A number of plants develop unique adaptive modifications to overcome the various environmental stress. These modifications include alteration of gross morphological features as well as anatomical structures.
Some examples of anatomical changes are given:
A. Loaf Anatomy of Nerium sp. (Fig. 8.1):
The T.S. view of Nerium leaf shows the following layers of tissues:
1. Epidermis:
Both upper and lower epidermal layers are multiseriate. They are composed of a few layers of compactly arranged isodiametric colourless cells. Both the epidermal layers are very strongly cuticularised. There are distinct sunken stomata on the lower epidermis. A good number of trichomes develops from the bordering cells and remain projected within the stomatal pit.
2. Mesophyll:
It consists of palisade and spongy cells. But unlike other dorsiventral leaves, palisade cells occur here both towards upper and lower epidermis and spongy cells are located in between them. Calcium oxalate crystals are scattered within the mesophyll cells.
3. Vascular Bundles:
The bundles are collateral and closed ones with xylem on the upper and phloem on the lower sides. They remain surrounded by parenchymatous bundle sheaths.
Adaptive Anatomical Features:
The specimen thus shows the features of dicotyledonous dorsiventral leaves.
The reasons are:
(a) Presence of stomata in lower epidermal layer.
(b) Presence of palisade and spongy mesophyll cells.
(c) Presence of collateral, closed, vascular bundles.
In addition, it bears a number of unique adaptive anatomical features.
These include:
(i) Presence of sunken stomata on the lower epidermis.
(ii) Presence of distinct stomatal pit with a good number of trichomes, to reduce water loss.
(iii) Presence of calcium oxalate crystals.
All these features are characteristics of xerophytic adaptations.
B. Anatomy of Hydrilla Stem (Fig. 8.2):
In T.S. view of the stem of Hydrilla sp., following tissue regions are noticed:
1. Epidermis:
It is single-layered and has thin-walled cells without cuticle. It contains chloroplasts. Thus it can function as a photosynthetic tissue.
2. Cortex:
It is composed of thin-walled parenchymatous cells with scattered air spaces (air cavities). There is a distinct endodermal layer encircling the stele.
3. Vascular Bundle:
It is very simple, concentric and closed type. The central part is occupied by a single xylem vessel surrounded by phloem tissues. Xylem tissue is very poorly developed. Phloem is principally composed of sieve tube elements.
4. Pith:
It is absent.
Adaptive Anatomical Features:
The specimen shows the features of a monocotyledonous stem with distinct hydrophytic adaptive features.
The characteristics are:
(i) Presence of simple and closed type vascular bundle. Xylem tissue is very poorly developed.
(ii) Presence of scattered air cavities in the cortical layer which is a hydrophytic feature.
(iii) Absence of mechanical tissues, stomata and cuticle in the epidermal layer etc. are all hydro-
phytic adaptive features.
C. Anatomy of Vallisneria Leaf (Fig. 8.3):
T.S. view of the leaf of Vallisneria sp. shows the following tissue layers:
1. Epidermis:
Both surfaces of the leaf are protected by epidermal layers. Each layer is composed of parenchymatous, chlorophyll-containing cells. The epidermal layers are devoid of cuticle and stomata.
2. Mesophyll:
It is composed of spongy cells, which are loosely arranged to form air cavities (aerenchymatous mesophyll).
3. Vascular Bundle:
It is situated almost in the middle of mesophyll tissue. It is covered by a distinct bundle sheath layer (parenchymatous). Vascular bundles are collateral and closed.
Adaptive Anatomical Features:
The specimen shows the feature of isobilateral leaf with some hydrophytic characters.
The reasons are:
(a) Epidermal layers are not distinguishable and devoid of stoma and cuticle.
(b) Presence of spongy mesophyll with air cavities.
(c) Presence of ill-developed xylem tissues in vascular bundles.
D. Anatomy of Nymphaea Petiole (Fig. 8.4):
A T.S. view through the petiole of Nymphaea sp., shows the following tissue regions:
1. Epidermis:
It is composed of parenchymatous tissue. It is single-layered with multicellular hairs.
2. Cortex:
It is distinguishable into thick-walled hypodermis and parenchymatous ground tissue with air-chambers scattered all over. Trico-sclereids are present, particularly within the air-chambers.
3. Vascular Bundles:
There are a number of vascular bundles scattered all over the ground tissue. Each bundle is composed of xylem and phloem — arranged in collateral fashion. Xylem is ill-developed. Vascular bundles are devoid of cambium, i.e., closed type.
4. Pith:
It is completely absent.
Adaptive Anatomical Features:
The specimen shows the features of hydrophytic plants due to the following reasons:
(i) Presence of thin-walled parenchymatous epidermis without cuticle.
(ii) Presence of air-chambers scattered all over the ground tissue.
(iii) Presence of trichosclereids in the ground tissue.
(iv) Presence of simple vascular bundles scattered all over ground tissue.
E. Anatomy of Eichhornia Petiole (Fig. 8.5):
In T.S. of Eichhornia petiole, the following tissue layers are noticed:
1. Epidermis:
It is single-layered, parenchymatous and devoid of cuticle, stomata and hairs.
2. Cortex:
It is distinguishable into a few layered hypodermis and parenchymatous ground tissue. There are a number of air-cavities in the ground tissue.
3. Vascular Bundles:
The vascular bundles are scattered in the ground tissue. They are very simple, collateral to concentric and closed type. Xylem tissue is very poorly developed.
4. Pith:
It is not at all recognizable.
Adaptive Anatomical Features:
The specimen shows the features of hydrophytic plants.
The reasons are:
(i) Presence of single-layered, cuticle and stomata-free epidermis.
(ii) Cortex is devoid of mechanical tissues.
(iii) There are a number of air-cavities in the ground tissue.
(iv) Vascular bundles are poorly developed and closed type.
F. Anatomy of Pneumatophore of Rhizophora sp. (Fig. 8.6):
In T.S. through the pneumatophore of Rhizophora sp., the following tissue layers are noticed:
1. Epidermis:
It is single-layered and cuticularised. It is ruptured at various points due to lenticle formation after cork development.
2. Cortex:
It is composed of parenchymatous cells with frequent air-chambers. In the peripheral layer, periderm is formed due to extrastelar cambial activity. Periderm is composed of cork layer and phellogen layer. Pheloderm layer is not distinguishable. Inner cortex is deliminated by endodermis, which encircles the stele.
3. Vascular Bundles:
In young stage, vascular bundles are of collateral and open type and arranged in a ring. Xylem is endarch. During secondary growth a cambial ring is formed and then a concentric ring of vascular tissues are formed, where xylem tissues are located inside the cambial ring and phloem tissues outside. Pericycle layer is distinct with a continuous ring of sclerenchyma cells.
4. Pith:
It is distinct and composed of parenchymatous cells.
Adaptive Anatomical Features:
The specimen shows the typical anatomical feature of stem with air- breathing properties.
The characteristics are:
(i) Presence of collateral open type vascular bundles arranged in a ring, which is enclosed by a pericycle layer.
(ii) Presence of parenchymatous cortex with air-chambers for air storage.
(iii) Endodermal layer distinct and composed of barrel-shaped cells.
(iv) Presence of lenticle after secondary growth of pneumatophore for air passage. The pneumatophores are negatively geoptropic, anatomically it bears the features of stem of dicotyledons.
G. T.S. of Casuarina Stem (Fig. 8.7):
In T.S. through stem of Casuarina sp. following tissue layers are noticed:
1. Epidermis:
It is composed of single-layered parenchymatous epidermis with cuticle. It is distinctly ridged and furrowed with sunken stoma in the furrow regions. There are long multicellular hairs in the furrow region.
2. Cortex:
It is composed of sclerenchymatous hypodermis, and compactly arranged thin-walled angular chlorophyll containing ground tissue (palisade assimilatory tissue), where leaf trace bundles are scattered. There are scattered storage tracheids adjoining leaf trace bundles.
3. Vascular Bundle:
The bundles are arranged in ring fashion at the central region of the stem. Each bundle is of collateral open type with distinct bundle cap (sclerenchymatous).
4. Pith:
It is distinct and parenchymatous.
Adaptive Anatomical Features:
The specimen shows the following xerophytic adaptive features:
(i) Presence of cuticularised epidermis, and sunken stomata in the furrows.
(ii) Presence of epidermal hairs in the furrows to reduce stomatal water loss.
(iii) Presence of hypodermal scierenchyma tissue showing its mechanical rigidity.
(iv) Presence of palisade assimilatory tissue in the cortical region, indicating the assimilatory properties of stem.
(v) Presence of collateral, open type vascular bundles arranged in a ring at the central region of the stem. This is a feature of typical dicotyledonous stem.
H. Anatomy of Aerial Root of Orchid (Fig. 6.9):
In T.S. the aerial root of orchid shows the following tissue layers:
1. Velamen:
It is multiseriate and composed of compactly-set dead cells, which often form a silvery outer coat. The walls are usually porose and fibrose, so that the cells work like a sponge. The outermost layer is known as limiting layer.
2. Cortex:
The outermost layer of cortex consists of a row of thick-walled cells forming an exodermis. There are some thin-walled cells (passage cells) in the exo-dermal layer. The bulk of the cortex, however, is composed of parenchymatous cells with scattered air-chambers. The innermost layer of cortex is endodermis which contains both passage cells and thick-walled cells.
3. Stele:
Vascular tissues are radially arranged without the pericycle layer. A good number of xylem and phloem groups occur alternately in the stele. Conjunctive tissues surrounding the phloem groups are sclerenchymatous. Xylem is arranged in exarch manner.
4. Pith:
It is distinct and parenchymatous.
Adaptive Anatomical Features:
The specimen reveals the features of monocotyledonous root with some adaptive features meant for absorbing moisture from air:
(i) Presence of velamen layer at the outer surface of the aerial root. During dry weather this layer remains filled with air and during rains it quickly absorbs moisture.
(ii) Presence of thick- and thin-walled cells in the endodermal layer.
(iii) Presence of several arches of xylem and phloem arranged alternately in radial fashion within the stele.
(iv) Presence of air-chambers in the cortex for storage of air.
(v) Xylem tissue arranged in exarch manner.