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Angiosperm Morphology

Introduction

     Angiosperms are a group of flowering plants that represent a major community in the plant kingdom. Angiosperms occur in all typical habits and constitute a very useful group of plants for animals and human beings. They include about 2,50,000 species distributed all over the world. They are the most highly evolved group of plants and appeared on Earth about 130 million years ago.

Root System

     Root is the descending portion of the axis of the plant. It is not normally green in colour. It is characterised by the absence of nodes and internodes.

Modifications of Roots

     Roots sometimes have special functions to perform and in such cases their form and structure differ from those of normal roots.

The Shoot System

     Shoot is the ascending portion of the axis and develops from the plumule of the embryo. It consists of the main axis or stem, branches and leaves. The stem is provided with nodes and internodes which may not be distinct in all cases. Leaves and branches develop from the nodes. The portion between two successive nodes is called an internode. The terminal or apical bud lies at the apex of the stem whereas the axillary bud is present in the axil of a leaf and the stem.

Stem Modifications

     Generally, the stems are aerial and grow above the soil surface. Sometimes, the stem becomes variously modified to perform special functions like perennation, vegetative propagation and food storage. The modified stems may be grouped under three heads viz, underground, subaerial and aerial.

Underground Stem Modifications

     The underground stems, by being situated below the surface of the soil, protect themselves against unfavourable conditions of weather and the attack of animals, and serve as store houses for reserve food, and in vegetative propagation. Their stem nature can be distinguished by the presence of nodes and internodes, scale leaves at the nodes, axillary buds in axils of scale leaves and a terminal bud. Further, the anatomy of the underground stem resembles that of an aerial stem. The underground stems are of four types namely rhizome, tuber, bulb and corm.

Subaerial Stem Modifications

     The runner arises from the base of the stem as a lateral branch and runs along the surface of the soil. It develops distinct nodes and internodes. At each node, the runner produces roots below and leaves above. In this way many runners are often produced by the mother plant and they spread out on the ground on all sides. If any accidental injury results in the separation of a runner, the severed parts are capable of leading an independent existence. E.g., Oxalis, Fragaria, Centella astatica.

Aerial Stem Modifications

     Tendrils develop as modifications of the stem in certain plants. The terminal bud gives rise to a tendril in Cissus quandrangularis and the axillary bud becomes modified into a tendirl in Passiflora.

Leaf

     The leaf is a green, flattened structure borne on the stem at the nodes and is a lateral appendage of the stem. A typical leaf is also known as the foliage leaf in order to distinguish it from other forms of leaves.

Stipules

     At the base of the petiole, in many plants, a pair of lateral appendages known as stipules are present. Leaves with stipules are called stipulate (e.g., Hibiscus) and the leaves without stipules are called exstipulate (e.g., Mango and all monocots).

Venation

     The arrangement of veins and veinlets in the leaf blade is known as venation.

Simple and Compound Leaves

     A leaf, in which the lamina is a single blade, is described as a simple leaf. It may be entire as in Mango or lobed as in Cotton. A compound leaf is one whose lamina is split into one or more distinct segments, called leaflets. The primary axis on which the leaflets are arranged is called rachis.

Pinnately Compound Leaf and Palmately Compound Leaf

     A pinnately compound leaf is one in which the midrib (rachis) bears laterally a number of leaflets. The arrangement of leaflets is either alternate or opposite. Following types can be recognized.

Palmately Compound Leaves

     A palmately compound leaf is one in which all the leaflets arise from a common point at the tip of the petiole.

Phyllotaxy

     The pattern of arrangement of leaves on the stem is called phyllotaxy.

Modifications of Leaves

     A normal leaf is thin, flat and green and performs the function of photosynthesis. In some plants certain special functions are performed by leaves, which become modified.

Insectivorous Leaves

     A few plants capture insects and absorb nitrogenous food from their bodies. Such plants are described as insectivorous plants or carnivorous plants. In these plants the leaves are modified for the purpose of capturing insects.

Inflorescence

     Flowers are the reproductive structures in angiosperms. Each flower is a modified shoot adapted for purposes of reproduction. Flowers may occur either singly or in clusters. When clustered, the flowers are generally arranged on branches, which are quite distinct from the vegetative branches. Such a floral branch system is known as inflorescence.

Racemose Inflorescence

     In racemose inflorescence the main axis is capable of continuous growth and it does not end in a flower. The flowers show acropetal succession on the main axis. The older flowers are towards the base and younger flowers are towards the apex. Sometimes the main axis becomes shortened and the flowers become clustered. In such cases the flowers are said to show Centripetal succession. Here, older flowers are towards the margin and younger flowers are towards the centre.

Cymose Inflorescence

     In cymose inflorescence, the main axis ends in a flower since the peduncle stops growing. The flowers show basipetal succession. The oldest flower is at the apex and youngest towards the base. When the flowers are clustered the arrangement is said to be centrifugal, Here, the oldest flower is at the centre and younger flowers are towards the margin. There are different types of cymose inflorescence.

Special Types of Inflorescence

     Here, the inflorescence is highly reduced. It has a cup shaped receptacle formed by the fusion of five bracts. On the surface of this cup, generally a yellow coloured nectar secreting gland is present. A single female flower arises from the centre of the cup. It is highly reduced and is borne on a long stalk. The male flowers are highly reduced and are produced in a scorpioid manner from the axil of each bract e.g. Euphorbia pulcherrima.

Flower

     The angiosperm plants produce characteristic structures called flowers for the purpose of reproduction. A flower is defined as a modified vegetative shoot meant for sexual reproduction. The flower consists of a very short axis on which whorls of different parts arise.

Parts of a Flower

      The Calyx forms the outermost whorl in the flower. Its component parts are the sepals. The sepals are generally green in colour. Sepals give protection to the inner parts of the flower when the flower is in bud condition. In some plants like Fuchsia and some Salvia species the calyx becomes brightly coloured and it is called petaloid.

Parts of a Flower - Continued

     Androecium is the male reproductive part and constitutes the third whorl in the flower. It is formed of one to many stamens. Each stamen consists of a filament and an anther at its tip.

Pollination

     Pollination is defined as the transference of pollen grains from the anther of a flower to the stigma of the same flower or another flower belonging to the same species. Two types of pollinations have been broadly recognised.

Cross Pollination or Allogamy

     Cross pollination is defined as the transference of pollen grains from the anther of a flower to the stigma of another flower of a different plant belonging to the same species.

Fertilization

     Fertilisation is the fusion of the female gamete (Egg or Ovum) produced in the embryosac and the male gamete produced in the pollen tube by the pollen grain.

Embryology

     It is one of the branches of biology which deals with the formation and development of the embryo. The events from initiation of the flower buds to be full development of the embryo are generally considered under embryology.

The Fruit

     After fertilisation the ovary develops into the fruit and the ovules into seed. The ovary wall develops into the fruit wall called the pericarp. In some cultivated varieties of plants like bananas, grapes, apples etc., the ovary may develop into fruit without fertilisation. As a result of this ovules do not develop into seeds. Thus the fruits produced become seedless. Such fruits are called parthenocarpic fruits. The pericarp may be thick or thin. When it is thick it may show two or three layers the outer epicarp, the middle mesocarp and the inner endocarp. Sometimes the pericarp is not differentiated into three layers.

Simple Fruits

     The fruit which develops from the syncarpous ovary of a flower with or without accessory parts is called a simple fruit. In simple fruit, the pericarp may be fleshy and juicy or it may be hard and dry. The former are called fleshy fruits and the latter dry fruits.

Aggregate Fruits

     A fruit that develops from the apocarpous ovary of a flower is called an aggregate fruit. It is a collection of simple fruits (fruitlets) all of which develop from the apocarpous ovaries of the same flower. After fertilisation, each carpel develops into a fruitlet and hence there will be as many fruitlets as there were carpels in the flower. Such an aggregate of simple fruits is also known as an etaerio.

Multiple or Composite Fruits

     It develops from the hypanthodium inflorescence. After fertilisation the hollow receptacle becomes fleshy. e.g., Ficus.

Summary

     The angiosperm plant body consists of an underground root system and an aerial shoot system. The shoot system contains vegetative parts and reproductive parts.



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