MODULE 8, LESSON 1

Hydrophytes and Its Characteristics, Ecological Adaptations and Anatomical

Features

Introduction

We all know plants are of different types. We have been made familiar with plants that are on land
but what about the ones underneath the water? They are plants that are meant to live underwater.
Aquatic plants are very fascinating and have unique characteristics.
In this lesson, you shall be introduced to the characteristics of aquatic plants which vary
depending on the type of plant and the aquatic environment in which it grows. Read the
discussion below. You are required to answer the given activities to assess your learnings.
Perform the activity and submit via messenger.

Learning Objectives
1. To summarize and discuss the features of aquatic plants that distinguish them from
vascular plants.
2. To have knowledge on the different types of aquatic plants.
3. To discuss the ecological adaptations and anatomical features of hydrophytes.

Learning Outputs
1. Summarize and discuss the features of aquatic plants that distinguish them from vascular
plants.
2. Distinguish different types of aquatic plants.
3. Provide representative plant species for each type of aquatic plants.
4. Discuss the ecological adaptations and anatomical features of hydrophytes.

Discussion

Aquatic plants are plants that have adapted to survive in aquatic environments. Also known as
macrophytes or hydrophytes, aquatic plants grow on or in water and are either submerged,
emergent or floating. They provide habitat and food for aquatic life, produce oxygen, and can
also be a food source for certain terrestrial animals. Aquatic plants require particular adaptations
to grow on the water's surface and cannot survive in terrestrial environments. For example, their
stems are spongy with air spaces to keep the plant vertical or floating above the water. Aquatic
plants have developed in different plant families and can be angiosperms or ferns. Seaweeds are
not typically classified as aquatic plants since they are multicellular algae. Some aquatic plants
are capable of growing in saltwater, saline, or brackish environments. Seagrasses are the only
angiosperms that can grow while completely submerged in seawater. Most aquatic plants
reproduce by flowering, while others depend on forms of asexual reproduction through turions,
rhizomes, and fragments. The Amazon water lily is among the largest aquatic plants, while the
minute duckweed is one of the smallest. Other familiar aquatic plants include water lilies, water
hyacinth, lotus, and floating heart. Factors that affect the distribution of aquatic plants include
the depth and duration of floods, distribution, and abundance of nutrients, salinity, grazing, and
disturbance by waves.

Types of Aquatic Plants

1. Emergent Plants

Emergent plants grow in water, but part of the plant remains above the water's surface. This
adaptation allows the leaves to photosynthesize more efficiently, providing more oxygen and
nutrients to submerged parts of the plant. In most cases, the leaves, flowers, and other
reproductive parts float above the surface to enable pollination by insects and wind. There are
numerous species of emergent aquatic plants, including the reed, wild rice, and flowering rush.
They can grow in fens and ponds, but do not thrive in these environments due to competition
from other aquatic plants. The purple loosestrife is an example of an emergent plant that can also
thrive in minimal water, such as fens and damp ground.

2. Floating-Leaved

Floating-leaved aquatic plants have roots that are attached to the bottom of the body of water in
which they grow, but their leaves float on the water's surface. They are characterized by broad
leaves, and examples of floating-leaved plants include pondweeds and water lilies.

3. Submerged

Submerged aquatic plants grow entirely underwater. They can either have roots that are attached
to the bottom of the body of water in which they grow or have no root system at all. Some
submerged plants remain attached to other aquatic plants.

4. Free-Floating

Free-floating plants remain suspended on the water's surface. They have broad leaves and stems
that attach to neighboring plants to create a dense net of free-floating plants. Free-floating
aquatic plants are not attached to the bottom of the body of water in which they grow, and are
therefore easily moved by wind and waves. Examples of free-floating aquatic plants include the
Nile cabbage, water cabbage, and water lettuce.

Characteristics of Aquatic Plants

The characteristics of aquatic plants vary depending on the type of plant and the aquatic
environment in which it grows. However, some features are universal. For example, they utilize
fewer resources to support their leaves and stems because they naturally stay afloat, and they
lack the cuticle layer that prevents water loss since there is no need to retain moisture. Aquatic
plants also lack the xylem to transport water since their entire bodies are submerged. The leaves
of immersed plants float on the water surface to access sunlight and air, while the roots float in
the water. Aquatic plants remain green throughout the year and do not drop their leaves since
they have a sufficient supply of nutrients and water.

Water Retention
Plants require water to survive, plants usually possess the ability to absorb and retain water to
keep the plant alive between watering cycles. Because aquatic plants are literally submerged in
water at all times, there is no need for the plant to water retention like non-aquatic plants. As
such, there is no need for aquatic plants to expend energy to regulate transpiration, which is the
loss of water in the plant due to evaporation.

Flat Leaves
Some aquatic plants float on the surface of water; most aquatic plants have flat leaves which act
as floatation to a portion of the plant. An example of such a plant is the water lily, Nymphaea
Attraction, which is a beautiful bright red water lily that can have up to a 12inch diameter leaf.

Feathery Roots
Hydrophtes are supported by water as compared to roots and stem structures; most hydrophtes
have roots that are small and feathery. These are designed to take in oxygen from the water, and
since the plant is fully submersed in water at all times, there is less need for a long and thick root
structure. One plant that has a feathery root is the Salvinia, which is a free-floating aquatic fern.

Air Sacks
Many hydrophytes have air sacks (chambers) that help the plant float on the surface of the water.
It is important to note that some aquatic plants will float slightly submerged in the water, such as
the buttercup. Others, such as water lilies, will float up on top of the surface since their leaves
distribute the weight across the surface of the water.

Let us make an in-depth study of the morphology and anatomy of the three groups of
hydrophytes.
I. Submerged Hydrophytes:
Entire body of submerged hydrophytes remain embedded in the water column and show
following morphological adaptation (Fig. 2.1):
(i) Root System:
1. Root system of submerged plants is greatly reduced.

2. Root-hairs are absent and the aquatic roots usually possess root pockets (e.g., Eichornia
speciosa).

3. The cuticle is either altogether absent (See fig. 2.2) or very poorly developed. This enables the
root to absorb water directly through its surface.

4. The cortex is very well-developed and possesses large air spaces. Air present in the air spaces
is used in respiration. Cells of the cortex are thin and parenchymatous.

5. The vascular tissue is feebly developed and does not show much differentiation of tissues.
Usually the cells of the vascular strand are thin walled. Sometimes the centre of the vascular
strand is occupied by an air cavity.

6. Roots may contain chlorophyll if they receive sufficient light.

(ii) Stem:
1. Stem is usually weak and flexible. Sometimes it is covered by a gelatinous sheath which
serves as protection against periodic desiccation.

2. The cuticle is either altogether absent or very poorly developed. The epidermis is always
single-layered and thin-walled; this character facilitates direct absorption of gases and mineral
salts dissolved in water.

3. The cortex is very broad and occupies bulk of the stem. The outer layers of the cortex are
parenchymatous and usually without inter-cellular air spaces, whereas the inner cortex is
aerenchymatous and possesses symmetrically arranged large air spaces. The air filled in these
cavities adds to the buoyancy of the plant and secondly facilitate the exchange of gases during
respiration and photosynthesis. (See Figs. 2.3 and 2.4)
4. The cells of the cortex contain chloroplasts and assist in carbon assimilation.

5. Usually there is no marked distinction of endodermis and pericycle. Sometimes the inner-most
layer of the cortex is regarded as endodermis (See fig. 2.3).

6. Vascular tissue is poorly developed and does not show marked differentiation of phloem and
xylem. An air cavity is mostly present in the centre of the vascular strand that adds to the
buoyancy of the plant. Sometimes, xylem is represented by a single strand present in the centre
of the stele (e.g., Hydrilla, Potamogeton, Elodea etc.)

7. There is no mechanical tissue present in the stem of the submerged plant. Water column itself
provides mechanical support to the plant.
(iii) Leaf:
1. The leaves are usually modified, thin, dissected or ribbon’ shaped. For instance, in Hydrilla
and Elodea (Fig. 2.5) the leaves are small and reduced, in Potamogeton and Vallisneria these are
long and ribbon- shaped, where as in Ceretophyllum the leaves are segmented.

2. Leaves do not possess cuticle, and stomata are also absent. Exchange of gases may take place
directly through surface layers.

3. There is no marked differentiation of tissues in the mesophyll. Sometimes, air spaces may be
present in the mesophyll (e.g., Potamogeton, Pontedaria). Chloroplasts are present in the
mesophyll cell thus enabling this tissue to conduct photosynthesis.

4. Leaf possesses a reduced fibro-vascular bundle.

Physiological adaptation : Submerged aquatic plants obtain oxygen used in respiration from that
dissolved in water or from their own photosynthesis. These plants may also respire anaerobically
for considerable periods of time, but will not thrive without free oxygen.

II. Free-Floating Hydrophytes:

Plants of this group remain floating on the surface of the water, but in some species the roots
remain embedded in the mud and only leaves and flowers float on the surface of water (e.g.,
Nymphaea, Limnanthemum, Victoria regia, Euryale ferox).

Following are the important adaptations of these plants:

(i) Roots:
The roots are feebly developed and are in the process of disappearance. For instance, in
Spirodela, there are several roots, but Lemna possesses only one root, and in Wolffia roots are
altogether absent.
2. Roots are mostly hairless but do possess characteristic root-pockets, which closely resemble
with the root caps of mesophytic plants. This character provides a club that hydrophytic plants
have probably evolved from mesophytic ones.

3. The epidermis is single-layered and thin-walled. It does not possess any cuticle and is thus
able to absorb water and minerals directly from the surrounding medium.

4. Cortex is well developed and possesses air cavities.

5. The mechanical tissue is mostly absent.

6. The centrally situated vascular strand is either undifferentiated or very poorly differentiated
into xylem and phloem. In Lemna, there is no distinct vascular tissue in the root.

(ii) Stem:
1. Internally the stem or rhizome may be very well differentiated. For example, in Nymphaea,
the stem shows well differentiated epidermis, hypodermis, cortex and vascular bundles.

2. The epidermis is thin-walled and single-layered. Cells of the epidermis are mucilagenous.
Cuticle is absent.

3. Cortex occupies bulk of the stem and possesses large air spaces. The air filled in these spaces
helps in respiration and adds to the buoyancy of the floating plant. The cells of the cortex are
always thin-walled and may even possess chloroplasts if light it available.

4. A large number of stellate sclereids may be present associated with the air spaces in the
cortex. Function of these sclereids is purely mechanical. No other mechanical structure or tissue
is present in the stem.

5. Vascular bundles are found scattered among the cortical cells. The vascular tissue is poorly
differentiated into xylem and phloem. The xylem vessels are thin- walled and the phloem is
represented by only few narrow sieve-tubes.

(iii) Leaves:
1. As the leaves are the chief organs of floating, very loose tissue with large air spaces is found
in the leaf blades. Such a tissue enables the leaves to float on the surface of the water. The
bulbous petiole of the water hyacinth is occupied largely by air spaces and presents a striking
case of the development of floating tissue (see Fig. 2.6).
2. The leaf possesses distinct epidermis, but the cuticle is always absent. Waxy coating may be
present on the upper surface of the leaf which prevents the wetting of upper surface and clogging
of the stomata by water.

3. The stomata are restricted to the upper epidermis of the leaf.

4. The mesophyll may be well differentiated into the palisade tissue and the spongy tissue (see
fig. 2.7). The spongy tissue possesses large air spaces filled with air.

5. The fibro-vascular bundles are poorly developed.

6. Stellate sclereids may be present scattered in the mesophyll; their function is purely
mechanical.
III. Emergent or Amphibious Plants:
Amphibious or emergent plants grow up into the air, but the roots of rhizomes are anchored in
the mud, perhaps beneath a foot or so of shallow water. Thus, plants of this group are adapted to
live partly in water and partly in air. The vascular plants of this group are the least specialized of
water plants and are closely related to mesophytes.

Development of the root system, number of root hairs and degree of branching is directly
proportional to decrease in water content of the habitat and increase in aeration. For instance,
Typha, an emergent plant, exhibits both mesophytic (i.e., well developed mechanical and
vascular tissues. See Figs. 2.8 and 2.9) as well as hydrophytic characters (i.e., prominant air-
cavities and storage parenchyma) in its roots (Fig. 2.10) and stem (Fig. 2.9). Because of the
abundance of mechanical tissue, the amphibious plants are able to grow erect without being
supported by water.
The rhizome, roots, and other submerged part of amphibious plants are in large degree
independent of the oxygen in the surrounding water. Most such organs are able to carry on
aerobic respiration because free oxygen diffuses downward from the aerial organs of the plant
through air passages in the leaves and stems. The oxygen is in part derived from atmosphere and
in part from photosynthesis.

The common amphibious plants are:

Ranunculus aquatilis, Sagittaria, Alisma, Limnophylla heterophylla, Myriophyllum
heterophyllum, Typha latifolia, Castalia, Cardenthera triflora, Proserpinaca palustris, Radicula
aquatica, Sium acutaefolium etc.

Functions of Aquatic Plants in the Ecosystem

Aquatic plants absorb dissolved nutrients from the water and act as purifiers. They are often
planted in artificial lakes and ponds to help reduce the toxicity of water and the organisms that
live in the water. They also reduce erosion by slowing down the movement of water. Some
aquatic plants such as wild rice, water caltrop, and Indian lotus are a food source for humans and
animals. Aquatic plants can also serve as an indicator of water quality. For example, a thriving
aquatic plant population indicates good or improving water quality, while a declining population
indicates contamination or pollution. Contamination may result from excessive use of herbicides,
turbidity, or salinization.

Summary

Aquatic plants are plants that have adapted to survive in aquatic environments. Also known as
macrophytes or hydrophytes, aquatic plants grow on or in water and are either submerged,
emergent or floating. Most aquatic plants have flat leaves, feathery roots and air sacks that help
the plant float on the surface of the water. There is no need for aquatic plants to expend energy
to regulate transpiration, which is the loss of water in the plant due to evaporation.
Aquatic plants provide habitat and food for aquatic life, produce oxygen, and can also be a food
source for certain terrestrial animals. They absorb dissolved nutrients from the water and act as
purifiers. They are often planted in artificial lakes and ponds to help reduce the toxicity of water
and the organisms that live in the water. They also reduce erosion by slowing down the
movement of water.