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In this article we will discuss about the characteristics of axillary bud in plants. This will also help you to draw the structure and diagram of the axillary bud in plants.
The lateral branch of seed plants, in their nascent state, is referred to as axillary bud that is commonly present on the stem at the axil of leaves.
The lateral bud primordia usually arise in close association with leaf primordia. Opinion varies regarding the place of origin of bud primordia. Foliar origin is suggested by Garrison (1955) in dicotyledons. In monocot (e.g. Musa, Tradescantia) the bud develops below leaf primordia and is axial in origin.
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In most cases bud primordia arise on stem at the axil of leaf primordia and sometimes may be displaced towards the leaf base during growth. Whatever may be the place of origin, i.e. foliar or axial; the buds arise exogenously in relation to superficial tissues. Champagnat (1961) reported epidermal origin of axillary buds.
Bud primordia usually arise later than the subtending leaves, which are commonly the second or third plastochron. [The term plastochron was adopted to designate the time interval between the formation of leaf primordium and the next on a shoot]. At this level the outer cells of apical meristem divide anticlinally and the cells, thus formed, delimit the meristem of axillary bud from apical meristem.
These cells possess all the characteristics of meristematic cells and divide in various planes, predominantly anticlinal and periclinal. As a result, the apical meristem of lateral branch is organized resembling that of the parent shoot and elevated above the surface. Initially the bud meristem is formed in continuity with shoot apical meristem.
Later the elongation of internodes separates them from each other. Vacuoles appear in certain cells situated above the axillary meristem and these cells separate the bud meristem from apical meristem.
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As growth continues the bud meristem begins to give rise to leaf primordia. Normal growth and development do not occur immediately. It happens when the axillary bud attains a position some distance away from shoot apical meristem due to normal growth of shoot.
In some species lateral bud is formed in the axil of older leaves. Such bud arises either from differentiated cell like parenchyma or from meristematic cells present at the internode. In the former case the cells owe their origin from the apical meristem.
In the derivatives of apical meristem, vacuoles appear and thus partly or fully differentiated parenchyma tissues are formed. At the time of bud formation, these parenchyma cells regain the power of cell division and form the meristem of axillary bud. This phenomenon can be interpreted as dedifferentiation when the meristematic activity is resumed in differentiated cells.
In the latter case (i.e. origin from meristematic cells) the cells are continuous with the shoot apical meristem. In some cells vacuoles appear and these differentiated cells separate a group of meristematic cells from the shoot apical meristem. This isolated group of meristematic cells is designated as detached meristem. This meristem remains dormant or inhibited until stimulated to form meristem of axillary bud.
In unusual cases axillary bud may arise from the subtending leaf of the first plastochron (ex. Hydrocharis). In the axil of all leaves of a shoot, bud may not be formed, but in some species they arise sporadically or in a regular sequence. In Alternanthera philoxeroides two axillary buds arise at a node from the opposite leaves. These buds are of unequal size and have different potentialities of growth. In Ulex the lateral buds develop as thorns.
In potato plant it is observed that, during the initiation of axillary bud, the divisions are quite regular. This orderly division occurs at base and on lateral side of the incipient bud. As a result a meristematic zone composed of a series of curved layers are formed. These layers are more or less parallel to each other and are designated as shell-zone because of its shell-like configuration (Fig. 24.1 A).
Regarding the function it is suggested that the cells of shell-zone divide and give an active upward thrust to the developing bud to protrude above the surface. The upward growth of the bud (Fig. 24.1 B) is followed by the appearance of the leaf primordia beginning with the prophyll.
Axillary buds are connected by vascular traces to the vascular strands of main axis. These vascular traces are referred to as bud trace or branch trace. If the bud primordia originate close to apex, the bud traces differentiate earlier than the vacuolation of peripheral meristem.
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At this stage the bud trace appears as meristematic strand lying between the vascular-tissues of main axis and protruding bud. Careful observation reveals that bud trace first originates in the bud primordium and then gradually extends towards the vascular strand of main axis, i.e. the differentiation of bud trace is basipetal.
In some species axillary bud arises in the axil of older leaves. In this case bud traces differentiate through partly differentiated or vacuolated ground tissues. The differentiation of bud traces in this case is also basipetal, i.e. traces originate in the bud and gradually extend towards the vascular strand of main axis.
Sometimes the dormant axillary buds are removed further as a result of secondary growth of the main shoot. Here also the basipetal differentiation of bud traces occurs by the addition of vascular elements formed by vascular cambium.
Factors determining the initiation and organization of cells into a bud meristem are mainly the hormone and the interaction between buds and subtending leaves. Complex hormonal regulations are definitely involved in the process. So long as the apical bud causes apical growth, the axillary buds do not develop —this phenomenon is known as apical dominance.
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It is suggested that apical buds dominate in auxin content than the lateral buds, but how auxin causes the inhibition of development of lateral buds is still controversial. At initial stage the development of axillary bud is stimulated by the leaf primordia and further growth occurs due to interaction between bud and its subtending leaf. Competition among the axillary buds for nutrients is also an important factor.
