Adaptive & protective system

 Epidermal tissue system

The outermost layer or covering of the plant body is formed

of the epidermal tissue system. It is derived from the
protoderm. The epidermal tissue system consists of the
following components.

i. Epidermis
ii. Stomata
iii. Epidermal appendages
Bikram Ghos
1. Epidermis 9831756144, 9804993221
Epidermis is the outermost layer made up of parenchyma cells. It possesses elongated and compactly
arranged cells and is single layered in most cases. A large vacuole and a thin layer of cytoplasm lining the
cell wall is present in the epidermal cells. In some plants, a few epidermal cells are modified to perform
special functions. For example, epidermal cells present in the aerial roots of orchids are modified to a
hygroscopic velamen tissue that absorbs water from the atmosphere. Similarly, in some monocot plants
like grasses, the epidermal cells are modified into large, thin walled cells with big water-filled central
vacuoles. Such cells are known as bulliform cells and help the leaves of these plants to roll up and reduce
the rate of transpiration by decreasing the exposed surface area.

 Cuticle
Cuticle is composed of the deposition of water-resistant substances such as cutin or suberin and protects
the epidermal cells from desiccation and mechanical injuries. The epidermal cells present in roots are
devoid of cuticle and are known as epiblema.
The cuticle is composed of an insoluble cuticular membrane impregnated by and covered with soluble
waxes. Cutin, a polyester polymer composed of inter-esterified omega hydroxy acids which are cross-linked
by ester and epoxide bonds, is the best-known structural component of the cuticular membrane. The
cuticle can also contain a non-saponifiable hydrocarbon polymer known as Cutan.

2. Stomata
The epidermis is interrupted at places by the presence of tiny pores known as the stomata. Stomata are
seen in the epidermis of leaves and young shoots. They can regulate
the process of transpiration and exchange of gases.
 Structure of stomata
Each stoma consists of a complex called stomatal apparatus. It consists
of the following:
a) Stomatal aperture or stoma: It is the pore present in between
the guard cells. Movement of gases takes place through the pore.
b) Guard cells: Cells surrounding the stomatal aperture or pore are called guard cells. The guard cells
possess chloroplasts and are capable of photosynthesising. The outer wall of a guard cell is thin and
flexible whereas the inner wall is thick and rigid. Guard cells regulate the closing and opening of the
stomata. When solute concentration in the guard cells is high, the water from neighbouring cells
enters the guard cells by endosmosis and the guard cells become turgid and expand. Due to the
 presence of a rigid inner wall and a flexible outer wall, expansion causes the guard cells to bulge
towards the outer side and results in opening of the stomatal aperture. This mostly happens during
the day when the guard cells produce sugars by photosynthesis. When the solute concentration in
the guard cells becomes less, the water from the guard cells leak out by osmosis. This causes the
guard cells to become flaccid and collapse which is why the stomatal aperture closes. Guard cells
are bean shaped in dicots. They are dumb-bell shaped in monocots.

a) Subsidiary cells: Subsidiary cells are specialised epidermal cells seen around guard cells. When
subsidiary cells lie above the guard cells, it is called sunken stomata.
Bikram Ghos
3. Epidermal Appendages 9831756144, 9804993221

The outgrowths present on the epidermis are called epidermal outgrowths. On the basis of the location of
epidermal appendages,

 Root hairs: Root hairs are the unicellular elongations from root epidermis or epiblema. They help in
absorbing water and minerals from soil and also help in anchorage.

 Trichome: Trichomes are unicellular or multicellular outgrowths from the shoot epidermis.
Trichomes can be soft or stiff. Trichomes occur in various forms -
i. Non-glandular hair - These can be unicellular or multicellular, branched or unbranched, stellate or T
shaped. These trap air on the surface of the leaves and help to reduce loss of water by transpiration
and also protect from very high or low temperatures. Eg.- Platanus sp., Rubus sp.

ii. Glandular hairs are usually multicellular in nature and secrete different substances such as
mucilage, salt, nectar, etc. Glandular hairs can also act as digestive glands, aromatic glands or
stinging glands. Eg.- Jatropha sp., Artemisia sp.

iii. Scales are multicellular, flattened, scale like trichomes that occur in pitcher plants and also form
the ramenta (hair-like structures present on the stem) of some ferns. Eg.- Nepenthes sp., Pteris sp.

iv. Unicellular trichomes represents root hairs and other hairs on stem. Eg.- Heliotropium sp.,
Onopordum sp.

 Adcrustation and Incrustation

 Adcrustation is the process of the addition of any substance on a surface or substance from inside.
It means that nothing is added on the surface to make the substances thick or big, rather the
growth comes from the whole body.The growth of fruits and subsequent parts is an example
of adcrustation.
 Incrustation is the process by which the substances grow with the addition of new layers on the
surface of the body. The cell walls of the plants grow by the process of incrustation.
 Bikram Ghos
 Anatomical Adaptations of Hydrophytes:
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1. Epidermis: It is present as a thin or a single layer, which comprises parenchyma cells that are non-
protective in function. The epidermis of the leaves include chloroplasts, which participate in the
process of photosynthesis. Mucilage encircles the epidermis of the submerged parts and protects the
plant against decay. The hypodermis is either absent or poorly developed. The submerged parts of
hydrophytes generally lack cuticle but can be present as a thin layer on the aerial parts.

2. Stomata: The submerged parts lack stomata, but the upper surface of floating leaves carries stomata
called “Epistomatous leaves”. Potamogeton is a hydrophyte consisting of non-functional stomata. The
emergent hydrophytes consist of scattered stomata on all aerial parts of hydrophytes.

3. Aerenchyma: It refers to the air cavities found between the differentiated mesophylls, which allows
the convenient diffusion of the gases. The diffused gases travel through the internal gas spaces of
young leaves, then forced down to the root by the aid of water pressure through the aerenchyma cells
of the stem. Older leaves do not support the pressure gradient, so the gases from the roots expel out
through the leaves.

4. Cortex: Hydrophytes possess a highly-developed and thin-walled parenchymatous cortex, which helps
the plants against mechanical stresses and also permits efficient gaseous exchange. The large air
cavities occupy the major portion. Hydrophytes comprise starch granules as the primary reserve food
material, which accumulate inside the cortex and pith.

5. Mechanical Tissue: Hydrophytic plants possess mechanical tissue (sclerenchyma and collenchyma).
Only the aerial or terrestrial parts possess the mechanical tissues, and the floating and submerged
parts completely lack or contain poorly developed mechanical tissues. Cystoliths or sclereids of variable
shapes are present in the tissues of leaves and other plant cells.

6. Vascular Tissue:Submerged hydrophytes have

poorly-developed xylem and tracheids. In
contrast, the amphibious plants contain well-
developed xylem (towards the central
region). Secondary growth in stems and roots
does not occur in hydrophytes. In
hydrophytes, the presence of endodermis and
pericycle are distinct.

7. Mesophyll Cells: Hydrophytic plants possess

undifferentiated mesophyll cells in the
submerged leaves and differentiated
mesophylls (palisade and spongy mesophylls)
with well-developed air cavities in both the
floating and emergent hydrophytic leaves.
 Bikram Ghos
9831756144, 9804993221
 Anatomical adaptations of xerophytes
1) Epidermis: Some xerophytes have multiple epidermises. Eg.- Nerium sp. . Epidermis is with thick
cuticle and deposition of waxes, resins etc. There are epidermal hairs especially in grooves
(furrows) that protect the sunken stomata. Mostly stomata are sunken and are in pits. Number of
Stomata is less than normal. Leaves that have the capacity to roll have specialized cells called
bulliform cells that help in rolling.
2) Hypodermis: Hypodermal layers of xerophytes are thick and well developed.
3) Ground tissue: In stems there is abundant mechanical tissue in the form of sclerenchyma as in
Casuarina stem. Since leaves are reduced, the stems usually have chlorenchyma. In succulent
plants, cortex is filled with water, mucilage, latex etc. In plants that have leaves, palisade
parenchyma is well developed. In Pinus mesophyll cells are modified. Intercellular spaces are
greatly reduced.
4) Conducting tissue or Vascular Tissues: Vascular
tissue (xylem and phloem) is very well developed
in xerophytes.
5) Root hairs and root caps are well developed
in Opuntia. Roots may become fleshy to store
water as in Asparagus.
6) Latex Tubes: In many xerophytic stems and leaves the
laticiferous canals are present (viz., Calotropis,
Euphorbia, Asclepias, etc.). Because of vicosity latex
the transpiration is reduced to some extent.
7) Reduced Leaf Surface: In many xerophytes, reduction
of the leaf surface partly checks water loss because
the total exposed surface of the plant body is
relatively small as compared with that of normal
mesophytes (viz., Casuarina, Asparagus, etc.).