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The following five points will highlight the five major ecological plants groups.
The five ecological plants groups are: (1) Hydrophytes (2) Hygrophytes (3) Mesophytes (4) Xerophytes and (5) Halophytes.
Group # 1. Hydrophytes:
Plant adapted to live nearly or quite submerged in water called as hydrophytes. Such plants have much reduced, or sometimes altogether absent, root system. The leaves, if completely submerged are usually very much dissected and thin.
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Stomata are always absent. The stem is usually very soft and weak; vascular tissue is very poorly developed. The bulk of the tissue is spongy provided with air passages. These plants may be submerged, free floating or amphibious (i.e., only partly submerged).
Their chief adaptations are:
(a) When Continuously Submerged in Water:
Hydromorphy of the vegetative organs and of the reproductive cycle; gas exchange with the water; chromatic adaptations.
(b) When Periodically Submerged:
The gas exchange of these plants can be carried on in water as well as in the air. Presence of gelatinous sheath in some plants (particularly algae] serves as a protection against periodic desiccation. Some flowering plants form two kinds of leaves; in others, the vegetative period is suspended in times of drought, and the life of the plant is carried on through resting organs.
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Examples of hydrophytic plants:
(i) Submerged:
Vallisneria, Hydrilla. Naias, Ottelia, Potamogeton etc.
(ii) Free-floating:
Wolffia, Salvinia. Hydrocharis, Utricularia, Ceratophyllum, Lemna, Pistia, Eichornia, Trapa, Neptunia, Azolla, Ceratopteris etc.
(iii) Amphibious:
Ranunculus aquatilis, Alisma plantago, Sagittaria, Limnophyllum heterophylla, Cardenthera triflora etc.
Group # 2. Hygrophytes:
The plants of this group are invariably moisture and shade-loving. Their root system and vascular tissue are poorly developed. Their stem and roots are soft and spongy and show stunted growth. The stem is mostly modified as underground rhizome which is also an organ for propagation.
The leaves are well developed and prominent, provided with stomata. Since these plants grow in moist and shady habitats, their rate of transpiration is very low, and the excess of water is expelled through special openings called water stomata or hydathodes. Common examples are ferns, begonias, aroids and certain grasses.
Group # 3. Mesophytes:
The majority of plant species thrives best on a moderate supply of water and is known as mesophytes. Living under conditions favourable for plant growth, they have well developed roots and leaves. They are usually large and fast growing. Their leaves are big and provided with cuticle and thin epidermis. Stomata are unprotected. Vascular tissue is very much developed and differentiated. Mechanical tissue is extensively developed. Their stem may be herbaceous or woody. There are certain mesophytes, such as deciduous trees, which are mesophytic during the summer and xerophytic during the winters.
Group # 4. Xerophytes:
Many plants have become so modified during the course of evolution that they are able to thrive under conditions where the available soil water is comparatively small in amount and where plants without special adaptive modifications would rapidly perish. Such drought-loving plants are known as xerophytes.
Important adaptations of xerophytic plants are given below:
(a) Growing in Uniform Dryness:
Plants, such as growing in deserts, have great capacity for resistance against changes of water content in the tissues, reduction of transpiration; protection against insolation; high osmotic pressure; high root pressure; deep roots; intracellular carbon dioxide cycle. The adaptations themselves can be sub-divided. Thus, succulents of the cactus type usually possess a low cell suction pressure, while other types have an extraordinarily high pressure.
(b) Periodic Dryness:
(i) Plants in the arid and semi-arid regions of steppes, tree steppes, savannahs, maquis, and in part, prairies are subjected to only period dryness. They have short vegetative period; ability to stand desiccation; xeromorphy of the aerial parts; deeply penetrating roots, resistant resting organs and sometimes water storage in the stem.
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(ii) Mediterranean vegetation also belongs to this category. They have evergreen vegetative organs, with winter assimilation period.
(iii) Inhabitants of Arctic ice deserts are also subjected to periodic dryness.
(iv) Rock plants also come in this category. Their adaptations are resistance against frequent and extreme desiccation, absorption of water through the whole vegetative body. Real rock plants include only lichens, certain mosses, and certain algae.
Some common xerophytic plants are— many species of Euphorbia; many cacti—such as Opuntia, Cereus, Pereskia; Aloe, Argemone, Capparis aphylla, Tamarix, Pedilanthus, Echinops. Carthamus. Amaranthus, Ziziphus nummularia, Portulaca, Basella, Salsola, Suaeda, Asparagus, Acacia, Tribulu terrestris, Trianthema, Gnaphalium, Kochia indica, Calatropis procera, Crotalaria burhia etc.
Group # 5. Halophytes:
The plants that inhabit saline soil with high concentrations of sails like NaCl, MgCl2, MgSO4 (hence physiologically dry soil), or saline water belong to this ecological group. On such a substratum only such plants can grow which can tolerate a relatively high concentration of these salts. These plants have succulent leaves and sometimes the stem is also succulent. In certain cases leaves are modified into spines. Such xerophytic adaptations are the result of reaction of excess of salts on the plants.
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Halophytes growing in marshy places near seashore form a special vegetation known as the mangrove or tidal woodland. These are also called helophilous halophytes. These plants belong to several different families. In India characteristic mangrove vegetation occurs by the side of almost all tropical seas.
Mangrove plants are characterized by the presence of downwardly curving adventitious roots, called stilt roots or prop-roots. The function of these roots is simply mechanical. They provide support to the plant to withstand tides.
In addition to the prop-roots special roots, called pneumatophores or respiratory roots are also produced in large number. These roots arise from the underground roots and grow straight up into the air above the surface of the mud. They have lenticels and large intercellular spaces.
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Through lenticels exchange of gases for respiration takes place. In Rhizophora, however, the prop-roots themselves serve as respiratory roots. In Gruguiera the horizontally running underground roots come out of the mud and bend down again thus assuming knee-like shape. This exposed portion of the root develops small pores and functions like that a respiratory root.
The halophytes show a peculiar phenomenon of seed germination which is known as vivipary. In this kind of germination, the seed does not undergo a period of rest and starts germinating almost instantaneously within the fruit itself, still attached to the parent plant and is nourished by the latter.
The radicle grows to a certain length and swells up at the lower end (see figure 1.7). Finally the seedling falls vertically downward into the water or the mud and the pointed end of the radicle may get stuck in the mud. Lateral roots arise from the radicle, with the help of which the young plant gets firmly attached to the soil.
Owing to their light weight and spongy’ nature, the fruits, seeds and seedlings of mangroves can float for a long time in water and are thus distributed over very wide areas. This is one of the main reasons why the mangrove vegetation right from Australia to East Africa includes similar species.
Typical Mangrove Plants are:
Rhizophora, Cerlops, Sanneratia, (Vern, keora), Heritiera (Vern., Sundri), Excoecaria (Vern„ Goe) etc. Besides mangrove plants, some halophytes occur in salt marshes, which, during the greater part of the year remain immersed in water. For a very short period these marshes remain dry. Plants growing on such tracts have fleshy jointed stem and are usually leafless. Examples of such halophytes are Arthrocaemum indicum, Salicarnia brachiata etc.
Xerophytic Features of Mangroves:
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1. The leaves are thick and somewhat fleshy e.g., Rhizophora and Avicennia.
2. The stomata are sunken.
3. Presence of water storage tissue in the leaves of certain plants, e.g., Rhizophora mucronata.
4. Palisade tissues in the leaf is the only chlorenchymatous tissue. Mesophyll is without intercellular spaces.
5. Surface of the plant may be pubescent i.e., covered with short, soft hairs, e.g., Avicennia.
6. Stone cells are sometimes present in between the palisade cells, e.g., Rhizophora, Sonneratia.