Spongy - In some aquatic plants such as water hyacinth, it becomes puffy and spongy, thus providing the plant with vitality (Fig. 3C). Lancet - Lancet-shaped, wider in the middle and narrower at the base and apex as in Nerium (Fig. 4C). Obovate - Broadest at the apex and tapering gradually towards the base as in jackfruit (Fig. 4K).
Runcinate - Lamina with triangular apex and small lateral lobes towards the base, as in Sonchus (Fig. 4R). Cuneate - Wedge-shaped, wider at the apex and narrower at the base, as in Pistia (Fig. 4S). Retuse - A broad apex with an apical notch as in Pistia (Fig. 6H). ix) Truncation - When the apex is cut abruptly, as in Paris polyphylla (Fig. 6I).
Imparipinnate - Petals are borne in unequal numbers and the rachis terminates in a leaflet as in rose (Fig. 8B). Insectivorous plants - In Nepenthes, the lamina changes into a pitcher and the tip of the leaf into the cap (Fig. 9H). Whorl : When at each node more than two leaves occur in the form of a whorl or cycle (Fig. 10D) e.g.
Alternate or spiral : When a single leaf arises at each node and the leaves are spirally arranged on the stem (Fig. 10A).
Early differentiation - As a result of continued cell division in cells of surface and subsurface layers, the leaf primordium protrudes from the shoot apex as a leaf buttress
The initial pericytes are also followed by further divisions of adjacent and underlying cells, ultimately leading to elevation of the leaf primordium above the surface of the shoot apical meristem. There was a search for the genes whose expression correlates with specific features of apical meristem functioning such as the initiation of leaf primordia. In maize, the knotted 1 gene is expressed in the corpus and not in the peripheral zone, whereas in Arabidopsis the knotted 1-like gene is expressed in the peripheral zone and not in the central zone of the shoot apex.
The cells with similar genetic constitution show different gene expression, but all cells are in constant communication with each other. The cells communicate to coordinate their activities in response to changing conditions such as darkness, light, temperature, etc. Much less is known about intracellular signaling mechanisms involved in cell communication in plants.
Of the various signaling pathways suggested, cell surface receptor kinases (eg CLAVATA in Arabidopsis) are said to be used in cell-to-cell communications. Early differentiation - As a result of continuous cell division in the cells of the surface and underground layers, the leaf primordium emerges from the shoot tip as a leaf buttress.
Origin and differentiation of tissues of the lamina - A strip of cells beneath the protoderm called adaxial meristem is often responsible for an increase in thickness of the
The regular arrangement of cell layers is interrupted by the development of the vascular bundles. Venation development - In dicotyledonous leaves, the procambium differentiates from the midvein into the leaf axis during the early stages of lamina development. Internally, the leaf consists of the dermal system, the ground tissue system and the vascular tissue system.
The compact arrangement of cells and the presence of cuticles and stomata are the main features of leaf epidermis. The leaf tissue that lies between the upper and lower epidermis excluding the veins is known as mesophyll. The palisade cells are located near the upper (adaxial) surface of the leaf where they receive more sunlight and perform the function of photosynthesis.
In some plants (e.g. Thymelea) the palisade parenchyma is found only on the underside (abaxial) side of the leaf. In certain plants (e.g. Dianthus caryophyllus), palisade parenchyma is present on both sides of the leaf. In leaves with parallel veins (most monocotyledons), all veins may have the same or different thickness.
The leaf veins are continuous through the petiole with the stem vein. Phloem transports prepared food from the leaves to other parts of the plant. Some structural changes appear in a certain area at the base of the leaves and eventually disappear.
Roots make up the lower part of the plant axis, which is green and typically grows against gravity. The main root, which is a direct extension of the radicle, is called the taproot. It is the sub-distal region of the root that constitutes small, densely colored, isodiametric cells that are compactly arranged.
The cells of the endoderm and pericycle divide rapidly, and an outgrowth emerges in the form of a knot-like nodule on the surface of the root. Such an apical cell was observed to be present in the root and shoot tips of most vascular cryptogams.
Both the root cap and root body are derived from a single apical cell and this type is commonly present in vascular cryptogams
The three histogens were named as dermatogen, periblem and pleroma which gave rise to the epidermis, cortex and central cylinder respectively. The histogen theory has received much criticism, mainly directed at the implied predestination of tissues to specific histogens. Virtually sharp distinction between dermatogen and periblem and between periblem and pleroma was not seen.
In some roots, a fourth histogen, calyptrogen, was also added, and much later the term columelogen was added to describe roots where the columella shows independent initials, ontogenetically unrelated to the marginal part of the root cap. Schüepp (1917) proposed the Korper-Kappe concept in relation to the cell pattern at apical meristems in roots. If the head of the letter T faces towards the tip of the root or the center of the root, it is called a Korper pattern, and if the head of the letter T faces away from the center of the root, it is called a Kappa pattern.
He described that at the distal tip of the root body there is a central region of cells that divide rarely or not at all. The peripheral cells surrounding this group are meristematic and considered part of the promeristem. Later, many researchers supported the presence of a resting center at the root pole, but it was also suggested that this is not a permanent feature of all roots.
Experiments with radioactive spores, irradiation of root tips, differential staining, counting of mitotic figures, etc. Clowes (1961) also put forward the concept of promeristem and described it as that part of the root apex which is capable of giving rise to all the tissues of the root. Four types of root apes can be recognized based on the mode of origin of the cap and relationship between initials/histogens and primary tissues of the root.
Here two groups of initials are present at the root pole, the inner group forming the plerome while the outer forming the periblem and cap. Thus the root cap and cortex have
In this type there are present three groups of initials arranged in three tiers at the root pole. The innermost tier forms the plerome, the middle one forms the periblem and the outermost
Here, there are three or four groups of initials arranged in three or four tiers. The innermost tier gives rise to the central cylinder and the second tier below this forms the cortex (in
In hydrophytes such as Hydrilla, etc., the parenchyma cells in the cortex have well-developed air spaces or air cavities which remain filled with air. Thick-walled parenchyma is found in the endosperm of Phoenix dactylifera, Asparagus and Coffea arabica. Parenchyma cells associated with xylem and phloem help in the conduction of water and food.
This tissue is present in the peripheral parts of the plant so it resists the bending and pulling action of the wind. In some plants collenchyma regains the power of division, e.g. in those plants where the cork cambium develops in the collenchymatous hypodermis. The average length of fibers is 1–3 mm in angiosperms. hemp, flax, jute, ramie, etc. the fibers are of excessive lengths from 20 mm to 550 mm. These long and thick-walled cells are used as fibers for commercial use.
They are usually found in the cortex and pith, but are also common in leaves, fruits and seeds of gymnosperms and dicots. Tracheary elements - Conduction in the xylem is carried out by two types of tracheary elements - tracheids and vessels. In most cases, the perforations in the sieve plates are surrounded by a carbohydrate of unknown chemical composition called the callose.
Companion cells and sieve tubes have close cytoplasmic connections through numerous pits and plasmodesmata present in the common walls. Secondary tissues can be of two types - vascular tissues, formed from vascular cambium in the stellate region, and cork and phelloderm, formed from phellogen in the extastelar region. The activity of these initials results in the formation of two types of systems in the secondary xylem, the axial (vertical) and radial (horizontal) systems.
Counting is done from the basal part because the number of annual rings in the upper part of the plant is smaller. Wood of dicotyledonous plants with vessels in the xylem is known as porous wood or hard wood. When vessels exist in the wood and show a ring-like arrangement in cross-section, the wood is said to be ring-porous.
Usually the cork cambium is differentiated in the epidermis, but sometimes it differentiates into the epidermis (apple), pericycle (Clematis) or phloem (Vitis). To replace it, some pores develop in the form of raised spots on the surface between the dead cells of phelem.
