These plants grow in saline soil or water, where there is abundance of salt in the soil; thus halophytes show some special characters. Most of halophytes have succulent leaves and certain possess succulent stems. Leaves may be modified into or provided with spines.
Halophytes growing in marshy places near the seashore, as in Sundarbans (West Bengal), form special vegetation called the mangrove. Mangrove plants produce a large number of stilt roots from the main stem and the branches.
In many cases, in addition to the stilt roots, special roots called respiratory roots or pneumatophores are also formed in large number, such roots develop from underground roots, and projecting beyond the water level they look like so several conical spikes distributed all-round the trunk of the tree.
They are provided with numerous pores or respiratory spaces in the upper part, through which exchange of gases for respiration takes place. Mangrove species also visible a peculiar type of germination. The seed germinates inside the fruit while it is still on the parent tree and is nourished by the same. Germination is almost immediate without any periods of rest. The radicle elongates to a some length and swells at the lower part. Ultimately the seedling separates from the parent tree and falls vertically down.
The radicle presses into the soft mud, keeping the plumule and cotyledons clear above the saline water. This type of germination of the seed inside the fruit while the latter still remains attached to the plant is known as vivipary. Typical examples of mangrove plants are—Rhizophora, Sonneratia, etc.
The mangrove plants exhibit several xeromorphic characters. The leaves are succulent. Sometimes the stems also become succulent. Several species are covered with hairs (e.g., Avicennia). The epidermis is thick-walled and strongly cutinized.
The sunken stomata are found beneath the epidermis. Usually aqueous tissue is present. The mesophyll is almost devoid of intercellular spaces and the palisade tissue in the main chlorenchyma (viz., Sonneratia).
The nerve ends dilate into water storing tracheids (e.g., Avicennia). Long stone cells or bast like mechanical cells are lodged in between the palisade cells (viz., Rhizophora Sonneratia, etc.) Mucilage cells also exist (e.g., in Rhizophora, Sonneratia, etc.).
T.S. of Stem of Sonneratia (A Mangrove Plant):
The young stem of sonneratia is provided with collenchymatous wings; the cork arises in the sub-epidermis in Sonneratia apetala Ham. Primary cortex includes spongy parenchyma. The outer part of cortex contains branched sclerenchymatous idioblasts. Xylem is found in the form of a continuous cylinder traversed by narrow rays.
The vessels contain simple perforations. The pholem includes sclerenchyma. The endodermis is inconspicuous. The pericycle is found in the form of patches over phloem. The intraxylary or inner phloem is observed next to xylem around the pith. The pith remains supported by sclerenchymatous elements. The calls of the pith and cortex possess tannin and oil drops. The secret or cells occur in the phloem as well as in intraxylary phloem, of Sonneratia apetala. According to Mullan (1933) crystals in vertical rows of special cells in the secondary phloem of Sonneratia apetala. The secondary cortex is made-up of tangentially elongated cells. Sclereids are also found in the secondary cortex. These cells also possess oil drops and tannin.
T.S. of Stem of Rhizophora (A Mangrove Plant):
Epidermis:
The epidermis is made-up of variously shaped cells appearing conical in transverse sections. The epidermis frequently consists of more than one layer, but a true hypodermis of 3—7 layers is also common. The cork in young stems usually arises superficially, usually in the hypodermis. The young stem has a very thick cuticle.
Cortex:
The primary cortex is lacunar. H-shaped sclerenchymatous idioblasts are present. The cells of cortex contain pitted walls and are of tannin and oil. Calcium oxalate crystals are also present.
The inner cortex has groups of branched sclereids which give mechanical strength to the lacunate cortex. The sclereids are lignified thick walled cells with narrow lumina. The endodermis is conspicuous. The endodermal cells contain starch grains.
Pericycle:
It consists of a sub-continuous composite ring of sclerenchyma including layers of cells.
Vascular Bundles:
The vascular bundles are conjoint, collateral, endarch and open.
Xylem:
The xylem is traversed by rays 2—3 cells wide in Rhizophora mucronata. The vessels contain scalariform perforation plates.
Crystals:
The crystals are generally clustered.
Secretory Elements:
Vertically elongated secretory cells possessing tannin and/or oil present in the cortex and the pith.
Anatomy of Root of Sonneratia:
The vertical, negatively geotropic portions of root system are called ‘breathing roots’ or ‘pneumatophores’. The terrestrial roots are also present.
The lenticels are present on the pneumatophores. The cork includes a succession of lamellae each composed of three layers of cells. The cells of the outer layer of the cork are rounded externally and not suberized.
The cells of the middle layer of the cork are tabular and suberized and those of inner layer are radially elongated, rounded on the inner side and suberized. Two layers of rounded cells, present between each succeeding cork lamella, facilitate separation at these points.
The cortex is made-up of rounded cells interspersed with schizogenous, intercellular spaces. The cortex also contains cells with crystals and sclerenchymatous, idioblasts. The cortex at the proximal end of the pneumatophore is supported by curved, lignified spicules. The phloem consists of isolated strands of fibre and vertical rows of crystalliferous cells.
The xylem is found in the form of a broad feeble lignified ring. The pith possess sclerenchymatous idioblasts at the distal end but not at the proximal end. The terrestrial roots of Sonneratia show the following characteristics.
Cortex is lacunar. It is composed of cells generally triradiate, and supported by thickening ridges and by vertically elongated cells with thick, pitted but lignified walls. Cork arises superficially.
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