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

BUD

A bud (gemma) is an embryonic SHOOT with shortened internodes that develops in a specific sequence on an axis, providing for its apical growth and branching.

A bud remains in a state of relative dormancy. It consists of a meristematic embryonic axis (embryonic stem) with an apex bearing a growing point, along with a series of embryonic metameres—leaf primordia of various ages in the axils of which embryonic lateral buds are located. The nodes in a bud are closely spaced because the internodes have not yet elongated. Externally, the bud is covered with young leaves or bud scales impregnated with cutin and resinous substances.

In terms of size, buds can be large, small, or medium; in shape, they may be round, ovoid, conical, elliptic, elongated, narrow, or fusiform; by apex shape (blunt, acute, pointed, obtuse); by color and surface features (glabrous or pubescent, glossy, shiny, sticky, resinous). These characteristics should be considered when providing a morphological description of buds.

By Structure, buds are classified as closed and open, or naked.

A bud possessing bud scales that protect the growing point from Temperature fluctuations and desiccation is called a closed bud. Such buds vary in their arrangement, number, degree of scale overlap, coloration, pubescence, stickiness, and other features. Closed buds are commonly found in plants of temperate and moderately cold zones.

Buds lacking protective covering scales (cataphylls) are called naked or open buds (e.g., in bugle, yellow archangel, barberry, buckthorn, glossy buckthorn, tropical plants, and annual herbaceous plants). The Development of covering scales is characteristic of overwintering buds. In open buds, protective Functions are performed by stipules, hairs, leaf sheaths, and similar structures.

By origin, buds are divided into endogenous and exogenous.

Exogenous buds include apical and lateral buds, whereas endogenous buds include adventitious buds.

By their position on the stem, buds are categorized into apical and lateral.

The apical, or terminal bud is located at the apex of the main and lateral shoots, ensuring their longitudinal growth (as in the majority of plants). The embryonic bud serves as the primary apical bud.

The lateral, or axillary bud is situated on the side of the stem in the leaf axils or near leaf scars, and it provides for shoot branching. It forms exogenously from the apical meristem and is typically observed in the axil of the third to fifth leaf primordium. If the apical bud dies, the lateral bud promotes shoot elongation. Several types of lateral buds are distinguished: axillary, cauline, and accessory or brood buds.

An axillary bud is located in the leaf axils. Such buds are characteristic of most plants. Among axillary buds, solitary and group buds are distinguished. Group buds are further divided into serial buds, if they are arranged vertically one above the other in the leaf axil (e.g., in honeysuckle, sensitive plant), verticillate buds, arranged in a circle (e.g., in certain plum species), and collateral buds, located horizontally side by side (e.g., in garlic, bamboo, onion).

A cauline bud is positioned outside the leaf axil, on the side of the stem.

An accessory or adventitious bud arises endogenously from internal Tissues (pericycle, cortical parenchyma, and medullary rays), meaning it develops without the participation of the shoot apical meristem and is located outside the leaf axil. Such buds can develop on shoots, stems, leaves, and roots. For instance, ROOT suckers in lilac, raspberry, blackberry, dandelion, sorrel, and other plants originate from adventitious buds that develop on the roots.

By function, buds are classified as vegetative, generative, vegetative-generative, regulatory-renewal, brood, enrichment, and dormant.

A vegetative or leaf bud is one from which a shoot with leaves develops. Such buds are formed in the majority of plants.

A generative, reproductive, or flower bud is one that develops into a flower or an inflorescence (e.g., in Forsythia, cherry, willow, etc.). A generative bud containing a single embryonic flower is called a flower bud or button (e.g., in snowdrop, narcissus, tulip, etc.).

A vegetative-generative or vegetative-reproductive (mixed) bud produces several shoot metameres as well as a flower or inflorescence (e.g., in lilac, elderberry, walnut, wild ginger, apple, pear, plum, grasses, etc.).

A regulatory-renewal bud produces new shoots after a certain period of dormancy, through which periodic renewal of the shoot system occurs after a pause (e.g., in most woody plants and perennial herbaceous plants). These include overwintering or dormant buds. A dormant bud is a lateral bud that remains in a dormant state for a prolonged period. It awakens following the death of the apical bud and an active influx of Auxins to its Location. When woody species are pruned or their branches suffer frost damage, dormant buds sprout and give rise to new shoots. In perennial trees, such shoots are known as Water sprouts or epicormic shoots (e.g., in aspen, poplar, apple, etc.).

When dormant buds are densely clustered on the trunks of trees, they form burls featuring massive broom-like growths (e.g., in birch, hornbeam, oak, robinia, etc.).

Renewal buds can be vegetative, generative, or mixed according to their function, and exogenous (lateral) or endogenous (adventitious) according to their origin. A renewal bud may remain dormant not only during the winter period but also during times of unfavorable environmental conditions.

Cauliflory is a phenomenon in which shortened, leafless floral shoots develop directly from dormant buds on old stems and trunks (e.g., in many tropical trees such as Ficus, papaya, cacao tree, etc.).

A brood bud is a specialized accessory bud adapted for vegetative propagation. Such buds are formed in the leaf axils or inflorescences of certain plants and, upon detaching, give rise to new plants. These are metamorphosed buds bearing rudiments of adventitious roots. Brood buds may be represented by: bulbils formed in the leaf axils of aerial shoots (e.g., in certain lily genera) and in inflorescences (e.g., in certain onion genera); cormels (e.g., in gladiolus); small tubers in leaf axils (e.g., in certain yams and pheasant's eye); buds developing in the notches of leaf margins (e.g., in Kalanchoe, Bryophyllum); 'eyes'—renewal buds on tubers accompanied by a portion of the stem (e.g., potato, Jerusalem artichoke).

A supplementary bud (or enrichment bud) is a bud that lacks a period of growth dormancy; that is, it remains functionally active alongside the growth of the parent shoot upon which it is formed (found, for example, in nasturtiums, beans, and certain species of cornflower and willow). Such buds develop into enrichment shoots that increase the leaf surface area as well as the number of inflorescences and seeds.

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Fig. 4. Arrangement of buds on the stem: 1 - alternate, 2 - opposite in maple, 3 - spiral, 4 - bicollateral, 5 - collateral.