Flowering plants or angiosperms are characterized by the enclosure of seeds within the fruit. Angiosperms (Gr., angeion = case, sperma = seed) are thus, closed seeded plants. They are the most highly evolved plants which bear flowers. Angiosperms are divided into dicotyledons and monocotyledons.
Morphology is the branch of science which deals with the study of external form and structure of living organisms. A critical study of the external morphology of higher plants is necessary in order to describe the plants, so as to distinguish them. A typical flowering plant has a root, stem, leaves, flowers, fruits and seeds.
The Root:
It is the non-green underground descending part of the plant body. In majority of dicot plants, the root system is formed by the direct elongation of the radicle into the primary root. Branches of the primary root are called secondary roots. These in turn branch to form tertiary roots.
The tip or apex of the main root is covered by a root cap, which is protective in function. Roots possess unicellular root hairs which help in increasing the surface area for absorption of water. The primary root and its branches constitute the tap root system. E.g. – mustard (Brassica).
In monocot plants, the radicle grows for some time and later dies. Then numerous roots arise from the base of the stem forming an adventitious root system as in grass, wheat, etc.
The Stem:
The shoot system or the stem is the main ascending axis of the plant. It develops from the plumule and epicotyl of the embryo. In flowering plants, it bears leaves, branches, flowers and fruits. It has nodes and internodes, even if it happens to be an underground stem.
Characteristics of the Stem:
1. The stem is the ascending part of the plant that grows away from the soil (negatively geotropic) and towards light (positively phototropic).
2. The stem apex bears a terminal bud. Stem apex is always apical in position.
3. The stem is differentiated into nodes and internodes.
4. Leaves and branches arise at the nodes on the stem. They are exogenous in origin.
5. Stem bears multicellular stem hairs called trichomes on its external surface.
The Leaf:
The leaf is a green, flat, thin, expanded lateral appendage of the stem which develops at a node and bears a bud in its axil. It is exogenous in origin. The green colour of the leaf is due to the presence of the chlorophyll pigment. Leaves help the plant to synthesize organic food by the process of photosynthesis.
Parts of a Leaf:
A leaf has three main parts: namely leaf base, petiole and lamina. In addition to this, leaves possess two lateral outgrowths called stipules at their bases.
Leaf-Base:
It is the basal part of the leaf by which it is attached to the stem. In monocots the leaf base is said to be sheathing, as it expands and partially or wholly surrounds the stem. In dicots the leaf-base bears two lateral outgrowths known as stipules. In Leguminosae, the leaf-base is swollen and is known as pulvinus. Leaves with stipules are described as stipulate and those without them are termed as exstipulate.
Petiole:
It is the stalk of the leaf. A leaf with a petiole is called petiolate and one without it is called sessile. A long petiole pushes out the leaf blade so as to provide it with sufficient sunlight.
Leaf Blade or Lamina:
It is the green expanded part of the leaf. It has a prominent median vein called the mid-rib which is continuous from the base to the apex. It produces thinner lateral veins which in turn branch to form veinlets. The lamina is the seat of photosynthesis, gaseous exchange, transpiration and other metabolic activities.
The Inflorescence:
The flower which represents the reproductive structure of a plant may occur individually or in clusters. When they occur in clusters, they are found on branches that are quite distinct from other vegetative branches. Such a branch with a cluster of flowers is known as inflorescence. The main axis of inflorescence is called peduncle. Individual flowers are attached to the peduncle, usually with a stalk of their own, called pedicel. Stalked flowers are described as pedicellate while, flowers without a stalk are called sessile.
Depending on the mode of branching, the different kinds of inflorescence are as follows:
(A) Racemose inflorescences.
(B) Cymose inflorescences.
(C) Special types of inflorescences.
The Flower:
The flower is a modified specialised shoot meant for sexual reproduction.
Parts of a Typical Flower:
The flower has a stalk called the pedicel. The tip of the pedicel is called as a thalamus or torus. In a typical flower, the thalamus is a slightly swollen (conical or convex in shape) structure. The calyx, corolla, androecium and gynoecium (pistil) are developed successively on it from the outside towards the centre.
The lower or outer two whorls comprising of the calyx and corolla are called as the accessory or non-essential whorls. They do not directly take part in reproduction. Androecium and gynoecium are described as essential whorls. They directly take part in reproduction. Seed setting is not possible without these whorls.
Position of the floral parts on the thalamus:
The different whorls of the flower arise on the thalamus.
There are three types of conditions based on the relative position of the first three whorls with respect to the ovary i.e.:
(a) Hypogyny:
In a hypogynous flower, the thalamus is conical, convex, flat or slightly concave and the ovary occupies the topmost position on the thalamus, while the stamens, petals and sepals are separately and successively inserted below the ovary. The ovary in a hypogynous flower is said to be superior, e.g., Brinjal, China rose.
(b) Epigyny:
In an epigynous flower, the margin of the thalamus grows further upward, completely enclosing the ovary and getting fused with it. The calyx, corolla and androecium arise above the ovary. The ovary in this case, is described as inferior, e.g., sunflower, cucumber.
(c) Perigyny:
In a perigynous flower, the margin of the thalamus grows upward to form a cup like structure, enclosing the ovary. On the rim of the cup are present all the other floral parts. Such an ovary is said to half-inferior (half-superior), e.g., plum (Prunus), rose.
Symmetry of a Flower:
Actinomorphic:
A flower is said to be symmetrical when it can be divided into two exactly equal halves by any vertical section passing through the centre of a flower. Such a flower is said to be regular or actinomorphic. E.g., mustard, Datura.
Zygomorphic:
Whenever a flower can be divided into two identical halves through only one particular vertical plane it is said to be zygomorphic. E.g., Ocimum, Cassia.
The Fruit:
The stimulus of fertilization brings about several changes in a flower. Usually all the parts of the flower wither away, except the ovary. The ovary enlarges and ultimately becomes the fruit which encloses the seeds.
A fruit is defined as a fertilized mature ovary. The fruit consists of two parts – the fruit wall called pericarp (ovary wall) and the seeds which are developed from the ovules. The pericarp generally consists of three parts—the outer part called epicarp, which forms the skin of the fruit; middle mesocarp and the inner part called endocarp.
A true fruit is one which is developed from the ovary. E.g., orange, tomato, pea. A false or spurious fruit (pseudocarp) is one in which other parts of the flower such as the thalamus; receptacle or calyx may also grow and form a part of the fruit. E.g., apple and pear.
Fruits can be broadly classified into three groups: simple, aggregate and multiple or composite fruits.
The Seed:
The aim of fertilization is to produce a seed. These seeds grow into new plants. Seeds are the most efficient means of propagation in plants.
After fertilization, the ovule or megasporangium becomes the seed. This is followed by increase in size of the ovule. Then, the integument slowly dries up and forms the protective seed coat. The outer integument forms the testa and the inner one forms the tegmen.
Physiological activities in the embryo are reduced to a minimum and the whole structure becomes dormant and dry. It is now called as a seed. Therefore, a seed is a mature integumented megasporangium.
Angiosperms are divided into monocotyledons and dicotyledons based on the number of cotyledons present in their seeds.
Based on the consumption of the endosperm by the embryo, there are two types of seeds in angiosperms:
(i) Exalbuminous or Non-Endospermic Seeds:
If the embryo completely fills up the seed consuming both endosperm and nucellus, the seed is said to be exalbuminous. Since endosperm is not present, food for the embryo is stored in the cotyledons. E.g., gram, pea, country bean.
(ii) Albuminous or Endospermic Seeds:
If the embryo has not completely consumed the endosperm, the seed is said to be albuminous. Stored food remaining in the endosperm would be consumed in the germinating stage. E.g., rice and maize.
Both dicotyledonous and monocotyledonous plants may be albuminous (endospermic) or exalbuminous (non-endospermic) according to presence or absence of endosperm.
Semi – Technical Description of a Typical Flowering Plant:
The plant is described beginning with its habit, vegetative characters – roots, stem and leaves and then floral characters, calyx, corolla, androecium gynoecium, fruit and seed in a proper sequence. The description has to be brief using technical words. The description is to be followed with floral diagram and floral formula.
Floral Diagram:
Floral diagram is the diagrammatic representation of the floral characters as seen in different transverse sections from base to top of the flower bud, and all these brought to the same level.
1. The axis to which the flower is attached is shown by a small circular dot or cross (posterior side of flower).
2. Bract is shown opposite to the axis (anterior side of flower) by an arc.
3. The bracteoles (if present) are drawn along both lateral sides by arcs.
4. Calyx is drawn as the outermost whorl by arcs representing the number of sepals. Corolla is drawn as the next whorl of arcs representing petals. The arcs representing sepals are shown with a midrib (an outward projection in the middle of the arc) whereas the arcs representing petals are without a midrib. If sepals and petals are free, the arcs are shown free. If they are fused, the margins of the arcs are joined.
Aestivation is shown by proper representation at the margins. Petals are shown alternating with sepals. The position of odd sepal and petal are shown in relation to the axis and bract. If perianth is found instead of calyx and corolla tepals are shown in a similar way as petals.
5. Stamens are shown inner to the whorl of petals in as many whorls as in the flower. If anthers are dithecous they are shown as a four lobed structure. If monothecous, shown as two lobed. Stamens if fused are joined by a line. If epipetalous joined to petal by a line, lntrorse, extrorse are also represented by showing the smaller lobes of anther towards inside or outside respectively.
6. Gynoecium is drawn in the centre of the floral diagram. If apocarpous, ovaries are shown by separate circles; syncarpous ovary is shown as in T.S. of ovary. The features like placentation, number of locules, number of ovules, presence of disc, etc., can also be shown in the floral diagram.
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