BIFACIAL LEAF:
It is also called dorsiventral leaf. This leaf is more strongly illuminated on upper surface than on lower surface. This unequal illumination induces a difference in the internal structure between upper and lower sides. Its internal structure shows:
(1) Upper Epidermis
This is single layer of cells with thick cuticle which checks excessive evaporation of water from the surface. It does not contains chloroplasts, stomata are also absent.
(2) Lower Epidermis
This is also single layer but with a thin cuticle. It bears many stomata, two guard cells of which contain some chloroplast, none are present in the epidermal cells. Internal to each stomata is large cavity called respiratory cavity. Lower epidermis of leaf is meant for exchange of gases (oxygen and carbon dioxide) between the atmosphere and the plant body. Excess water also evaporates from lower epidermis.
(3) Mesophyll
The ground tissue lying between upper epidermis and the lower one is known as mesophyll. It has palisade parenchyma and spongy parenchyma.
(a) Palisade parenchyma consists of one to two or three layers of elongated cylindrical cells closely packed with long axes at right angels to the epidermis. The cells contain many chloroplasts and manufacture sugar and starch in the presence of sunlight.
(b) Spongy parenchyma consists of oval rounded or irregular cells loosely arranged towards the lower epidermis enclosing numerous large intercellular spaces and air cavities. They fit closely around the vein or vascular bundle. Cells contain few chloroplasts. Spongy cells help diffusion of gases by empty spaces left between them. They manufacture sugar and starch to some extent.
(4) Vascular Bundles
Each vascular bundle (vien) consists of xylem towards the upper epidermis and phloem towards the lower. Xylem has various kinds of vessels (annular and spiral), trachieds, wood fibres and wood parenchyma. Xylem conduct and distributes water and raw food material to different parts of leaf blade.
Phloem consists of some narrow sieve tubes, companion cells and phloem parenchyma. Phloem carries prepared food material from leaf blade to growing and storage regions.
Around each vascular bundle is compact layer of thin walled cells called border parenchyma or bundle sheath. Sclerenchyma occurs as a sheath around bigger bundle or as patches with xylem and phloem.
ISOBILATERAL LEAF:
It is equally illuminated on both sides, (isos = equal, bio = two, lateris = side).
It is also called unifacial or isolateral leaf. Stomata are present in both the epidermal layers and may be called amphistomal leaf. Mesophyll is not differentiated into palisade and spongy tissue but consists of parenchyma cells with chloroplast and inter cellular spaces e.g. maize,.
Upper and lower epidermis in Monocot leaves though are similar in structure, former shows greater variation in structure. It is made up of thin walled compactly arranges cells. In maize bamboo and in many grasses in addition to normal cells there are present groups of bigger cells called bulliform cells or motor cells.
In water plants cells of upper and lower epidermis have chloroplasts. Epidermal cells may also contain wax. In date palm more than one layered epidermis is present.
Mesophyll layers in most of monocot leaves form spongy mass of chlorophyll cells and leaves form spongy mass of chlorophyll cells and have a continuous communication system of intercellular spaces to the outside through the stomata.
On one or both sides of veins there may be present sclerenchymatous patches that continue upto epidermis and are called bundle sheath extensions. Vascular bundles or veins in monocot leaves are arranged in parallel manner. In most cases all the veins are of equal size or they may differ in size in some cases. They have common structure being conjoint collateral and enclosed by a bundle sheath. Xylem is towards the upper side and phloem on lower side. In some cases bundle sheath is 2 layered of which inner layer is sclerenchymatous e.g. wheat.
.jpg)