Water is the most important ecological factor in the life of an organism. The amount of rainfall greatly influences the vegetation.
Warming classified the plant communities on the basis of the dependence and relation of plants to water -
three major ecological groups, they are 1. Hydrophytes, 2. Mesophytes and 3. Xerophytes.
Hydrophytes: - live in partly or wholly submerged in water or in wet places
Xerophytes: live in the habitat deficient in water or soil is physiologically dry
Mesophytes: live in habitat with moderate supply of water.
Hydrophytes
Hydrophytes are the plants which live in water or on sufficiently moist soils. The way they develop and their relation to water and air, they are grouped into 5 categories:
1. Free floating:
Plants which float freely on the surface of water but are not rooted in the soil. These hydrophytes are in contact with both water and air.
-e.g. Eichhornia, Pistia, Wolffia, Lemna
2. Rooted hydrophytes with floating leaves:
The leaves of these plants float on the surface of water but the plants remain attached to the substratum by their roots. They have long, flexible petiole, which keep the leaves floating on the water surface.
- e.g. Victoria regia, Nymphaea, Nelumbo
3. Submerged suspended hydrophytes:
These plants are completely submerged in the different water depths but remain free as they are not rooted in the soil.
e.g. Ceratophyllum, Najas, Utricularia, Hydra
4. Submerged rooted hydrophytes:
These plants are completely submerged in water but they are attached by root to the muddy substratum.
eg: -Potamogeton, Vallisneria,
5. Rooted emergent hydrophytes:
These plants grow in shallow waters. They live partly in water and partly in air. They are rooted in the soil.
e.g., Ranunculus, Typha
6. Amphibious plants:
These plants are adapted to both aquatic and terrestrial modes of life. Grow around water bodies with the water current occasionally touching them. They are rooted in the soil. Can also survive some dry periods
E.g., Marsilea, Polygonum
Morphological Characters
Roots:
Due to availability of water in plenty, the roots are of secondary importance, and are very poorly developed. Roots may be fibrous, adventitious, unbranched or poorly branched.
May even be absent as in Ceratophyllym, Wolffia.
Root hairs are absent or poorly developed. Root caps usually absent.In Pistia, Eichhornina, balancing roots are present which have the root pockets
Some plants, such as Jussiaea repens develop two types of roots - Normal Adventitious roots and floating roots, aerial negatively geotrophic Respiratory roots.
Stems:
In submerged forms such as Hydrilla and Potomogeton, stem is long, slender and flexible. In free-floating forms it may be slender, floating horizontally on water surface as in Azolla, or thick, short, stoloniferous and spongy as in Eicchornia.
Leaves:
In submerged forms, leaves are thin, long, ribbon-like as in Vallisneria, Hydrilla, or - finely dissected as in Ceratophyllum, Ranunculus aquatilis, these roots offer less resistance. Floating leaves are large, and flat (Nymphaea, Nelumbo) with their upper surfaces coated with wax.
Their Petioles are long, flexible and covered with mucilage which can adapt to varying depths of water. Bulbous petiole are present in Eichhornia.
Heterophylly is seen in Ranunculus, Sagittaria, Limnophilla.Below water the roots are – narrow, long, dissected while above water they are broad, small and entire.
Anatomical Characters
Arenchyma:
All hydrophytes contain Arenchyma in all plant parts. The of Sponginess of tissues is due to disintegration or seperation of group of cells which forms the intercellular spaces. Arenchyma help in buoyancy, facilitate gaseous exchange.
Reduction of protecting structure –
Epidermis in hydrophytes is not protecting layer. The epidermal cell walls are thin. The epidermis is useful for absorption and assimilation as contain it contains chloroplasts
Cuticle absent or very thin
Stomata are totally absent in submerged hydrophytes as exchange of gases takes place directly through the thin cell walls by the process of diffusion. Non functional stomata are found in Potamogeton. In plants with floating leaves like Nelumbo, the leaves are epistomatous with stomata present only on the upper surface. These leaves show waxy coating to prevent retention of water on the surface.
There is no differentiation into pallisade and spongy parenchyma, so mostly the are leaves are isobilateral.
Reduction of Mechanical tissues:
As the plants are not generally subjected to stress and strain, hence, mechanical tissues like collenchyma, sclerenchyma are more or less absent.
Reduction of Conducting tissues:
As the absorption of water takes place through all over the surface of plant body, the Xylem is poorly developed
Rigidity in the plant body is due to turgidity of parenchyma cells.
No Secondary growth is present in the hydrophytes.
Xerophytes
The plants which live in the habitat deficient in water or soil is physiologically dry are called xerophytes.
Based on the nature of soil in which they grow, availability of water and other physical factors xerophytes are generally classified into two types:
• Physical Xerophytes: These plants grow in soils which are physically dry. Physical dryness of soil may be due to low rainfall or poor water holding capacity and high temperature Ex – Casuarina, Optunia
• Physiological Xerophytes: These plants grow in soils having abundant water, but the water contains a high concentration of salts. Due to this, plant can’t utilize water. Ex – Mangrove plants or Halopohytes– e.g., Rhizophora, Avicenia
Based on morphology, physiology and life cycle pattern, xerophytes are generally classified into the following three categories –
1) Ephemerals
These are short living plants which complete their life cycle in 6-8 weeks during favourable season. During the unfavourable season these plants disappear and remain as seeds. These don’t know dryness, hence they are called drought evaders or drought escapers.
2) Succulents
These plants absorb large quantities of water and store it in different plant parts in form of mucilage. As a result, plant parts like stem, leaf or root become fleshy or succulent. This stored water is very sparingly and economically used durng dry periods. Succulents suffer dryness only externally. Hence, plants are called drought avoiding plants.
e.g, - Stem Succulents – Ex – Optunia, Euphorbia tirucoli,
Leaf Succulents - Ex – Aloe, Agave, Begonia,
Root Succulents – Ex - Asparagus, Ceiba parviflora
3) Non- Succulents
These are called True Xerophytes which are perennial plants which can withstand or tolerate prolonged drought conditions. They experience the dryness both externally and internally, hence they are called drought resistant xerophytes. These plants undergo a number of morphological and physiological modificiations.
Morphological Characters
Roots
Root system is extensively developed. It may be many times longer than shoot system. Roots are very much branched, spread over wide areas. Root cap and root hairs very well developed. Root hairs function for a long time. Roots may store water in Ceiba.
Stem
Stems are usually stunted, woody, hard and covered with thick bark. In some xerophytes, stem becomes underground, where as in Optunia, it is modified into fleshy, green, leaf – like (phylloclade or cladodes). Stems may be covered with hairs or waxy coatings as in Calotropis.
Leaves
Leaves may be reduced, small, scale like as in Casuarina, Asparagus. Leaves may be reduced to spines as in Opuntia. Lamina may be long, narrow, needle like as in Pinus. Lamina may be divided into many small, narrow leaflets as in Acacia.
Leaves may become thick fleshy, succulent or tough and leathery in texture as in Aloe, Agave. Leaf surface are generally coated with wax as in Calotropis. Leaf surface are generally shiny, glazed to reflect light and heat.
In some monocots such as Amnophilla, leaves become folded and rolled in such a manner that the sunken stomata become hidden and thus the rate of transpiration is considerably minimized.
In some plants such as Euphorbia, Ziziphus stiupules get modified into spines.
Anatomical Characters
Root
Root shows extensive development of Xylem and Phloem. The cortex wide especially in storage roots (Asparagus) where its cells become large and filled with water.
Stem
In Succulent fleshy xerophytes, stems possess water-storage region.
In stems of most of the Non-Succulent xerophytes, such as Casuarina, the following characteristics are found
– thick cuticle
– Epidermis is well developed with thickened cell walls.
– Hypodermis is several layered and sclerenchymatous.
– Sunken stomata
– Vascular tissues well developed
– Mechanical tissues well developed
Leaves
In Succulent leaves of some xerophytes like Peperomia – epidermal cells serve as water storage organs.
Similarly, Succulent Leaves of Aloe have prominent water storage regions in their mesophyll. Moreover, in such leaves cuticle is thick with outer epidermal wall heavily deposited with cutin
Leaves of Non Succulent xerophytes show following characteristics -
– Well developed thick cuticle
– Multi layered epidermis – Nerium and several-layered, sclerenchymatous hypodermis as in Pinus.
– Vascular tissues well developed
– Mechanical tissues well developed
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