Halophytes are special types of xerophytes which grow in saline soils. These soils are called physiologically dry because of high salt concentration. Halophytes are plants of saline habitats which not only have the ability to tolerate high concentration of salts in their rooting medium but are able to obtain their water supply from the same. They grow in saline soils, mangroves, coastal dunes and tidal marshes. They have a high osmotic pressure, a minimum of 40 bars. Most halophytes possess succulence as well.
Plants of saline soil show characteristics of xerophytes, e.g., Suaeda, Salsola, Salicornia, Tamarix, Atriplex, Spartina. They may have succulence in leaves, stems or both. The succulent parts have large-size cells which store water and mucilage. Though it is a means of storing and retaining water, succulents also help in diluting salts. Many halophytes actually secrete salts through chalk or salt glands, e.g., Tamarix, Atriplex, Spartina. Tamarix also, excretes salts but directly through epidermis. Thick cuticle, sunken stomata, anthocyanins, and tannins occur to reduce insulation and prevent desiccation. Proline and other organic solutes may also occur.
Halophytes are placed into following four categories.
- Lithophilous, plants growing on solid rocks.
- Psammophilous, plants growing in sandy swamps.
- Pelophilous, plants growing in saline soil.
- Helophilous, plants growing in swampy, saline deltas. Helophilous plants, also termed as halophytes, grow in swamp forests. These include mangroves which occur in swamp forests of littoral regions in tropical and sub-tropical climates e.g., Rhizophora, Sonneratia, Avecenia, Salsola, etc.
Mangroves are marshy areas found in tropical deltas and saline ponds near sea shore. The areas have not only excess salt but also excess water or anaerobic condition besides difficulty in anchoring and seed germination. Plants growing in mangrove are halophytes. Many of them excrete salts with the help of the salt secreting glands present over their leave, e.g., Avicennia, Aegiatilis. A few mangrove plants secrete salts from their roots. Some have water storage tissues to dilute salt, e.g., Rhizophora (red mangrove), Avicennia. Cuticle is thick. Some leaves have corky cells, presence of sunken stomata, hair may occur. Anthocyanins, tannin and oil are present to reduce insolation. Proline and sorbitol are organic solutes often present in these plants for osmoregulation.
A green alga, Dunaliella found in hypersaline lakes, possesses a lot of glycerol in its cells for osmoregulation. Mangrove plants are supported in marshy areas by supporting roots such as horizontal roots, knee roots, buttress roots, stilt roots, prop roots etc. Lenticels occur for gaseous exchange.
A number of plants possess small negatively geotropic vertical roots termed as pneumatophores. Pneumatophores have lenticel, for gaseous exchange. They are connected with internal aerenchymatous tissue.
Another adaptation of mangrove plant is vivipary or seed germination while attached to plants. The seedling grows sufficiently before it falls into the saline marsh. Only the radicle part passes into mud or water while the plumule part remains above the surface of water, e.g., Rhizophora, Aegiceras, Ceriops.
Morphological features of halophytes
Following are some of the important morphological features of halophytes.
The special roots termed as pneumatophores or knees are present. They are negatively geotropic and come out of the soil. The part of the root outside the soil or swamp consists of perforations for exchange of gases i.e., oxygen and CO2.
These are mostly succulents or fleshy e.g., Salicornia, Suaeda, etc.
Leaves are always evergreen, small and also leathery. The leaves of submerged marine halophytes are thin. The leaves of coastal aero halophytes cover their surfaces with trichomes.
In this case, the seed germinates while it is inside the fruit, which remains attached to the plant. The radicle of the soil grows rapidly and comes out piercing the fruit wall. It elongates and becomes stout. Later on, due to its weight, the fruit gets detached from the plant and falls vertically down. The elongated portion of the radicle pierce into the soil and soon develops lateral roots which gives anchorage to the growing seedling. The plumule grows upwards and gives rise to the shoot for example, Rhizophora etc.
Anatomical characters of halophytes:
Following are the common anatomical characters of the halophytes:
Two types of roots are present in the plants, aerial and subterranean. The subterranean roots show the following anatomical characters:
- Multiseriate cork.
- Stellate or star-shaped cells in the cortex.
- Storage of oil and tannin in some of the cells of the cortex.
- Lignified and pitted cells in the pith.
Transverse section of the stem shows the following typical anatomical characters:
- A thick cuticle is present even in the young stem.
- Epidermal cells are thick and filled with tannin or oils.
- Hypodermis consists of many layers and the cells are thick-walled.
- The outer cortex shows large intercellular spaces.
- These cells are filled with oil, tannins, crystals of calcium oxalates etc. H-shaped spicules are also present in the cortex which provide mechanical support to the stem.
- Inner cortex is made of compactly arranged cells. Numerous branched sclereids occur in this region. These give mechanical strength to the stem. Endodermis is distinct. Starch grains are also present in the cells.
- Pericycle is sclerenchymatous . It is three to four layers deep.
- Cortex is highly lacunate.
- H-shaped spicules occur in the large parenchymatous pith also.
- Vascular tissue is well developed.
Transverse section of the leaf shows following anatomical characters:
- Upper and lower epidermis possesses cuticles.
- Lateral walls of epidermis are thick.
- Presence of crystal of calcium oxalates in the epidermal cells.
- Sunken stomata present only in the lower epidermis.
- Sub-epidermal aqueous tissue is present just below the epidermis. It is three to four layers deep. The cells forming these tissues are large but thin walled. The outer cells of these tissues possess tannins and oils.
- The lower most layer of the aqueous tissue is miscellaneous and remains embedded in the palisade.
- Spongy parenchyma is present near the lower epidermis. It is six to ten layers deep.
- Palisade tissue is present near the upper epidermis. It is three to four layers deep.