Characteristics Of Hydrophytes
Hydrophytes are plants that are specifically suited for growing in water. Specific characteristics of hydrophytes allow them to grow in water or watery surroundings like swampy areas. They may also be called macrophytes, to distinguish them from algae which are often tiny. Eg. Utricularia, Vallisneria, Hydrilla, etc.
What makes hydrophytes different from the algae are their specific characteristics. Characteristics of hydrophytes depend on their category or habitat. Each group has its specialized features.
Classification of Hydrophytes
According to their relation to water and air hydrophytes are classified into the following categories.
- Submerged hydrophytes
- Floating types
- Amphibious
Characteristics of Hydrophytes
Submerged hydrophytes
Plants that grow below water level and are not in contact with the outer air are called submerged hydrophytes. Such plants are generally anchored to the bottom of lakes, ponds, and other water sources—Eg. Vallisneria, Hydrilla, Chara, and many other aquatic microbes. Submerged water plants show many characteristics of shade-loving plants as they are not exposed to high-intensity light.
Floating Hydrophytes
Plants that float either on the surface or slightly below the surface of water are called floating hydrophytes. They may or may not be rooted in the soil. In this group, the floating plants have been divided into two groups.
- Free-floating plants float freely on the water surface but are not rooted in mud. Eg. Wolfia, Pistia, Azolla, etc.
- Free-floating but rooted hydrophytes are submerged plants that are rooted in the muddy substrate of lakes and ponds but their leaves as well as shoot floats on the water surface. Eg. Nelumbium, Nymphaea, Victoria regia, etc.
Amphibious Hydrophytes
These are plants that are adapted to both aquatic and terrestrial modes of life. They grow either in shallow water or in muddy substratum. Roots and some parts of the stem may be submerged in water but some leaves, branches, and flowering shoots will remain above the surface of water so they are partly in air and partly in water.
Aerial parts of these amphibious plants show mesophytic or sometimes xerophytic features while the submerged parts develop hydrophytic characteristics. Eg. Limnophila heterophylla, Sagittaria, Marselia, etc.
Adaptations of Hydrophytes
The water plant shows a large number of peculiar adaptations mainly due to the high water content around, deficient supply of oxygen for submerged plants, low temperature, etc. These hydrophytes show anatomical, morphological, and physiological adaptations. Some adaptations of hydrophytes mainly include,
- Lack of water retention
- Having flat leaves to help them float on the water’s surface
- Feathery roots in increased number to help absorb oxygen and nutrients
- Air chambers inside the leaves and stems to maintain their buoyancy
Morphological Adaptations of Hydrophytes
The morphological adaptations of hydrophytes are seen in all parts of the plant such as roots, leaves, stems, etc. However, due to the uniformity of the aquatic environment, they require very few adaptive features.
Root System
Hydrophytes have poorly developed root systems or are sometimes completely absent. The root system in hydrophytes is very much reduced in size or may be completely devoid of branching. They do not need specialized roots since the entire plant body can absorb water, gases, and minerals.
In some floating plants such as Utricularia, Wolfia, etc, and some submerged plants like Ceraceracophyllum, roots are absent. Root hairs are usually absent except in those plants which grow in mud.
In Azolla, Lemma, etc., root pockets fit over the ends of roots that have no root cap. The function of root pockets is not clearly understood. Root sheaths protect their tips from injuries. The roots are mostly devoid of root hairs (Lemna, Nymphaea) so that the entire root can absorb water and dissolved minerals.
Salvinia and Ceratophyllum do not have roots. Rooted plants like Hydrilla, Elodia, Valusneria, etc are partially dependent on their roots for their nourishment.
Jussiaea has two types of roots- normal and floating. The floating roots are negatively geotropic and spongy. It helps the plant stay floated. The inflated and spongy petiole of Trapa stores air that helps them float.
Adaptations in the Leaves
The leaves of most hydrophytes are thin and smooth. The upper surface of the leaves is often exposed to the air while the lower surface stays in contact with water.
In some plants such as Water Lily (Nymphaea), the floating leaves are characterized by a waxy coating which checks the wetting of the upper surface and clogging of stomata with water. This waxy coating will also protect the leaves from chemicals present in water.
Heterophylly
Some hydrophytes show heterophylly having both submerged and floating leaves. When the same plant produces leaves of different forms, it is called heterophylly which is found in Limnophila heterophylla, Ranunculus aquatilis, Sagittaria, etc. But when the plant bears only one form of leaf, the condition is called homophyllous. The variation in their two forms of leaves is due to the following reasons.
- Heterophylly helps the plants reduce transpiration.
- The broad leaves on top reduce the intensity of light passing through to the submerged leaves.
- During drought, the submerged leaves compensate by absorbing water so that the plant does not show adverse responses to the conditions.
- The floating leaves balance or regulate hydrostatic pressure with an increase in transpiration.
- It also provides variation in habitat and life forms.
Submerged leaves are usually long, narrow, ribbon-like, finely divided, or dissected. The plants that occur in slow-moving streams have ribbon-like leaves as in Hydrilla and Vallisneria or finely divided as in Ranunculus aquatilis and Ceracophyllum. The floating leaves are usually broad, circular, and lobed. They bear stomata only on the upper surface.
In the submerged plants, transpiration in the strict sense is excluded but excretion of water takes place by guttation through water pores or hydathodes. The dissected leaves of Utricularia offer less resistance to undercurrents and stress and strain of water. If the leaves were entire and broad they might have been torn to shreds and will cause permanent damage.
In amphibious plants, the exposed leaves show mesophytic features such as being tougher than other hydrophytes. Water enables seed dispersal and pollination in hydrophytes. Their pollens and seeds are lightweight so that they can float.
Petioles
In some aquatic plants like Nymphae, the petioles are long and spongy and they can adapt themselves to the varying depths of water, thus they keep their leaf lamina on the surface of the water. They appear green or yellow.
Petioles of lotus grow indefinitely to keep their laminae on the surface of water. In Hyacinth, the petioles swell and become spongy to make it buoyant.
Adaptations in the Stem
In some aquatic plants, the stems are slender, delicate long, soft, and spongy which can bend easily towards every direction needed. They are green or yellow in color.
Sometimes they may also be modified into runners or rhizomes as well. Nymphaea produces a rhizomatous stem that firmly attaches the plant to the mud.
Moreover, vegetative propagation is a characteristic feature of several hydrophytes. It is rapid fragmentation in Elodea and runners in Eichhornia. It is a general biological cause that humidity opposes the production of sexual organs.
Anatomical Adaptations Of Hydrophytes
The anatomical adaptations of hydrophytes enable them to survive in the aquatic environment without any damage or injuries. Whether it is the submerged plants or the floating types, every species develops unique characteristics that help them survive the environmental conditions.
Such features of hydrophytes are,
- Reduced protective structures
- Increased aeration
- Reduced mechanical tissues
- Reduced vascular tissues
Reduction in Protective Structures
- The cuticle layer is absent in the submerged parts of the plant. It may be present in the form of a thin film on the surface of plant parts that are exposed to the atmosphere.
- Epidermis is thin, single-layered and it is not a protecting layer but it absorbs minerals and gases directly from the aquatic environment. Extremely thin cellulose walls in the epidermal cells facilitate the absorption process.
- Epidermal cells contain chloroplast which helps these cells function as photosynthetic tissue.
- Hypodermis in hydrophytes is poorly developed and has extremely thin walls.
Anatomical adaptations of hydrophytes in Stomata
Stomata are usually absent in submerged parts of hydrophytes. In some plants, there may be some functionless vestigial stomata present. Due to the lack of stomata, gaseous exchange takes place through the cell walls.
Stomata when present are in limited number and are confined to the upper surface of floating leaves. Amphibian plants have scattered stomata on the aerial parts. These stomata may be larger in number than what is seen on floating leaves.
Air Chambers
The two other important external factors that influence the submerged water plants are poor supply of air and weak light. These plants, just like other plants, require oxygen for respiration and carbon dioxide for photosynthesis.
Hydrophytes have solved this difficulty by the extensive development of a system of air spaces or air chambers. There is aerenchyma in their comparatively large cortex of root, stem, and in leaf mesophyll, especially that of submerged parts so that the tissues can obtain enough oxygen for respiration by internal circulation.
The air chambers are separated by chlorenchyma. The oxygen produced by photosynthesis is stored in air chambers and is used for respiration and the resultant carbon dioxide is utilized to perform photosynthesis. Air chambers have perforated septa called diaphragms for better aeration and to check floating.
Aerenchyma in the plant parts gives mechanical support and buoyancy. The air chambers are abundantly present in the fruits of water plants thus facilitating dispersal by water. At the same time, the development of air chambers in plants is governed by the habitat.
Reduction in Supporting or Mechanical Tissues
On account of the buoyant influence of water, the mechanical tissue is not needed in hydrophytes and is therefore reduced or underdeveloped. It is the same in both floating and submerged hydrophytes.
The stem is soft and tender when present. Among the submerged plants there are no woody plants. The chlorenchymatous cortex is much larger when there is no development of collenchyma and sclerenchyma.
The larger chlorenchyma tissues facilitate increased photosynthetic activity. In amphibian plants, there may be sclerenchyma in the cortex of aerial and terrestrial parts.
The endodermis and pericycle are usually absent. In water lilies and some other plants certain star-shaped lignified cells called astero sclereids develop which give mechanical support to the plants. The thin-walled, long, and loosely arranged spongy cells provide the needed support.
In Water Lily, there are specialized star-shaped lignified cells called asteroscleroids for mechanical support. Lignification occurs very little or not at all. The buoyancy of the plants protects them from injuries.
Reduction of Vascular Tissues
Conducting tissues are very poorly developed in hydrophytes since the absorption of water and minerals takes place through the entire surface of submerged plants. There is little need for vascular tissues.
When the xylem is present, it is highly reduced and consists of only tracheids with no other xylem elements. In some aquatic plants, a lacuna is present in the center in place of the xylem.
Phloem tissue is also poorly developed in most of the aquatic plants. The sieve tubes are smaller but there is extensively developed parenchyma.
They have properly defined endodermis. The smaller vascular bundles are aggregated towards the center. Secondary growth in thickness does not take place in aquatic stems and roots.
Physiological Adaptations Of Hydrophytes
The peculiar nature of the hydrophytic habitat greatly modifies the physiology of plants. This leads to the development of certain physiological adaptations of hydrophytes.
Hydrophytes are confronted with the difficulty of getting enough air for root respiration. Hence they develop plenty of air spaces where air is stored.
However certain hydrophytes have developed the ability to respire anaerobically. In plants like water lilies, the air spaces of submerged parts communicate with the air outside to enable a smooth exchange of gases during respiration and photosynthesis is possible. Similarly, there are various other physiological adaptations of hydrophytes.
- The absorption of water by hydrophytes is very peculiar. Thus all hydrophytes show a reduction in their vegetative growth.
- Cells of the root are often thin-walled. This facilitates the absorption through any region of the root.
- Absorption is also possible through other parts of the plants. That is why hydrophytes show a tendency to reduce the root system. A separate absorptive region becomes non-essential in certain hydrophytes where the whole plant body is absorptive.
- Transpiration is correlated to absorption. Transpiration is also very much reduced because of low absorption.
- In hydrophytes, there is very little conduction since their xylem is reduced. The osmotic pressure is also very low and plants rapidly wilt when removed from water.
Adaptations in Submerged Plants
In submerged plants, transpiration is absent. These plants excrete water through guttation through water pores called hydathodes.
Submerged water plants receive only weak-intensity water and are absorbed mostly by the solid particles seen in water. Therefore, such plants become weak, etiolated (internode increases in length), and thin. Moreover, the chlorphyllated tissues of mesophyll show little differentiation.
In many cases, leaves are thin and linear or they are highly dissected. This increases the total absorptive surface of leaves. This is necessary for the absorption of carbon dioxide and to receive diffused light from the water.
Chloroplast is abundant even in the epidermis, especially in the submerged plants. Since it is present in the epidermis, these plants can perform photosynthesis even though light penetrates only up to the epidermis.
Their chloroplasts are large and mobile so that they can move up within the palisade and receive weak light. But in floating leaves, the palisade is well developed. In some cases as in Trapa, the cortex of roots contains chloroplast and is photosynthetic in function.
Importance of Hydrophytes
Hydrophytes play an important role in the elimination of greenhouse gases and nitrogen. Hydrophytes such as plankton are the highest producers that help control the concentration of carbon dioxide in the atmosphere. They help control the greenhouse gases.
Hydrophytes are used in treatment plants to denitrify water. These plants can utilize nitrogen compounds and reduce their concentration in the water.
References
- Zhang, Y., Sun, S., Gu, X., Yu, Q., & He, S. (2023). Role of hydrophytes in constructed wetlands for nitrogen removal and greenhouse gas reduction. Bioresource Technology, 388, 129759. https://doi.org/10.1016/j.biortech.2023.129759
- Shukla, R.S. and Chandel, P.S. (2001) Plant Ecology. S. Chand and Company Ltd., New Delhi.
- https://www.biologydiscussion.com/plants/morphological-and-physiological-adaptations-of-hydrophytes/4583
