PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 5. VEGETATIVE PLANT ORGANS

5.2. Formation of the Aerial Part of a Plant

5.2.1. METABOLISM/2.html">THE CONCEPT OF the SHOOT. The Bud as an Embryonic Shoot

The aerial part of angiosperms is represented by a system of shoots—vegetative and floral (peduncles). A vegetative shoot is a stem bearing leaves and buds. In addition to leaves, floral shoots develop flowers or inflorescences. Both vegetative and floral shoots develop from buds. Thus, a bud is an undeveloped (embryonic) shoot.

At different Stages of Ontogeny, plants form embryonic buds and renewal buds (Fig. 80). Embryonic buds develop in the seed embryo, whereas renewal buds arise on various parts of perennial plants (roots in ROOT-suckering plants, rhizomes, tubers, bulbs, aerial shoots, etc.). The General structural plan of embryonic buds and renewal buds is similar. The central part of an embryonic bud consists of an embryonic stem with a well-developed apical meristem. One to two pairs of embryonic leaves are formed on the embryonic stem. The central part of a renewal bud is represented by an embryonic stem, the apical region of which consists of a meristem forming the shoot apical meristem. The number of leaves on the embryonic stem varies among different plant species. Leaves, like the stem, are formed through The activity of the shoot apex. In most temperate plants, renewal buds are covered with bud scales. Bud scales (cataphylls) are arrested leaves that perform a protective function, shielding vital PARTS OF THE bud from low temperatures and sharp Temperature fluctuations; consequently, bud scales vary in Structure. They may undergo varying degrees of suberization (common lilac, Syringa vulgaris) or secrete resinous substances (balsam poplar, Populus balsamifera). In many plants, bud scales are densely covered with hairs (white poplar, Populus alba). These structural features are adaptive in nature.

Class="center">Fig. 80. Structure of buds in red elderberry (Sambucus racemosa): I—vegetative buds; II—floral buds: A—general appearance and arrangement on the stem; B—longitudinal section: 1—bud scales; 2—shoot apical meristem; 3—embryonic inflorescence; 4—embryonic leaves; 5—embryonic stem

The number of bud scales is also adaptive. In winter-hardy species, the number of bud scales is small. For instance, various willow species have only a single closed cap-like scale, while elderberry has 2 to 4. In contrast, most temperate plants have more scales, showing a quite distinct transition from bud scales to embryonic leaves. Embryonic buds lack bud scales, with their protective function being performed by cotyledons and the seed coat.

5.2.2. FORMATION AND STRUCTURE of the Shoot

Under favorable conditions, buds begin to develop and grow into shoots. Shoots have a definite structure. The stem serves as the axis of the shoot, while the leaf represents a lateral outgrowth (lateral organ) attached to the stem by its base. The point of leaf attachment to the stem is called a node, the section of the stem between two adjacent nodes is an internode, and the upper (adaxial) angle between the stem and the leaf is the leaf axil (Fig. 81). Thus, a shoot consists of metameres (from Greek meta—after, meros—part, portion)—homogeneous segments repeating along the axis.

Fig. 81. Structural diagram of elongated (A) and shortened (B) shoots: 1—node; 2—internode; 3—apical bud; 4—axillary bud; 5—leaf axil

During the growing season, shoots increase in length either through the meristem of the shoot apex (apical growth) or via intercalary meristem (intercalary growth). Apical growth is characteristic of vegetative shoots. Such shoots grow actively throughout the growing season; therefore, their internodes are well-developed, as the nodes are situated at considerable distances from one another. Shoots with well-defined internodes are termed elongated shoots, or auxiblasts.

Intercalary growth is predominantly characteristic of reproductive (floral) shoots (cereals, daffodils, tulips). Since the lengthwise growth of floral shoots in woody plants (poplar, birch) and shrubs is limited, the nodes appear closely crowded and the internodes are barely distinguishable.

Shoots with poorly defined internodes are called shortened shoots, or brachyblasts. In fruit trees, they are referred to as spur shoots (fruit spurs).

Pines possess shortened vegetative shoots. Each such shoot bears from two to five needle-like leaves.

Shoot growth is most intense during the spring, slows down by autumn, and eventually ceases altogether. In perennials that retain their aerial parts over winter (trees, shrubs), The formation of renewal buds is fully completed by autumn. In herbaceous perennials, the aerial part dies back, and renewal buds are formed on surviving underground structures.

The aerial part of a plant is formed through branching, which can be monopodial, sympodial, or false-dichotomous (pseudodichotomous). The branching pattern determines the habit of the plant—its general appearance, and in woody plants, the crown shape. To a certain extent, the branching character defines the life form of many plant species.

The Concept of the shoot as an aerial axial organ was first introduced by the French plant morphologist C. Gaudichaud (1789–1864) in 1841. However, this idea received its greatest development in the work of the British botanist Agnes Arber (1879–1960), *Natural Philosophy of Plant Form*, published in 1930. Subsequently, her concept of the shoot as an axial aerial plant organ was supported by A number of leading morphologists—both foreign (A. Eames, K. Esau) and Russian (N. V. Pervukhina, T. I. Serebryakova). At the same time, the majority of botanists adhere to the classical view of vegetative Organs, which is also reflected in educational curricula.

The leading role in shoot formation belongs to the stem apex, more specifically the apical meristem of the shoot tip. Through the division and differentiation of meristematic Cells of the shoot apex, the stem, leaves, and buds are formed.

5.2.3. Classification of Buds

Buds are an essential component of the shoot. Despite sharing a general or similar structural plan, buds differ in a number of features, which is reflected in their classification. Thus, buds are classified according to their structure, position on the shoot, formation characteristics, duration of the resting period, positioning within the leaf axil, and presence of protective structures.

Based on their structure, buds are categorized into vegetative, reproductive (floral), and mixed buds (see Fig. 80).

A vegetative bud consists of an embryonic stem and embryonic leaves, and gives rise to a vegetative shoot (e.g., aspen, Populus tremula). In a reproductive (floral) bud, a flower or inflorescence is initiated from the apical meristem of the shoot tip. Embryonic leaves are either absent in a floral bud or occur in very small numbers (red elderberry, Sambucus racemosa). In mixed buds, both the vegetative and reproductive parts of the shoot are equally well-developed (common lilac, Syringa vulgaris).

Depending on their position on the shoot, buds are classified as terminal, lateral (axillary) (see Fig. 81), and adventitious. Unlike terminal and axillary buds, which occupy a strictly defined position on the stem, adventitious buds are located on various parts of the internode. Terminal buds enable the annual longitudinal growth of the shoot. Axillary and adventitious buds give rise to lateral shoots, thereby facilitating branching.

Buds also differ in their formation patterns. Terminal and axillary buds develop exogenously, whereas adventitious buds arise endogenously within the pericycle, developing in a manner similar to lateral and adventitious roots.

Due to their varying origins, buds in perennial plants differ in the duration of their dormancy period. Resting, or wintering, buds have a short dormancy period dictated by the length of the cold autumn-winter season. Such buds "awaken" annually with the onset of spring warming. Both terminal and adventitious buds are resting buds, while adventitious buds act as "dormant" buds. They can develop within the stem Tissues for a long time, emerging to form shoots only under specific conditions—typically when apical dominance is lost due to branch pruning or the death of the terminal buds. Prolonged growth of adventitious buds inside the stem leads to the formation of woody outgrowths known as burls. Such naturally occurring burls are characteristic of Karelian birch. Cross-sections of these burls reveal a highly attractive grain pattern, making them highly valued and sought after for various luxury handicrafts (Fig. 82).

Fig. 82. Numerous burls on a tree trunk

The number of buds formed both at the apex and in the leaf axils can vary. The production of A large number of buds

in a specific region of the shoot is either genetically predetermined or influenced by environmental factors (such as a sharp transition from drought to a period of heavy rainfall). Most commonly, a single bud develops in a leaf axil; these are referred to as solitary buds. However, serial and collateral buds may also form within the leaf axil (Fig. 83).

Fig. 83. Arrangement of buds in the leaf axil: A—solitary buds of bird cherry (Padus avium); B—serial buds of fly honeysuckle (Lonicera xylosteum);

C—collateral buds of common apricot (Armeniaca vulgaris)

Serial buds are arranged one above the other in vertical rows (such as in plum, cherry, honeysuckle, and raspberry), whereas collateral buds form in horizontal rows (found in elderberry and forsythia). Because neither serial nor collateral buds develop simultaneously, the buds within each group vary in size and do not sprout into shoots at the same time. The largest bud—specifically the one that formed first—"awakens" first, while the remaining buds typically fail to develop and either wither or persist in a latent state.

The formation of serial and collateral buds in certain plants is most likely an adaptive trait. If the main bud is damaged, which can occur during winter due to sudden temperature fluctuations, new shoots can develop from the supplementary buds.

As noted previously, renewal buds are usually protected on the outside by bud scales, in which case they are termed enclosed buds. However, some tropical plants possess naked buds lacking scales. Among plants cultivated as ornamentals in Belarus, naked buds occur in the wayfaring tree (Viburnum lantana). Naked buds are also formed in the butternut (Juglans cinerea) and other walnut species introduced to Belarus.

In naked buds, the protective function is performed by heavily pubescent outer rudimentary leaves.