MEDICAL BOTANY - A.G. Serbin - 2003

SECTION 2. MORPHOLOGY. REPRODUCTION

MORPHOLOGY OF GENERATIVE ORGANS

The primary generative organ of angiosperms is the flower, which develops from a generative bud. Flowers may be solitary at the apex of the stem or in the axils of leaves, but more frequently they are borne on a specialized SHOOT and form an inflorescence.

INFLORESCENCE (INFLORESCENTIA)

An inflorescence is a specialized generative shoot bearing flowers and bracts. It consists of a main axis and lateral axes of the first, second, and subsequent orders. Flowers may be pedunculate or sessile, and accompanied by green or modified bracts, also known as floral leaves.

Various classifications and morphological descriptions of inflorescence types are based on the following features:

✵ position on the shoot (terminal and axillary inflorescences);

✵ mode of shoot growth and branching (monopodial and sympodial inflorescences);

✵ degree and order of branching (simple, complex, and mixed homogeneous and heterogeneous inflorescences);

✵ presence, absence, and nature of bracts (foliaceous inflorescences with green bracts are frondose, whereas those with scaly bracts are bracteose; leafless or naked inflorescences have reduced bracts or lack them entirely);

✵ sex of the flowers comprising the inflorescence (monogamous — pistillate or female; staminate or male; and polygamous — containing both unisexual and bisexual flowers);

✵ apex activity (determinate or closed inflorescences, where the shoot apex ceases growth early and terminates in a flower that opens before the lower flowers; indeterminate or open inflorescences, where the main axis exhibits unlimited growth and does not end in a flower, with flowers opening sequentially from the base upwards).

Monopodial, or botryose, inflorescences are characterized by lateral branching and monopodial growth.

Simple monopodial inflorescences may be open or closed, with sessile or pedunculate flowers (Fig. 2.18, 1 — 8). Let us review the most widespread types of inflorescences. Raceme — the main axis is elongated, and flowers are alternate and pedicellate; a raceme can be unilateral, bilateral, dense, loose, interrupted, erect, pendulous, etc. Spike — the main axis is elongated, and flowers are sessile and alternate (Plantago). Spadix — the main axis is elongated and thickened, with sessile flowers (Acorus). Catkin (ament) — the main axis is pendulous and deciduous, with sessile, unisexual flowers. Umbel — the main axis is greatly shortened, and pedicels are approximately equal in length, arising seemingly from a single point (cherry, onion). Corymb — the main axis is moderately developed, flowers are alternate, and the pedicels of the lower flowers are longer than those of the upper ones, bringing all flowers to approximately the same level (pear, hawthorn). HEAD (capitulum) — the main axis is shortened and thickened, with very short-pedicellate flowers (clover). Head/capitulum (basket) — the main axis expands horizontally into a common receptacle, bearing small, sessile flowers with characteristic corolla types (fam. Asteraceae).

Complex homogeneous monopodial inflorescences (Fig. 2.18, 9 — 12) feature lateral axes of the second and subsequent orders bearing flowers or partial (elementary) monopodial inflorescences (e.g., a double raceme) branching off the main axis.

Class="center">Fig. 2.18. Inflorescences. 1— 8 — simple botryose: raceme (1), spike (2), spadix (3), catkin (4), umbel (5), corymb (6), head (7), capitulum/basket (8); 9 —12 — complex homogeneous botryose: panicle (9), compound umbel (10), compound spike (11), compound corymb (12); 13 —16 — complex heterogeneous botryose: panicle of umbels (13), raceme of capitula (14), spike of capitula (15), corymbose panicle of capitula (16); 17 — 22 — cymose: simple monochasium (17), cincrinus (18), bostryx/helicoid cyme (19), dichasium (20), pleiochasium (21, 22); 23 — thyrse — panicle of dichasia; a — main axis; b — pedicel; c, d, e — lateral axes of the first and subsequent orders; f — flowers; g — bracts

A panicle consists of an elongating main axis with profusely branched higher-order axes bearing flowers, racemes ( racemose panicle), or corymbs (corymbose panicle). A compound umbel is composed of simple umbels, or umbellets (fam. Apiaceae). A compound spike consists of elementary inflorescences known as spikelets (fam. Poaceae). A compound corymb consists of simple corymbs (fam. Rosaceae — rowan, chokeberry).

Examples of complex heterogeneous (aggregate) monopodial inflorescences (Fig. 2.18, 13 — 16) include: a panicle of umbels, a raceme of capitula, a spike of capitula, a corymbose panicle of capitula, and a panicle of compound spikelets.

Sympodial, or cymose, inflorescences are characterized by lateral branching and sympodial growth. They are typically determinate, because an early-forming terminal flower halts the further Development of the main axis. Growth of the inflorescence is continued by one or more underlying lateral shoots of subsequent orders, which also terminate in flowers. Cymose inflorescences are subdivided into the following morphological types: monochasia, dichasia, and pleiochasia (Fig. 2.18, 17 — 22). A simple monochasium has a single lateral axis or two alternate axes (fam. Ranunculaceae). A complex monochasium has several lateral axes, each giving rise to only one continuing axis of the next order. Varieties of complex monochasia include the cincrinus, bostryx, and glomerule. Cincrinus, or scorpioid cyme — the lateral axes of the second and subsequent orders are directed to one side (potato, henbane). Glomerule — the lateral axes of a cincrinus are greatly shortened, resulting in congested flowers (beet). Bostryx/helicoid cyme — the lateral axes of the second and subsequent orders are directed alternately in opposite directions (iris, gladiolus). Dichasium, fork, or false umbel — two lateral axes of the second and subsequent orders are positioned opposite each other (fam. Caryophyllaceae). Pleiochasium, or false umbel — the lateral axes of the second order are whorled, bearing flowers, dichasia, or monochasia (fam. Euphorbiaceae, Apocynaceae). Verticillasters (false whorls) are pleiochasia or dichasia with short peduncles arranged around the stem in a single plane (fam. Lamiaceae).

Thyrses, or mixed heterogeneous inflorescences (Fig. 2.18, 23), are characterized by a main axis with monopodial growth and lateral axes with sympodial growth, where the degree of branching of the lateral axes decreases from the Base of the inflorescence to its apex. Thyrses can be determinate or indeterminate, with alternate or opposite arrangement of partial inflorescences. Examples of morphological variations of the thyrse include: a raceme of cincrini (horse chestnut), a raceme of double helicoid cymes (figwort); a corymbose thyrse (Tormentil), a spicate thyrse (mullein), an amentaceous thyrse (birch, oak), an umbellate thyrse (geranium), and a pleiothyrse (nettle).