (heads). The compound raceme of goldenrod (Solidago virgaurea) is likewise made up of capitula. A rare type of complex inflorescence is the compound head. In the globe thistle (Echinops sphaerocephalus), the capitate inflorescence is formed by single-flowered heads.

Complex cymose inflorescences in which the main axis branches monopodially while the partial lateral inflorescences branch sympodially are termed thyrsoid inflorescences, or thyrses. Functionally, this is a compound raceme with different branching patterns for the main and lateral axes (horse chestnut — Aesculus hippocastanum) (Fig. 146, A).

Complex cymose inflorescences in which both the main and lateral peduncles branch sympodially—a feature characteristic of the general inflorescence in members of the borage family (alpine forget-me-not — Myosotis alpestris, comfrey — Symphytum officinale)—are called cymoids (Fig. 146, B). Thus, the terms "thyrsoids" and "cymoids" require significant clarification in morphological descriptions. Complex cymoid inflorescences consisting of dichasia are rare (Fig. 146, C). In certain spurges (sun spurge — Euphorbia helioscopia), a complex cymose pleiochasium is formed, consisting of specialized structures known as cyathia. A cyathium comprises a single naked pistillate flower and several staminate flowers. Much like the pistillate flower, each staminate flower lacks a perianth and consists of a single stamen.

Fig. 146. Complex cymose (determinate) inflorescences: A — thyrse; B — cymoid; C — catkin of silver birch (Betula pendula)

The question of the ancestral inflorescence type remains unresolved to this day; consequently, all presented schemes of evolutionary relationships among various inflorescence types are hypothetical. The notion of whether a single flower or an inflorescence came first is equally speculative. Scientists express diametrically opposed views on this matter. It is entirely possible that both evolutionary directions of adaptive modification occurred across different lineages of flowering plants.

From a biological standpoint, inflorescences offer clear advantages over solitary flowers. The sequential development and blooming of A large number of flowers ensures that any damage to an individual flower does not impair seed production. Thus, inflorescences provide much more reliable conditions for Pollination and Fertilization, and consequently for fruit and seed set, which is crucial for the reproduction of flowering plants and the maintenance of stable populations across various species.