12th Class Biology Organisms and population Ecological Plants Groups

Water as an important ecological factor was first recognized by Warming. On the basis of water requirement plants are divided in to 3 major types :

(1) Hydrophytes : They live in abundance of water. They require abundance of water to complete their life cycle. They are of following types :

(i) Rooted submerged : The hydrophytes, which are rooted in mud at bottom and remain under water. e.g., Hydrilla, Vallisnaria.

(ii) Submerged floating : They are not rooted in the soil but completely submerged and floating e.g., Ceratophyllum, Utricularia.

(iii) Rooted with floating leaves : They are rooted in the soil but the leaves are floating on the surface of water e.g., Nelumbo, Trapa, Victoria.

(iv) Free floating : They are not rooted in soil and mud. These hydrophytes float freely on the surface of water e.g., Wolfia (Smallest angiosperm), Lemna, Spirodella, Pistia, Azolla, Salvinia.

(v) Rooted emergent : Roots are in soil shoots or leaves are partly outside and partly inside the water. Plants show heterophilly (Amphibious plants) e.g., Typha, Ranunculus, Sagittaria, Cyperus.

Morphological adaptations

(i) Roots of hydrophytes are poorly developed or completely absent in Wolfia, Ceratophyllum etc. Root hair absent but root pockets may be present e.g., Pistia, Eichornia, Trapa.

(ii) Stem is reduced in free floating plants e.g., Pistia, narrow and slender in submerged plants e.g., Hydrilla, Ceratophyllum and well developed in amphibious plants e.g., Typha.

(iii) Petioles become long, swollen and spongy for floating.

(iv) Leaves are usually long ribbon like e.g., Potamogeton, or finely divided e.g., Ranunculus or thin and broad e.g., Nelumbo, Victoria.

(v) In some hydrophytes leaves of different forms are produced by same plant. Aerial leaves are not dissected but submerged leaves dissected (Heterophilly) e.g., Ranunculuc, Limnophila.

Anatomical adaptations

(i) Cuticle absent or poorly developed.

(ii) Stomata are absent in submerged plants. Floating hydrophytes have stomata on upper surface e.g., Lotus (epistomatic).

(iii) Air spaces are extensively developed in root, stem and leaves. Well developed aerenchyma helps in buoyancy and gaseous exchange.

(iv) Leaves have spongy tissues and palisade is poorly developed. As light difuses from all palisade and spongy tissue. Epidermal cells contain chloroplasts for maximum capturing of difused light.

(v) Mechanical tissues like sclerenchyma (lignified tissues) and collenchyma are poorly developed or absent.

(vi) Vascular tissues are poorly developed.

Physiological adaptation

(i) Water and mineral nutrients are absorbed through general body surface.

(ii) Osmotic concentration or osmotic potential of cells is equal to or is slightly higher than external water.

(2) Xerophytes : They are adapted to grow in dry habitats. On the basis of pattern of life cycle, xerophytes are of three types :

(i) Ephemerals : They complete their life cycle in a very short period, evade dry season by disappearing, leaving their seeds. They are referred as drought escapers or drought evaderes e.g., Cassia toria, Argemone maxicana, Solanum xanthocarpum.

(ii) Succulents (Fleshy xerophytes) : They absorb large quantities of water during rainy season and store water in different body parts. They are common in deserts and referred as drought avoiding xerophytes e.g., Opuntia, Bryophyllum, Euphorbia, Mesembryanthemum (ice plant).

(iii) Non succulents : They are true xerophytes and called as drought resistant. They can with stand long drought periods e.g., Acacia, Calotropis, Casuarina, Nerium, Capparis, Prosopis.

Xerophytes are further divided in to different types :

(a) Oxylophytes : Plants growing on acidic soil.

(b) Psychrophytes : Plants growing on cold soil.

(c) Halophytes : Plants growing on saline soil.

(d) Lithophytes : Plants growing on rocks.

(e) Chersophytes : Plants growing on waste land.

(f) Eremophytes : Plants growing in deserts and steppes.

(g) Psilophytes : Plants growing in savannah.

(h) Helophytes : Plants growing in mud.

Morphological adaptations

(i) Roots of xerophytes are extensively developed to increase water absorption. Roots are much more longer than the shoots. Root hairs and root caps are well developed. The roots reach to great depth.

(ii) Stems of xerophytes is usually stunted (dwarf), woody, dry, hard and covered with thick bark. Stem is modified into flat leaf like phylloclades or cladodes e.g., Opuntia, Ruscus, Asparagus.

(iii) Leaves of xerophytes are usually thick may be reduced to spines e.g., Opuntia, scales e.g., Casuarina or may become needle like e.g., Pinus (Microphyllous) or may absent e.g., Capparis. Leaves and stem become fleshy (Malacophyllous) e.g., Bryophyllum.

Anatomical adaptations

(i) Stomata are sunken and generally on the lower surface of leaves.

(ii) Epidermal cells thick walled and covered by hairs (Trichophyllous). e.g., Calotropis. Epidermis may be multilayered (Multiple epidermis) e.g., Ficus, Nerium.

(iii) Palisade generally on both sides (surfaces) of leaves e.g., Nerium.

(iv) In leaves spongy parenchyma are absent.

(v) Water storing parenchyma, conducting tissues and mechanical tissues are well develop.

(vi) Bulliform or motor cells are found in between the cells of upper epidermis. These cells cause rolling and unrolling of leaves e.g., Poa, Amnophila (grasses).

(vii) In Nerium leaf, upper as well as lower epidermis are multiseriate or multiple and are covered with thick cuticle. Mesophyll is different into palisade and spongy parenchyma palisade tissue occurs near both the epidermis while spongy parenchyma is located in between the palisade.

(viii) In Ficus leaf, upper epidermis is multiseriate and is thickly cuticularised. Cystoliths are present is the cells of inner layers of this epidermis.

Physiological adaptations

(i) Osmotic concentration or osmotic potential of cell sap is high.

(ii) They have resistance to dessication and mucilage to hold water.

(iii) They show less transpiration.

(3) Halophytes : They are special types of xerophytic plants which grow on saline soils with high concentrations of salts like \[NaCl,MgC{{l}_{2}},MgS{{O}_{4}}\](Physiologically dry soil). Most of these are succulents. They have negatively geotropic roots for gaseous exchange called Pneumatophores. Halophytes show Vivipary (germination of seeds inside the fruits).

Halophytic communities growing on swamps are called helophilous halophytes which are of two types :

(i) Salt swamp and salt desert.

(ii) Littoral swamp forests which are most extensive in all tropical areas.

Swamp forest forms a characteristic vegetation called mangroves e.g., Rhizophora, Sonneratia, Avecenia, Heritiera, Salsola.

In India mangroves are quite common in sea shores of Bombay and Kerala, an in Andamans and Nicobar Islands.

(4) Mesophytes : Plants growing in places of moderate water supply. These plants cannot live for a long time either in water saturated or in moisture deficient soil. e.g., garden plants and crops.