Abscission and Healing of Wounds in Plants!
The plants commonly lose various parts periodically. Lower vascular plants usually do not shed their parts, which remain attached to the plants and ultimately get decayed.
But in the higher vascular plants the fall of the parts—particularly of the leaves, floral members and fruits—is mainly caused by the formation of a distinct zone where usually the part gets detached from the mother plant. The process is referred to as abscission, and the zone as the absciss zone.
Periodic defoliation is a notable feature of the gymnosperms and most of the woody dicotyledons. The deciduous plants shed their leaves at the approach of winter. The evergreen plants have no fixed season for fall of leaves; but they may shed their leaves in any season.
Some structural modifications actually take place at the base of the leaves, and finally they are abscissed. The formation of absciss zone is usually initiated before the leaf-fall. In fact, the leaves now become mature, turn yellowish in colour due to disintegration of chlorophyll and preponderance of carotinoid pigments.
Moreover, the waste products are transferred to the leaves, so that the plants may get rid of them with the leaf-fall. It is really a vital phenomenon involving special changes in the cells occurring at the base of the petiole, what is evident from the fact that the leaves of the dead branches are not shed, but they get dried up and decayed in course of time.
The absciss zone is usually formed at the base of the petiole in case of simple leaves. In compound leaves the zone appears at the base of the rachis and also at the base of individual leaflets. It is a narrow zone composed of a few layers of cells (Fig. 660).
Before the fall of leaf a distinct separation layer is formed in the absciss zone which may be considered the immediate structural reason for the fall of leaf.
Besides this, there develops a protective layer within the absciss zone which protects the exposed surface after leaf-fall from loss of water and attacks of micro-organisms. Periderm is formed later and provides effective protection.
The absciss zone is the weakest spot on the petiole of the leaf. In some cases it becomes slightly constricted and thus rendered externally visible. The vascular bundles become reduced here and the strengthening tissues are minimum.
Collenchyma cells are absent and the sclerenchyma, if present, is weakly developed. The separation layer consists of at least two rows of cells which are different from those occurring above and below the absciss zone in cell size and cell contents.
The cells of this layer are smaller in size, have denser protoplasm and abundant starch grains. The vessels usually become clogged by gums and tyloses below the zone. Distinct changes take place in all the parenchyma cells of the separation layer.
The middle lamella and the outer walls of the cells swell, become gelatinous and eventually break down. Calcium pectate of the middle lamella is converted into pectic acid and that into water-soluble pectin.
Thus the cells now become separate from one another either along the middle lamella or the walls actually break down. Now that the separation layer is thus formed, the leaf remains attached to the stem only by the vascular elements.
It breaks in time either by the mechanical weight of the leaf or slight swaying due to wind. During rains, the leaf gains in weight and gelatinisation of the cell wall is hastened, resulting in quicker abscission. The separation actually starts at the periphery of the petiole and extends inwards.
It continues in the living parenchyma cells of the vascular bundles, but the reduced vascular elements break by mechanical weight. It has been suggested that middle lamella of the cells of the vascular tissues also disintegrate.
After the fall of leaf the surface is exposed which has to be protected from desiccation and outside injuries. A few cells in the absciss zone just beneath the separation layer now constitutes the protective layer which remains as a cover to the scar caused by leaf-fall.
The walls of the cells of this layer undergo suberisation, and possibly partial lignification, and thus protect the exposed surface. That the substance defined as lignin is really wound gum giving reactions similar to lignin, is the opinion of some workers.
Now phellogen appears on the inner side of the protective layer, and periderm is ultimately formed which becomes continuous with the periderm of the stem.
Healing of Wounds:
Plant organs often suffer from injuries from diverse external sources when the living cells lying internal to the wounds are exposed. They have their own arrangement of healing of the wounds, so that the exposed living cells may not suffer from loss of water and infection of fungi, bacteria and other micro-organisms.
In case of simple lower plants the thallophytes, often portions get fragmented from the mother plant. Walls are secreted at the points of injuries, and thus both the fragment and the mother plant are regenerated and carry on independent existence. That is how vegetative multiplication is secured.
In higher plants wounds are healed up by different methods, depending on the nature of the wounds. In case of superficial wounds the underlying exposed cells die and dry up. The layer of cells beneath the wound becomes impregnated with protective substances like suberin.
Plants with laticifers heal up the simple wounds by secretion of latex, which coagulates and serves as a protective cover. In some cases the living cells beneath the injury take up meristematic nature and serve as a secondary cork cambium. Those cells cut off cork cells on the outer sides and thus the living internal portions are protected.
In case of deep wounds the uninjured living cells beneath the wound become meristematic and go on producing a mass of undifferentiated parenchyma cells known as callus. The callus actually fills up the wound.
Very soon a cork cambium arises in the peripheral layers of callus, which produces a layer of cork, referred to as wound cork, on the outer surface and thus the wound is healed up. Very often callus overgrows the wound, and as a result, characteristic swellings or knots are noticed in the trees (Fig. 661).
Knots are also frequently found in the wood of the trees which have considerable increase in thickness. When a branch is either cut off or broken down, the wound surface is covered by the callus.
But with secondary increase in thickness, the secondary xylem elements gradually extend over the stump of the branch, so that it eventually gets embedded in the wood. In the mean time all the living cells of the base of the branch die, shrivel up, and the stump remains as a cone embedded in the woody portion. They appear as knots in the timber.
The wound may be very deep so as to injure the cambium of the vascular bundle. In that case callus is formed as usual and the wound is filled up. Then a few parenchymatous cells of the callus become meristematic to form a new cambium, so to say, in a line with the uninjured cambium of the organ.
The new cambium joins up with the existing uninjured one, and thus continuity is established, so that the formation of secondary tissues is not disturbed.
The horticulturists take advantage of this process in budding and grafting, where the cambia of the two shoots, referred to as stock and scion, brought together during grafting, unite, provided of course the two are of the same or closely-related species.