NEET Biology Notes Anatomy of Flowering Plants Tissue system
On the basis of their structure and location, there are three types of tissue systems. These are epidermal tissue system, the ground or fundamental tissue system and the vascular or conducting tissue system which are described below :
- Epidermal Tissue System
The epidermal tissue system forms the outermost covering of the whole plant body and comprises epidermal cells, stomata and the epidermal appendages, i.e. the trichomes and hairs.
Epidermal cells are parenchymatous with a small amount of cytoplasm lying below the cell wall. The stomatal aperture, guard cells and the surrounding cells all together called stomatal apparatus. Guard cells in dicots are kidney-shaped and in monocots (grasses) are dumbell-shaped.
Epidermal cells in some monocot leaves, become larger thin-walled, having vacuole and are called bulliform cells.
These cells bring about rolling of leaves during dry season, thus reducing rate of transpiration,
e.g. Ammophila. The trichomes in the shoot system usually multicellular. They prevent water loss due to the transpiration. The cells of epidermis bear a number of hairs.
- Ground Tissue System
All tissues except epidermis and vascular bundles constitute the ground tissue. It consists of simple tissues such as parenchyma, collenchyma and sclerenchyma. In leaves, the ground tissue consists of thin-walled chloroplast containing cells and is called mesophyll.
- Vascular Tissue System
The central column of root or stem is called stele which is made up of a number of vascular bundles.
It consists of phloem and xylem. In dicot stems, cambium is present between phloem and xylem. Such vascular bundles because of the presence of cambium possess the ability to form secondary xylem and phloem tissues, hence are called open vascular bundles.
In monocot stems, the vascular bundles have no cambium, hence they do not form secondary tissues, so they are referred to as closed. When xylem and phloem within a vascular bundle are arranged in an alternate manner on different radii, the arrangement is called radial, such as in roots.
Anatomy of Stem and Root Anatomically, stem and root of dicot and monocots are very different. The tissue organisation of stems and roots can be studied better and conveniently by studying the internal structure.
- Anatomy of Leaves
Based on the internal structure, arigiospermic leaves can be dorsiventral (bifacial), isobilateral (equifacial) and concentric (unifacial).
- Anatomy of Dicot Leaves
Most of the dicot leaves are dorsiventral having reticulate venation, in which, mesophyll is differentiated into palisade and spongy parenchyma. Palisade is usually restricted towards dorsal (upper) surface. The spongy parenchyma lies below the palisade parenchyma and above the lower epidermis
- Anatomy of Monocot Leaves
Most of the monocot leaves are isobilateral, in which, there is no differentiation of mesophyll into palisade and spongy parenchyma. The leaves usually show parallel venation (veins running parallel to each other).
- Concentric leaves are cylindrical, which show no distinction of dorsal (upper) or ventral (lower) surface.
- Multilayered epidermis
Begnonia, Nerium, etc. is found in the leaves of Ficus, The multiple epidermis of Peperomia has as many as 14-15 layers.
Lithocytes are the specialised epidermal cells containing cystoliths, the crystals of calcium carbonate. Lithocytes frequently found in Apocyanaceae, Acanthaceae, Moraceae
(e.g. Ficus), Cucurbitaceae and Urticaceae families.
- Secondary Growth
Secondary growth is the growth in the girth of stems and roots in dicots produced by division of secondary meristem, resulting in woody tissues. The cambium is involved in secondary growth.
The increase in the diameter or thickness is due to the formation of secondary tissues as a result of the activities of primary and secondary lateral meristems, namely vascular cambium (fascicular cambium) and cork cambium (phellogen) respectively.
The secondary ‘growth begins by the formation of a continuous cambial ring. In stems, there is already a fascicular cambium between the xylem and phloem of the vascular bundles, which becomes joined up by interfascicular cambium.
- Cork Cambium
The cork cambium or phellogen cut off cells both outer side and inner side. The cells cut off on outer side are phellem or cork cells and that of inner side are phelloderm or secondary cortex. Phellem, phellogen and phelloderm collectively constitute periderm.
The phellem or cork cells are dead, which have deposition of a fatty substance called suberin (i.e. cork cells are suberised). Suberin is impervious to water. The commercial cork is obtained from the cork tissue of Quercus suber, which yields bottle cork.
- Vascular Cambium
The meristematic layer that is responsible for cutting off vascular tissues, i.e. xylem and phloem is called vascular cambium. In young stem, it is present in patches as a single layer between the xylem and phloem. Later, it forms complete ring.
- Secondary Xylem and Phloem
The cells of the cambium ring divide periclinally and results in new cells both outside and inside. The cells that cut of out side constitute secondary phloem, while inner side secondary xylem. Secondary growth is the characteristic feature of dicots. It remains absent in monocots.
- Anomalous Secondary Growth
In some plant like Aristolochia, etc. Secondary growth does not occur as described above. The deviating types of secondary growth are called anomalous secondary growth. Anomalous or abnormal secondary growth is found in some monocot stems such as Yucca, Dracaena, Aloe, Agave, Sensiviera, etc.