Plants of hot deserts are adapted to survive in dry conditions of soil and high temperature. Places where available water is not present in adequate quantity are termed xeric habitats. The plants growing in xeric conditions are xerophytes.
Xeric habitats may be of following types:
(i) Habitats physically dry; where water retaining capacity of the soil is very low and the climate is dry, e.g., desert, rock surface, waste land, etc.
(ii) Habitats physiologically dry; places water is present in excess amount but it is not such as can be absorbed by the plants easily. Such habitats may be either too salty or too acidic, too hot or too cold.
(iii) Habitats dry physically, as well as physiologically, e.g., slopes of mountains.
Drought Evading Plants:
The plants which evade dry conditions are known as ephemerals. They are short lived plants. During critical dry periods, they survive in the form of seeds and fruits which have hard and resistant seed coats and pericarps respectively.
For example, in desert areas of Rajasthan, many annual plants germinate from seeds, complete their life cycle quickly during the rainy season and survive in the form of seeds in the following dry season.
Thus, plants remain unaffected by extreme dry conditions. Such plants (ephemerals) are quite common in the semiarid zones where rainy season is of short duration.
The common examples are:
Astragalus, Artemesia members Zygophyllaceae, Boraginaceae, grasses, etc.
Deep Rooted Plants:
Some plants have deep tap roots, which can reach even upto water table, in arid climates; hence they are capable of absorbing water from deep soil.
Some examples of deep rooted plants are:
Prosopis (mosquito), palms and some species of Acacia.
Plants having Deciduous Leaves:
Many shrubs and trees of dry climate, such as Acacia nilotica and Capparis decidua have deciduous leaves. But in the majority of the plants the leaves are generally reduced to seeds, as in Casuarina, Ruscus, Asparagus, etc.
Leaves with Sunken Stomata:
For example, stomata are found is sunken pits in the common Nerium plant, reducing the rate of transpiration.
In many xerophytes, the leathery leaf surface and waxy cuticle also check the transpiration.
Sometimes the leaves may be reduced to spines, e.g. in Ulex, Opuntia, Euphorbia, Capparis, Acacia, etc.
Succulence:
Succulents are those plants in which some organs become smaller and fleshy due to active accumulation of water, or the bulk of the plant body is composed of water storing tissues. Water stored in these tissues is consumed up during the period of extreme arid conditions when the soil becomes depleted of available water. In cacti leaves are reduced to spines, while stems are modified into fleshy and spongy structures.
In leaf succulents, the leaves swell remarkably and become very fleshy owing to storage of excess amount of water and latex in them. Plants with succulent leave generally develop very reduced stems. The examples of leaf succulents are: Sedum, Aloe, Mesembryanthenum, Kleinia and several members of family Chenopodiaceae.
Trichophylly:
In some xerophytes, especially these growing well exposed, to strong wind, the undersurfaces of the leaves are covered with thick mats of hairs which protect the stomatal guard cells and check the transpiration. These xerophytes which have hairy covering on the leaves and stems are known as trichophyllous plants, e.g., Zizyphus, Calotropis, Banksia, Nerium, etc.
Rolling of Leaves:
The leaves of many xerophytic grasses, roll tightly under dry conditions. In these grasses, the stomata are combined to the ventral surface of the leaf, so that when the leaf edges roll inward, the stomata are effectively shut away from to outside air.
As the stomata are located on the inner surface of the leaf, the air enclosed by the rolled leaf soon becomes saturated with water and the outward diffusion steps, e.g., Ammophila, Erica, Spartina, etc.
Other Significant Adaptations:
Many tropical plants, especially grasses which grow in hot and arid climates, have C4 pathway of photosynthesis. The plants having C4 pathway of photosynthesis perform better in low soil water environments. Thus, such plants use less water to achieve higher rates of photosynthesis, at higher temperatures.
Many other xerophytes, such as cacti and succulents close their stomata during the day and open them in the night, hence reducing transpiration. Crassulacean acid metabolism (CAM) occurs in succulents and other plants that normally grow in dry conditions.
Many xerophytes accumulate proline (an amino acid) in the cells which maintain osmotic and water potential in their leaves.
On the other hand, the heat shock proteins (chaperonins) provide physiological adaptations to plants to high temperatures. Such protein helps other protein to maintain their structure and avoid denaturisation at high temperature.
Characteristics of Xerophytes:
The peculiar characteristics of xerophytes are as follows:
1. They grow in deserts or in very dry places; they can withstand a prolonged period of drought uninjured, for this purpose they have certain peculiar adaptations.
2. The xerophytic plants have to guard against excessive evaporation of water; this they do by reducing evaporating surfaces.
3. Plants produce a long top root which goes deep into the sub-soil in search of moisture.
4. To retain the water absorbed by the roots, the leaves and stems of some plants become very thick and fleshy, e.g., Aloe, Agave, etc. Water tissue develops in them for storing up water; this is further facilitated by the abundance of mucilage contained in them.
5. Multiple epidermis sometimes develops in the leaf, e.g., in Nerium.
6. Modification of the stem into phylloclade for storing water and food at the same time performing functions of leaves is characteristic of many desert plants, e.g., Opuntia and other cacti.
7. In xerophytes, certain structural features are also common. Leaves are thick and leathery, with a well-developed cuticle and abundant hairs.
8. Well differentiated mesophyll is also present, and there is often more than one layer of palisade tissue, e.g., in Nerium and Hakea.
9. The walls of epidemal and sub-epidermal cells are frequently lignified and a distinct hypodermis may be present.
10. They have well developed vascular system and often an abundance of sclernchyma, either in the form of sclereids or fibres, e.g., in Hakea and Ammophila.
11. The leaf is sometimes cylindrical or related. This organisation is to protect the stomata, which may occur in furrows.
Adaptations in Aquatic Environments:
Plants which remain permanently immersed in water are called hydrophytes. Such plants grow in water or very wet places. They may be submerged or partially submerged, floating or amphibious. Their structural adaptations are mainly due to the high water content and the deficient supply of oxygen.
The various adaptations are as follows:
Epidermis:
In aquatic plants, the epidermis is not protective but absorbs gases and nutrients directly from the water. The epidermis in the typical hydrophyte has an extremely thin cuticle, and the thin cellulose walls permit ready absorption from the surrounding water.
Commonly chloroplasts are found in epidermal cells of leaves, especially when the leaves are very thin; these chloroplasts utilize the weak light under water for photosynthesis.
In submerged plants, stomata are not present, and exchange of gases takes place directly through cell walls. The floating leaves of aquatic plants have abundant stomata on the upper surface.
Reduction of Absorbing Tissue:
The root-system in hydrophytes is feebly developed and root hairs and root caps are absent. In some floating plants, such as Utrieularia, Ceratophyllum, etc., no roots are developed, and in submerged plants, such as Vallisneria, Hydrilla, etc., water dissolved mineral salts and gases are absorbed by their whole surface.
An aquatic plant is in reality, submerged in or floating upon a nutrient solution. In hydrophytes the root-system is functioning chiefly as holdfasts or anchors and a large part of the absorption takes place through leaves and stems.
Aerenchyma:
Chambers and passeges filled with gases are commonly found in the leaves and stems of hydrophytes. The air chambers are large, usually regular, intercellular spaces extending through the leaf and often for long distances through the stem, e.g., in Potamogeton, Eichhornia, Pontederia, etc. The spaces are usually separated by partitions of photosynthetic tissue only one or two cells thick.
The air chambers prepare an internal atmosphere for the plant. These air chambers on the one hand give buoyancy to the plant for floating and on the other they serve to store up air (oxygen and carbon dioxide).
The carbon-dioxide that is given off in respiration is stored in these cavities for photosynthesis, and again the oxygen that is given off in photosynthesis during the daytime is similarly stored in them for respiration.
The specialised tissue frequently found in aquatic plants that gives buoyancy to the plant parts on which it occurs is aernechyma. This tissue helps to transport oxygen produced during photosynthesis and permits its free diffusion to other parts, including roots located in anaerobic soil.
Presewa of inflated peteioles in Eichhornia (water hyacinth) keeps the plants floating on the surface of water.
Roots are poorly developed or absent in free floating hydrophytes, such as Wolffia, Solvinia and Ceratophyllum, or poorly developed, as in Hydrilla. Nymphora and Nelumbo are examples of rooted hydrophytes which grow in fresh water pond.
Examples of aquatic plants are:
Submerged:
Vallisne- ria, Potamogeton, Hydrilla, etc.
Floating:
Cerato- phyllum, Pistia, Utricularia, Trapa, Eichhornia, Neptunia, etc.
Plants with floating leaves:
Nymphaea, Nelumbo, Victoria, Euryale, Limnan- themum, etc.
Amphibious plants:
Ranunculus aquatilis, Sagittaria, Typha, Alisma, Cardenthera, Myriophyllum etc.
Adaptations in saline environments:
Some plants grow and complete their life cycle in the habitats with a high salt content. They are called salt plants or halophytes. Halophytes occur in tidal marshes and coastal dunes, mangroves and saline soils. They show some special characters.
The halophytic plants, under hot and dry conditions, may become succulent and dilute the ion concentration of salts with, water they store in cells by stems and leaves. Leaves may be modified into or provided with spines.
Typical examples of such plants are:
Suaeda maritima, Salsola, Acanthus ilicifolius. Chenopodium and some species of family Asclepiadaceae.
Halophytes growing in marshy places near the seashore, as in Sundarbans (West Bengal), from a special vegetation known as 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 still roots, special roots called respiratory roots or pneumatophores, are also produced in large numbers.
Such roots develop from underground roots, and projecting beyond the water level, they look like so many conical spikes distributed all-round the trunk of the tree. Such roots are provided with numerous pores or respiratory spaces in the outer part, through which exchange of gases for respiration takes place.
Some species of mangroves can excrete salts through the salt glands on the leaves. Some mangroves can exude salts from the roots by pumping excess salts back into soil.
Many mangrove plants have high levels of organic solutes, such as proline and sorbitol, to cope with conditions of high salt concentration and osmotic potential. Dumaliella, a green alga found in saline lakes, can tolerate saline conditions by accumulating glycerol in the cells, helping in osmoregulation.
Mangrove species also show a peculiar type of germination. The seed germinates inside the fruit while it is still on its parent tree and is nourished by the same. Germination is almost immediate without any period of rest. The radicle elongates to a certain 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 kind of germination of the seed inside the plant is known as vivipary.
Typical examples of mangrove plants are:
Rhizophoni, Avicennia, Sonneratia, Ceriops, Heritiera, etc.
Avicennia and Rhizophora are the domiant species in mangrove forests. The pneumatophores are capable to take up oxygen from the atmosphere and transport it to the main roots. The stilt roots give support to the mangrove plants in wet and muddy substratum, while vivipary permits plants to escape the effect of salinity on seed germination.
Adaptations to oligotrophic soils:
The oligotrophic soils are those which contain lone amounts of nutrients. Such soils are generally found in geologically stable areas, such as soils of tropical rain forest region. Due to intense weathering and high rates of leaching, such soils have a poor nutrient retention capacity.
However, in nutrient poor soils, nutrient accumulation in vegetation is high.
Mycorrhizae:
Many plants that grow in nutrient-poor soils have mycorrhizae or fungus roots which have mutualistic association of roots with fungi.
These are two types of mycorrhizae:
(i) Endomycorrhizae, and
(ii) Ecotomycorrhizae.
In endophytic mycorrhizae, the fungal hyphae are found within the tissues of roots in several vascular plants. While in ectomycorrhizae, the fungal mycelium forms a mantle of mat outside the root of vascular plant.
Ectomycorrhizae generally occur in roots of shrubs and trees of temperate regions:
Mycorrhizae help in efficient absorption of nutrients, such as phosphorus and other nutrients, thus fulfil the scarcity of nutrient-poor soils.