Let us make in-depth study of the displacement, protection, modification and prefoliation of buds.
A bud is a compact young shoot consisting of a short stem overlapped by crowded immature leaves which effectively cover up the growing tip.
Thus, the growing stem tips are as well protected as the root tips covered by root caps.
Flower buds also are buds but they arc modified to carry on the special function of reproduction. Sometimes, a bud may be merely a mass of undifferentiated meristematic tissue not showing the leaf-primordia. The plumule is the first bud of the plant.
It grows vertically up opening the leaves from below and the apical bud always remains at the apex. While the stem is growing, lateral buds grow at every leaf axil in an acropetal succession. These are the axillary buds.
The first axillary buds often develop at the axils of cotyledons which are the first leaves. shows a potato seedling in which the buds at the axils of the cotyledons develop the first tubers. The axillary buds grow to develop the branches. Each branch, as it will grow, will have its apical bud and will give rise to lateral axillary buds in the same way as the main stem. Each axillary bud at the axil of every leaf does not grow as no plant needs so many branches.
Most buds remain dormant, i.e., they remain in very rudimentary forms. Buds may also remain dormant because of bad nutrition. Dormant buds may resume growth at a later stage when conditions for their development are fulfilled, e.g., when the main shoot is cut off. In that case the nutrient sap which would have gone to the main shoot would be diverted towards the dormant buds and would cause their development into new, strong branches.
Such branches are deferred branches which look out of position as they do not fit into the acropetal order. Some buds fall off after formation so that they may be called deciduous buds. The disposition of the buds determine the final shape of the shoot, e.g., as the lateral buds do not develop the palm stem assumes the typical unbranched form.
A. Displacement of Buds:
When a bud develops from a usual position (i.e., terminal or axillary) it is a normal bud. In some cases, although the bud arises from the normal axillary position, during its growth it is carried to some distance along the lamina of a leaf or along the inter-node as the tissues of the two organs get fused together so that the bud appears to have developed at the development of buds at cotyledon middle of a lamina or at some such extra-axillary position.
Organs developed from such a displaced bud also look displaced . Such displacement of a bud by fusion may be called ad-nation.
Accessory Buds:
While only one bud is found in a normal position, sometimes there may be more than one. The apical bud is single in practically all Angiosperms. But, in some lower plants, it splits into two giving rise to two branch shoots. This is known as dichotomy.
It is more common to find more than one bud at the axil. The additional axillary bud is then called an accessory bud as found in the common hedge plant Duranta) .
Accessory bud may be placed one above another (superposed or serial buds) as in Aristolochia and walnut , side by side (collateral) as in many Liliaceae and Solanaceae (brinjal, chilli, etc.) plants and also in maple , or in a cluster.
Sometimes, a number of buds thus occurring in a group may fuse to form a composite fasciated branch. In Cuscuta (dodder) several buds may appear to be in the same axil because of the branching of the original single bud.
Adventitious Buds:
When a bud grows from a position other than the normal (i.e., apex or axil) then it is an adventitious bud. Some adventitious buds may arise from extra-axillary positions on the stem (cauline buds), from roots (radical buds) or from leaves (epiphyllous buds).
When a plant is pollarded, i.e., when its main stems are cut off, the surplus flow of sap not only causes many dormant buds to develop but also causes some completely new adventitious buds to grow on the cut ends of the stem.
Buds on the root are known in many plants. When the trunk is cut off, some plants like woodapple (Aegle marmelos) and poplar grow buds on the roots. Root buds are largely used for the propagation of some agricultural crops like-sweet potato (Ipomoea batatas) or palwal (Trichosanthes dioica) where root cuttings are used as’ seed’.
Similar buds with roots are known to grow on the margin or the surface of leaves of Begonia, Kalanchoe, Gesnera, Gloxinia, etc. These again are greatly utilised by horticulturists.
Young plantlings are obtained at the angles of the crenate margin of mature and Kalanchoe leaves when they are placed on moist soil. Similar plantlings grow near incisions made on Begonia leaves .
B. Protection of Buds:
Buds protect the delicate growing points of the stem. Sometimes this protection has to be accentuated. This may be done by a covering of hairs (e.g., Zizyphus), resinous sccretions, etc.; by the leaf bases themselves or by stipules and, more conspicuously, by the bud-scales .
These bud-scales are prominently seen to cover the buds of jack-fruit, different Ficus and Magnolia species, etc. The scales are tough, modified leaves of the nature of calaphylls (discusscd under types of leaves). They may also be modified leaf bases, petioles or stipules.
The true morphology of bud-scales (i.e., which part of leaf they represent’! varies from plant to plant and even on the same plant where transitional forms may be seen. Bud-scales do not serve as foliage leaves but fall off as soon as the bud opens.
Buds with scales are sometimes called scaly buds. Most plants in tropical climates bear buds that are not covered at all, i.e., naked buds. On the other hand, plants in very cold countries bear specially protected buds called winter buds .
During winter the leaves fall off but the abscission layers do i not touch the axillary buds. These winter buds remain well covered by a series of scale leaves and lie almost dormant during the winter.
During spring time these buds burst out of their coverings and the new leaves open out. Some of the winter vegetables of our country remain in the bud stage during winter. Cabbage, cauliflower and Brussels sprouts are buds of this type.
One whole cabbage is nothing but a gigantic leaf bud and a cauliflower is similarly a reproductive bud. Brussels sprouts arc formed of leafy buds. All these buds open out at the close of the winter giving rise to leaves and flowering shoots.
C. Modification of Buds:
While leaf axils are normally occupied by buds, sometimes other structures like thorns or tendrils occupy their positions. Such thorns or tendrils are then modifications of the buds or, perhaps more correctly, of stem.
Most peculiar, however, is the modification of whole buds into swollen structures known as bulbils which serve for the propagation of these plants. The bulbils borne on the axils of Dioscorea bulbifera are swollen structures which may serve as famine food.
When these get detached from the parent plant and fall on the ground they germinate into new plants and thus serve as excellent means of vegetative propagation. Lilium bulbiferum also shows similar bulbils.
The large inflorescence of Agave (Century plant) shows, many of its floral buds transformed into bulbils which germinate while still on the inflorescence—thus showing vivipary.
Similar transformation of flower buds is seen in the inflorescences of Globba bulbifera , Allium sativum (garlic), Crassula and other plants. The common Oxalis shows bulbils occurring like adventitious buds just above the swollen root.
Such adventitious bulbils are also seen round the crown and the base of some pineapples (Ananas). The type of modified stem known as bulb (discussed under ‘stem’) may also be considered as a modification of buds.
D. Prefoliation:
The arrangement of the young leaves in the bud prior to opening is known as pre-foliation. It is also called vernation.
It can be studied from two aspects:
(a) The manner how each individual leaf is folded or rolled—known as ptyxis, and
(b) The arrangement of all the leaves in relation to one another—vernation proper.
(a) Ptyxis (Arrangement of individual leaves independent of others):
Leaves may be folded in different ways, e.g.,
(1) Reclinate or inflexed—upper half on lower half as in some garden croton;
(2) Conduplicate—folded lengthwise along the midrib as in guava, custard apple, magnolia, Bauhinia, etc.;
(3) Plicate—folded longitudinally several times along some prominent veins as in palms.
In other cases the leaves are rolled in different ways, viz.;
(4) Circinate—leaf rolled up from apex to base like the tail of a dog
as in ferns where it is a characteristic feature;
(5) Convolute—leaf rolled up from one margin to the other like the rolling up of a sheet of paper as in banana and Canna;
(6) Involute —the two margins (or round margin in peltate leaves) are rolled up towards the midrib or the centre of the leaf as in Antigonon, lotus, water-lily, etc.;
(7) Revolute—the leaf is similarly rolled down towards its lower surface as in Nerium (oleander). Finally,
(8) The leaf may be crumpled in an irregular manner as in cabbage.
It should be remembered that, besides all these, the bud leaf may lie completely fiat without showing any rolling or folding whatsoever.
(b) Vernation Proper (Prefoliation arrangement of all the leaves in the bud as a whole):
When the individual leaves are flat or slightly convex the vernation may be:
(1) Valvate—when the leaves are arranged almost in a circle while their margins may just touch one another without Overlapping;
(2) Imbricate—when the margins overlap one another but not in any particular order;
(3) Contorted or twisted or spiral—such overlapping is regular in one direction so that one margin of every leaf overlaps the leaf on one side while the other margin is overlapped by the leaf on the other side thus giving a twisted appearance to the bud;
(4) Induplicate is a variety of valvate where the edges of the leaves instead of slightly touching one another are bent inwards for a little distance. When the leaf is folded or rolled the arrangement may be;
(5) Equitant—conduplicate leaves standing face to face and one completely overlapping the other by both the margins;
(6) Half-equitant or obvolute—when such overlapping is incomplete so that only one-half of a blade is inside the other;
(7) Supervolute—when a convolute leaf encloses another presenting a completely rolled up bud.
The vernation proper of floral buds is known as aestivation or prefoliation and will be studied in that connection: Examples also will be found to be more common among flowers.