In this article we will discuss about the arrangements of tissues in leaves.
Leaves are generally flat organs and to differentiate the two surfaces different terms are used. The surface that faces the stem upwardly is designated as upper surface or adaxial surface whereas the other surface is the lower or abaxial surface. Anatomically a leaf comprises the usual three main tissues systems-the dermal system, the ground tissue and the vascular system.
The dermal system consists of upper and lower epidermises, the ground tissue comprises the mesophyll and the vascular system is represented by vascular bundles that form the veins in a leaf.
A cross section of leaf reveals the followings (Fig. 18.3):
(i) Upper or adaxial epidermis;
(ii) Mesophyll where at the central region the vascular bundles appear to be embedded, and
(iii) Lower or abaxial epidermis.
Epidermis (abaxial and adaxial):
Usually the epidermis is uniseriate, but the leaves of Ficus, Piper and Nerium have multiseriate epidermis where the number of layers of cells varies from two to 16. It is regarded that the multiple epidermis protects the inner mesophyll from desiccation. In the leaves where both epidermises are multiseriate the adaxial epidermis consists of more number of layer of cells than abaxial one.
In multiple abaxial epidermis a sub-stomatal cavity is present (e.g. Piper), which is a space between the guard cells and mesophyll. In Nerium abaxial epidermis is multiseriate and the stomata are present in stomatal crypts only where the epidermis is uniseriate.
Stomata may occur either on the adaxial (e.g. Nymphaea) or abaxial (e.g. Ficus) or both surfaces. Their positions in relation to other epidermal cells may be either at the same level or raised above or sunken (e.g. Hakea) or restricted to stomatal crypts (e.g. Nerium).
Usually epidermal cells are devoid of chloroplastids except the guard cells, but in aquatic plants like Ranunculus, Phyllospadix etc. chloroplastids occur in the epidermal cells. In the epidermis glandular or non-glandular hairs, crystals, cystoliths may occur. The epidermal cell walls usually contain cutin, occasionally lignin and wax.
Ground tissue of a leaf is the mesophyll that lies internal to epidermises. Mesophyll consists of parenchyma cells. In mesophytic dicotyledons these parenchyma cells are of two types: palisade parenchyma and spongy parenchyma.
In transverse section of leaf the cells of palisade parenchyma are cylindrical, elongated or rod shaped. They are densely packed together and may be arranged in one or more rows. In longitudinal section of a leaf these cells appear as spherical and either separated or slightly attached to each other.
Each palisade cell contains abundant chloroplasts. The palisade cells are usually present immediately below the adaxial uniseriate or multiple epidermis, but sometimes the epidermis and palisade may be separated by the presence of hypodermis. Though palisade occurs on the adaxial side only, the leaves of Thymelaea have abaxial palisade.
Palisade parenchyma is present on both adaxial and abaxial sides in the leaf of Dianthus, Atriplex, and Artemisia etc. In these plants spongy parenchyma occurs in the form of strips at the centre of lamina. A leaf is said to be isobilateral or isolateral when palisade parenchyma occurs on both adaxial and abaxial surface.
Usually monocotyledonous leaves belong to this category, and the abaxial palisade cells are less developed. The cylindrical leaves of Hakea have palisade tissue situated all round the periphery. When the palisade parenchyma occurs on the adaxial surface only the leaf is termed as dorsiventral or bifacial.
Spongy mesophyll occurs in the abaxial surface of a dorsiventral leaf and may be of various shapes. They may be elongated resembling the palisade or spherical or irregular. The cells contain abundant chloroplastids and are arranged in such a manner that there occur large intercellular spaces.
Vascular bundles are situated at the central region of a leaf and they have inverse orientation of vascular tissues in contrast to stem. Xylem and phloem occur respectively on adaxial and abaxial surface.
In some monocotyledons (Cyperus, Saccharum, Zea etc.) the vascular bundles are surrounded by parenchyma cells that are morphologically distinguishable from adjacent mesophyll cells. They are usually larger, thick walled and may contain fewer chloroplasts.
This sheath of parenchyma around the vascular bundle of a leaf is termed as bundle sheath. These cells are concerned in conduction and storing of food materials.
The bundle sheath is regarded either as endodermis due to occasional occurrence of casparian strips or as starch sheath, since these cells sometimes contain starch. Later study with improved techniques reveals that the C4-dicarboxylic acid pathway, known as Hatch-Slack pathway occurs in the cells of bundle sheath.