The leaf both morphologically and anatomically is the most variable plant organ. They have been grouped as —foliage leaves, cataphylls, hypsophylls and cotyledons. Of these the foliage leaves are the principal photosynthetic organs. The cataphylls are the scales that appear on the buds, and on underground stem for their protection.
The hyposophylls are the various kinds of bracts accompanying the flowers. Their function is protection or if coloured to provide attraction for the insects. The cotyledons are the first leaves of the plants. The floral organs are also considered as leaves.
Normally when we talk of a leaf, we mean a foliage leaf which generally has a flattened part, lamina and a stalk called petiole for its attachment to the stem. Though the leaves have lot of variability still they all have the same tissue system as the root or stem, the dermal, the ground and the vascular tissue systems. The foliage leaves usually does not have a storage tissue, a periderm and mainly have primary tissues.
On the anatomical basis the leaves may be grouped as follows:
(1) Dorsiventral leaves,
(2) Isobilateral leaves.
1. Dorsiventral Leaf:
It is also called a bifacial leaf and may be differentiated internally into a dorsal region and a ventral region. Most of the dicotyledonous leaves are of such type.
A typical dorsiventral leaf e.g., mango, in transverse section, shows the following structures:
[A] Epidermis:
It is formed of a single layer of cells, which are closely fitted and have outer thick walls. The outer wall is usually cutinized. The chloroplast and stomata generally not present.
[B] Mesophyll:
It forms the main bulk of leaf and includes parenchyma which Is differentiated into two regions:
i. Palisade Tissue:
It is made-up of one or two layers of closely arranged columnar cells. A large number of chloroplasts are present in each cell. The palisade parenchyma is present just below the upper epidermis and is meant for photosynthesis.
The palisade tissue though is compact itself remains exposed to the intercellular spaces for a considerable part to receive the proper gaseous supply. In certain xerophytic plants palisade tissue can be present on both sides of the leaf viz. Eucalyptus.
ii. Spongy Tissue:
Just below the palisade tissue there are present loosely packed, parenchymatous cells having irregular outlines with branches extending from one cell to the other. It looks like a net. The intercellular spaces are quite prominent and therefore, the name spongy parenchyma is given. These cells possess less or no chloroplast. This tissue helps in the exchange of gases between the leaf and the atmosphere.
[C] Vascular Bundle:
The vascular bundles are irregularly distributed in the spongy parenchyma. In mango and certain other cases a midrib is present. The structure of the midrib and other bundles is similar to each other. A large number of vascular strands form an interconnected system in the median plane of the blade. These vascular bundles in the leaves are known as veins.
A large number of these small veins arise from the mid vein and form a network. Each bundle is conjoint, collateral and closed. Xylem is present towards the upper epidermis and the phloem towards the lower. The xylem includes annular or spiral vessels, tracheids, wood fibres and xylem parenchyma. The entire bundle is enclosed in a parenchymatous bundle sheath, which is made up of a layer of compactly arranged cells. The cell walls of bundle sheath can become suberized or lignified also in certain cases.
[D] Midrib Region:
In the midrib region, generally in place of palisade tissue, there is present certain collenchyma followed by parenchyma above the vascular bundle. On the lower side also there is present parenchyma. Just below the upper and lower epidermis scleranchyma can also be present.
[E] Lower Epidermis:
This is similar to upper epidermis in structure. There are present numerous stomata in this layer. Each stoma is surrounded by two guard cells and is followed by a substomatal cavity. Such type of leaf having stomata only on the lower side are known as hypostomatic leaf. The guard cells of stomata may be situated either in the level of epidermal cells or can be placed in pits.
2. Isobilateral Leaf:
This cannot be differentiated internally into two regions. The structure on both the surfaces is quite similar (isos = equal; bi = two; lateris = side). It is also called a unifacial leaf or isolateral leaf. The stomata in this case are usually present in both the epidermal layers and thus, it may be known as amphistomatic leaf. The mesophyll is not differentiated into palisade and spongy tissue, but consists of parenchyma cells with chloroplast and intercellular spaces, viz., maize.
The upper and lower epidermis in the case of monocotyledon leaves, though, are almost similar in structure, the former visible greater variation in structure. It is generally made up on these walled compactly arranged cells. In maize, bamboo and good number of grasses in addition to the normal cells there are present groups of bigger cells known as bulliform cells or motor cells.
In Ammophila these cells are restricted in the depressions and help in the rolling of leaves. In water plants, viz., Potamogeton, the cells of both upper and lower epidermis have chloroplasts. The stomata, though equally distributed in both the epidermis, in floating leaves these are present only in the upper epidermis. This leaves are known as epistomatic. The epidermal cells may also be impregnated with wax. In certain cases e.g., Datepalm more than one layered epidermis is present.
The mesophyll cells in most of the monocotyledon leaves produce a spongy mass of chlorophyll bearing cells and have a continuous communicating system of intercellular spaces, to the outside through the stomata. On one or both sides of the veins, there can be present sclerenchymatous patches; that continue upto the epidermis and are called bundle sheath extensions. In certain cases, viz. Lilinm, there may be seen the differentiation of a palisade like layer on the upper surface of the leaf. The sclerenchyma patches can also be present at other places below the epidermis e.g., Datepalm.
The vascular bundles or veins in monocoryledon leaves are arranged in parallel manner. In most of the cases all the veins are of almost equal size or they can differ in size in some cases. They have a common structure, being, conjoint collateral and enclosed by a bundle sheath.
The xylem is towards the upper side and the phloem on the lower side. In certain cases the bundle sheath is 2 layered of which the inner layer is sclerenchymatous viz., Wheat.
Various isobilateral leaves are epidermis shown in Fig., 10.2, 10.3 and 10.4.
Petiole:
The petiole in transverse section visible a groove towards the upper surface (posterior side) or is flat. It resembles more or less to the stem in internal structure. The structures of epidermis, ground parenchyma, collenchyma and sclerenchyma, both in distribution and composed are similar to stem.
A typical Petiole Shows:
i. Epidermis:
It is single layered and includes elongated compact and barrel- shaped cells. Stomata may be present in it.
ii. Hypodermis:
It is usually few layered, collenchymatous and uniformly distributed. In some cases it is present only in patches and the rest part is composed of chlorenchyma.
iii. Ground Tissue:
Just below the hypodermis there is present a uniform, parenchymatous tissue with intercellular spaces; endodermis and pericycle are not differentiated. The cell can have chloroplasts.
iv. Vascular Bundles:
The bundles are arranged in a semi-circular manner and are of different sizes in the same petiole. Generally the lowest bundle is of the biggest size. Each bundle is similar to that of leaf in structure and orientation, that is, there is present xylem towards the upper or inner side and phloem on the lower or outer side. There is a great variation in the distribution of the vascular tissues within the petiole (Fig. 10.5).
The bundles frequently form a continuous or a multistranded is open towards the adaxial side of the petiole (Fig. 10.6 B.D; Olea, Euconymus, Nerium, Mahonia). They can form a circle (Ricinus, Platanus), sometimes with additional bundles within the circle outside it (Fig. 10.5 F). In some cases they can be numerous and arranged in several superposed arcs viz., Canna, or may be scattered viz., Rumex.
Description of the structure of some of the petioles is given below:
(1) Foeniculum Petiole:
It is more or less circular with a ring of vascular bundles and a cavity in the center. Hypodermis is partly chlorenchymatous and partly collenchyma (Fig. 10.7).
(2) Trapa Petiole:
Trapa is a floating aquatic monocotyledon plant.
The petiole is swollen and includes the following internal structures:
a. Epidermis:
It is single layer of parenchyma. The cuticle is lack.
b. Hypodermis:
It consists of a few layers of compact parenchyma.
c. Aerenchyma:
The hypodermis is followed by a large aerenchyma including numerous air cavities, separated by one-layered diaphragms. Here and there are present certain crystals or calcium oxalate.
Cortex is represented by one or two layers of parenchyma just outside the stele.
d. Vascular Tissue:
In the center is present a C-shaped, vascular tissue surrounded by endodermis. Below the endodermis are visible a few patches of xylem alternating with phloem. In the center is present the parenchymatous pith.
(3) Nymphaea Petiole:
Nymphaea is a free-floating hydrophyte.
The petiole is circular and visible the following internal structures:
i. Epidermis:
It is single-layered parenchyma. The cuticle is not present.
ii. Hypodermis:
It is a few layered sclerenchyma.
iii. Ground tissue:
The hypodermis is followed by a large undifferentiated parenchyma. In the center, there are present very big-sized air cavities separated by few-cell-thick diaphragms. A few sclerotic cells can be present in the cavities.
iv. Vascular Tissue:
There are present four big bundles, one each in every diaphragm. A few more bundles are dispersed in the outer part of the ground tissue. Each bundle is conjoint and collateral. The xylem cavities are quite large. Above each bundle a little of sclernchyma is present.
Development of Leaf:
The leaf is initiated by one or two periclinal divisions in one or two layers beneath the protoderm of the shoot apex slightly below the apical meristem. Then certain anticlinal divisions take place in these layers and also in the protoderm to form a protrusion known as leaf ‘buttress’. This leaf primordium then extends as a bladeless conical or peglike protuberance and is known as the leaf axis. The growth is more on abaxial side making the leaf to develop towards the shoot apex. The dorsiventral character also may be seen in its leaf primordium.
The growth in width takes place because of localization along two margins of the leaf axis. The primordium above the base differentiates into a midrib and two panels of leaf blade. Lateral veins then evolves as procambial strands from the midvein and the blade continues to widen. The apical growth of the leaf increases its length. Therefore, the growing primordium has an apical meristam at the tip and two marginal meristems along the margin of the axis.
The derivatives of the cells of these two meristems in their turn again divide and may be known as intercalary meristem. These are distributed at random and make the major contribution to the increase in size of the leaf.
Phyllode:
It is a flattened petiole and resembles in shape with lamina. It is isobilateral in structure.
Australian Acacia forms the typical instance of a phyllode and has the following internal structure (Fig. 10.11):
a. Epidermis:
It is single layered with well evolved cuticle and sunken stomata. On the margins, the cells are radially elongated.
b. Mesophyll Tissue:
This is differentiated into palisade tissue and the parenchyma. Just below the epidermis on both sides, there are present one or two layers of palisade cells for photosynthesis. In the center, a little of parenchyma is present.
c. Vascular Bundles:
The bundles are conjoint, collateral and arranged in a ring below the palisade. The marginal and central bundles are big sized and the surrounded by sclerenchyma.
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