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Consortium for
Educational
Communication
Module on
Morphological and
anatomical responses of
plants to water
By
Shauket Ahmed Pala
Research Scholar, Department of
Botany, University of Kashmir
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Text
Availability of water is a major selective force in the evolution of
the plants ability to respond to moisture stress. Mosses, algae,
fungi and lichens have no protective mechanism against water loss
because their internal water status tends to match atmospheric
moisture conditions. At the condition of water scarcity, their cells
shrink without disturbing the fine protoplasmic structures and the
vital processes are suppressed. With the improvement of moisture
condition, they imbibe water and cells resume normal functioning.
Such organisms are called poikilohydric as they restrict their growth
to moist periods. Other plants such as ferns and seed plants are
able to maintain stable water balance within limits independent
of fluctuations of atmospheric moisture levels and are called
homohydric. The ability is supported by vacuoles that store water
within the cell, a protective cuticle slowing down evaporation,
stomata to regulate transpiration, a combination of osmotic pressure
and turgor pressure of water within the cells and the extensive root
system. All features of specialization that help plants to grow under
different habitats are referred to as ecological adaptations. The
term ‘adaptation’ can be defined as ‘the structural and functional
characteristics of the living organisms which develop over a
period of time and enable them to survive and reproduce in
a particular environment or habitat’.
It is quite evident that water is an important ecological factor in
the life of plants. Tissues of growing plants may contain from 60 to
95 percent water by weight. Water is the prime factor responsible
for the distribution of vegetation and its structure. Warming (1909)
classified the plants on the basis of their water requirement into
three ecological groups. They are; a) Hydrophytes, b) Xerophytes
and c) Mesophytes
A. Hydrophytes
Hydrophytes are plants that grow in regions where there is plenty
of water supply such as lakes, ponds, rivers, streams and marshes
or wet soils. The organisms found in aquatic habitats experience
a variety of physical factors. These include the availability of
oxygen and light, pressure fluctuations, resistance to motion,
salt concentration, etc. To adjust to the prevailing conditions, aquatic
plants have various types of adaptations. According to the way in
which they develop in water, they are divided into the following five
categories:
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(a) Free-floating hydrophytes: These plants float freely
on the surface of water but are not rooted in the mud. These
remain in contact with water and air, but not soil. Very small such
hydrophytes float on the surface of water by their flattened bodies
and surface tension of water. The larger plants possess special air
storing organs for balancing them over the surface of water. Examples
of free floating hydrophytes are: Eichhornia, Pistia, Wolffia, Lemna,
Salvinia, Azolla, etc.
(b) Rooted hydrophytes with floating leaves: These
hydrophytes are rooted in the mud but their leaves and flowering
shoots float on or above the water surface. Such plants usually
grow in shallow water and posses elongated and flexible petioles
or stems to bring the lamina on the surface of water. Victoria
regia, Nymphaea, Trapa, Nelumbo nucifera and Marsilea are
some of the examples of rooted hydrophytes with floating leaves.
(c) Submerged floating hydrophytes: This group of submerged
hydrophytes include those plants which are not rooted in mud and
remain completely suspended in water body. They neither reach
the surface nor are in contact with the bottom. Examples include
Najas, Utricularia and Ceratophyllum
(d) Submerged rooted hydrophytes:
These plants are
completely immersed in water and rooted in the mud. However,
the root system is poorly developed and helps in the absorption
of some nutrients from the bottom. Examples of such plants are
Hydrilla, Vallisneria, Potamogeton pectinatus, Isoetes, etc.
(e) Rooted emergent (amphbious) hydrophytes: This group
includes those plants which grow in shallow water with their shoots
extending above the surface of water. These plants are rooted in
the mud. Their basal parts including roots, rhizomatous stem,
basal part of stem and some leaves may be submerged in water
but the upper part of stem, branches, some leaves and flowering
shoots are exposed to air. These plants show both hydrophytic
and mesophytic characters. The aerial parts of these amphibious
plants show mesophytic characters, while the submerged
parts develop hydrophytic characters. Rununculus aquaticus,
Limnophylla heterophylla, Typha, Sagittaria, Myriophyllum
heterophylum are some of the examples of such plants.
I Morhological adaptations
1. Roots.
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Due to availability of water in plenty, roots (the principal organs of
water absorption) in such plants become of secondary importance
and less significant. Their overall development is usually very poor
and insignificant in most of the hydrophytes.
(i) Roots may be entirely absent as in Wolffia, Utricularia, Salvinia
and Ceratophyllum or poorly developed as in Hydrilla. In Salvinia
submerged leaves compensate for roots. However, in emergent
forms, as in Rununculus which grow in mud, roots are well
developed with distinct root caps.
(ii) Root hairs and root caps are generally absent in floating forms.
In Eichhornia and Lemna, the root caps are replaced by fingerglove like root-pockets which help in balancing.
(iii) Roots, if present, are generally fibrous, adventitious, reduced
in length, unbranched, or poorly branched.
(iv) In Jussiaea repens two types of roots develop. Some of
them are normal, while others are negatively geotropic floating
roots, spongy in nature and keep the plants afloat.
2. Stems.
(i) In submerged forms as Hydrilla and Potamogeton stem is long,
slender, spongy and flexible. In free floating forms it may be
slender, floating horizontally on water surface as in Azolla
or thick and short, stoloniferous and spongy as Eichhornia.
(ii) In forms which are rooted with floating leaves like Nymphaea
and Nelumbo with their upper surface coated with wax, the
stem is a rhizome. These rhizomes live for many years and
produce leaves every year.
(iii) In Neptunia oleracea, the stems are swollen and store gases
to function as floats.
3. Leaves.
(i) In submerged hydrophytes leaves are thin, and are either long
and ribbon shapped as in Vallisneria, or long and linear as in
Potamogeton, finely dissected as in Ceratophyllum and Utricularia.
(ii) Leaves of floating forms are large, flat and entire as in
Nymphaea and Nelumbo with their upper surfaces coated with
wax. The wax coating protects the leaves from mechanical and
physical injuries and also prevents clogging of stomata by water.
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Petioles of floating leaves are long, spongy and flexible and often
covered with mucilage. In Eichhornia and Trapa petioles become
swollen and spongy.
(iii) In some amphibious plants Ranunculus and Sagittaria the
leaves show heterophylly, i.e., the plants develop two different
types of leaves. The submerged leaves are usually dissected or
ribbon shaped offering little resistance against the water currents,
and absorbing dissolved carbon-dioxide from water, and aerial
leaves are entire, rounded or lobed and show typical mesophytic
features.
4. Vegetative reproduction is common. Vegetative propagation by
runners, stolons, stem, root tubers, offsets etc., is most common
and extensive method of reproduction.
5. Flowers and seeds are less common in submerged forms.
Pollination (e.g. Vallisneria) and di s p e r s a l o f f r u i t s a n d seeds
are accomplished by the agency of water. Seeds and fruits of
many hydrophytes are light in weight and they easily float on the
surface of water.
6. All the submerged parts are surrounded by mucilage which
functions as lubricant, prevents desiccation, and protects plants
from epiphytes and attack of pathogens
II Anatomical adaptations:
The hydrophytes show following anatomical features:
1. Cuticle is completely absent in submerged parts of the plants
but may be present as a thin film on surface of parts exposed to
atmosphere
2. The epidermis is usually single layer of thin walled cells,
not protective in function. Epidermal cells of leaves contain
chloroplasts and they can function as photosynthetic tissue,
especially where the leaves and stem are very thin e.g. Hydrilla.
The epidermal cells of submerged parts are mostly covered on
outer side by mucilage to protect the plant body from decay
under water.
3. Wax coating is present on the upper surface of floating leaves.
4. The aerial parts of some hydrophytes bear hydathodes to lose
excess water that enters into the plant due to endosmosis.
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5. Stomata are totally absent in submerged forms and exchange
of gases takes place through diffusion. In floating leaves stomata
are confined only to the upper surface (epistomatous leaves) e.g
Nelumbo. Non-functional stomata are seen in Potamageton. In
amphibious plants stomata may be scattered on all the aerial
parts. Cortex is well developed, thin-walled and parenchymatous,
major portion of which is occupied by well developed prominent
air cavities- the ‘aerenchyma’ which offers resistance to bending
stress, increases buoyancy and allows a rapid gaseous exchange.
6. Mechanical tissues, i.e. sclerenchyma and collenchyma, are
absent or poorly developed in floating and submerged parts. They
are, however, present in aerial and terrestrial parts. Branched or
star shaped sclereids and idioblasts are found in the aerenchyma
of Nymphaea, Eichhornia and related plants. I t g i v e s mechanical
support to the plants.
7. In the vascular tissue xylem is poorly developed in hydrophytes,
especially in submerged forms like Potamogeton, as the water
absorption takes place all over the surface of the plant body. In
xylem vessels are less common but tracheids are generally present.
In amphibious plants, the xylem and phloem are well developed and
are usually aggregated towards the centre. Generally secondary
growth in thickness does not occur in aquatic stem and roots.
8. Mesophyll is undifferentiated in submerged leaves as in
Potamogeton, but is well differentiated into palisade and spongy
parenchyma with air cavities as in floating leaves (e.g. Nymphaea)
and emergent leaves (Typha).
B. Mesophytes
These are very extensive on the land surface. They are such
land plants which grow in habitat where water is not scarce or
not abundant and need well aerated soils. They prefer soil and
air of moderate humidity and avoid soil with standing water or
containing a great abundance of salts. In some respects they
stand in between the hydrophytes and xerophytes. They generally
lack special structural and physiological adaptations found in
hydrophytes and xerophytes. The general morpho-anatomical
features of mesophytes are as follows.
1. Root system is well developed. Roots are generally fairly
branched, with root caps and root hairs.
2. Stems are generally aerial, solid and freely branched.
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3. Leaves are generally large, broad, thin and varied in shapes;
generally oriented horizontally, green, without hair or waxy coating.
4. Cuticle in aerial parts is moderately developed.
5. Epidermis well developed, without any hair or waxy coatings
and cells without chloroplasts.
5. Stomata are generally present on both surfaces of leaves.
6. Mesophyll in leaves is differentiated into palisade and spongy
parenchyma, with many intercellular spaces.
7. Vascular and mechanical tissues fairly developed and well
differentiated.
C. Xerophytes
Xerophytes are those plants which grow in xeric habitats i.e., under
deficient supply of water and extremely dry conditions. Under such
habitats the conditions are favourable for excessive loss of water
from aerial parts of plants (transpiration) and the water is not
available for absorption. Xeric habitats are characterized by:
1. High temperature of atmosphere and soil
2. Deficiency of water in the soil or presence of water deep in the
soil
3. High intensity of light and wind velocity.
The plants facing such adverse conditions develop certain
morphological, anatomical and physiological adaptations.
Adaptations in xerophytes are of two types:
(i) Xeromorphic adaptations are those which are inherited whether
the xerophyte grows in xeric conditions or not. For example, a
Cactus has the same feature, whether it is in a desert or in a
normal land.
(ii) Xeroplastic adaptations are the ones that are induced temporarily
but disappear when the conditions are favourable.
On the basis of morphology, physiology and life cycle patterns,
xerophytes are generally classified into following three categories.
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1. Ephemeral annuals
They are also called ‘drought evaders’ or ‘drought escapers’. They
are mostly found in arid zones. They are annuals, which complete
their life cycles within a very short period of 6-8 weeks and thus
escape dryness. Generally they germinate and grow when the
habitat remains moist during the short rainy season. They flower
and set seeds before the habitat becomes dry. They pass the
period of drought by means of their seeds. Examples of such
plants are Solanum xanthocarpum, Argemone mexicana, Cassia
tora etc.
2. Succulents
They are also called drought enduring xerophytes. These
plants suffer from dryness in their external environment only.
Their succulent, fleshy organs (stems, leaves, roots) serve as
water storage organs which accumulate large amount of water
during the brief raining season. Examples include Agave, Aloe,
Euphorbia, Opuntia, Asparagus etc. In some plants, stem becomes
succulent which are called the ‘F l e s h y xerophytes; as in Opuntia
and Euphorbia. Those succulent xerophytes in which leaves
become fleshy are also known as Malacophyllous xerophytes, such
as Aloe, Yucca, Agave, Bryophyllum, Tradescantia etc.,
3. Non- succulent perennials
They are also called drought resisting xerophytes and are the true
xerophytes. They possess a number of morphological, anatomical and
physiological characteristics which enable them to withstand critical dry
condition. They are the plants that suffer from dryness both in
their internal as well as external environments. Examples of such
plants are Calotropis procera, Acacia nilotica, Zizyphus jujuba,
Capparis aphylla, Casuarina, Nerium, Saccharum etc.
I Morphological adaptations
1. Roots
(i)In contrast to hydrophytes, roots of xerophytes develop under
water deficient conditions. Thus, in order to secure water, which is
present in less amount and moreover in deeper layers of soil, roots
in xerophytes become the principle organ of primary importance.
The plants develop extensive and deeply penetrating root system
which in some cases is several times longer than the shoot. Root
hairs and root caps are well developed.
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(ii). In some cases the roots become fleshy and succulent to
store large quantity of water and mucilage e.g. Asparagus
(iii). Some fleshy and succulent xerophytes develop shallow root
system to obtain water from light showers as well as dew e.g.
Opuntia
2. Stem
The stem remains stunted, thick and heavily branched giving
a bushy appearance. They also develop thick bark to escape
transpiration. In some xerophytes, the stem becomes fleshy and
succulent e.g. Euphorbia. Sometimes the stem is modified into
green, flat, leaf like structure called phylloclade, covered with
spines and performs photosynthesis.
3. Leaves
(i) The leaves in xerophytes exhibit comparatively more variation
than any other part. In many cases they are either reduced or
scale like modified into spines to check transpiration. In some
plants leaves are permanently oriented in a vertical position to
avoid direct sun rays. Some plants shed their leaves during the
dry periods to reduce loss of water through stomata.
(ii) The foliage leaves of some xerophytes are thick, fleshy and
leathery. They are the main storing organs. They have shining
surfaces to reflect light and heat and protect plant from desiccation.
(iii). In some grasses such as Ammophila and Poa leaves become
folded and rolled in such a manner that the sunken stomata
become hidden, and thus rate of transpiration is considerably
minimized.
5. In some xerophytes, the leaves and young parts of plant are
covered over by dense coating of hair which act as shade against
Sun to reduce transpiration e.g. Calotropis
9. In some of xerophytes such as Euphorbia, Acacia nelotica,
Zizyphus jujuba and Capparis aphylla, stipules become modified
into spines.
II Anatomical modifications
1. The epidermis of leaves and other delicate parts is covered
with thick cuticle. In addition some xerophytic plants (e.g.
Calotropis) possess protective coating of wax and silica. The wax
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is impermeable to water vapours, where as silica protects against
solar radiations. Epidermis is multilayered in some xerophytic
plants e.g. Nerium
2. The epidermal cells of some xerophytes are coloured due to
presence of anthocyanin and betacyanin in their cell sap. The
pigments act as light screen and reduce insolation.
3. The stomata are generally confined to lower epidermis of leaves
called hypostomatous (except some grasses). They are sunken
below the level of other epidermal cells. In some cases, they are
located in deep pits which are filled with hairs. In some xerophytes
the stomatal pores are plugged with wax or resins during the period
of drought. The stomata of mature and old leaves are usually non
functional due to greater thickening.
4. Leaves of some xerophytic grasses possess bulliform or motor
cells in their epidermis. These cells are thin-walled and greatly
enlarged. These cells are generally present at the basses of grooves
and furrows in upper epidermis. In moist conditions these cells
remain turged and keep the leaf surface flat. But when they lose
their turgidity in the period of drought they get flaccid and cause
rolling of leaves.
5. In many xerophytes the hypodermis is one to several layers
thick consisting of sclerenchymatous tissue.
6. The mesophyll is differentiated into palisade and spongy
parenchyma. In some xerophytes palisade is located on both upper
and lower surfaces of leaves. The succulent leaves of Aloe and
Salsola have prominent water storage regions in their mesophyll.
7. Mechanical tissue system is very well developed in xerophytes.
8. The water conducting system (i.e. xylem) is well developed.
It also provides mechanical support. The tracheids are smaller in
size where as vessels are larger with lignified thick walls. Annual
rings are also prominent.