A flower has four whorls: 1. Calyx 2. Corolla 3. Androecium 4. Gynoecium or Pistil.
Part # 1. Calyx:
It is the outermost whorl of a flower. It is made up of individual units called sepals. The sepals are usually green, resembling the leaves. They have veins and stomata like ordinary leaves, but are thicker in nature. When the sepals are free from one another, the calyx is described as polysepalous. E.g., Caesalpinia. If the sepals are partially or completely united with one another, then such a calyx is described as gamosepalous. E.g., Crotalaria.
Functions of the Calyx:
i. Sepals are protective in function and protect the other floral parts in the bud condition.
ii. Since they are green in colour they help in photosynthesis.
iii. Modified sepals perform extra functions.
a. Petaloid Sepals:
In Mussaenda, four sepals are normal while the fifth one is large and petaloid. It helps in attracting insects for pollination.
b. Pappus:
In Tridax, the calyx is modified into scaly, feathery structures called pappus. It helps in dispersal.
Part # 2. Corolla:
This is the second whorl of the flower that arises inner to the calyx. It is made up of individual units called petals. They are usually brightly coloured and fragrant. This makes them highly attractive to insects, which help in pollination.
Polypetalous and Gamopetalous Conditions:
The corolla is described as polypetalous, if the petals are free from one another. E.g., Hibiscus, gulmohar.
The corolla is described as gamopetalous, if the petals are fused with one another. E.g., sunflower, Ipomoea.
Functions of the Corolla:
i. Petals provide protection to the inner whorls of a flower.
ii. Since they are brightly coloured, they attract insects, birds, etc., which are the agents of pollination.
Aestivation:
Aestivation is the arrangement of either the sepals or the petals in a flower bud in relation to one another. It is of considerable importance in the classification of plants.
It is of the following types:
1. Valvate:
In valvate aestivation, one margin of a petal or a sepal just touches the other at the margin without overlapping. E.g., custard apple (Annona).
2. Twisted or contorted:
In twisted aestivation, one margin of a sepal or petal overlaps the next member, and the other margin is overlapped by the preceding petal giving a twisted appearance to the bud. E.g., corolla in china rose, cotton, oleander (Nerium).
3. Imbricate:
In imbricate aestivation, one of the sepals or petals is internal being overlapped on both the margins, and one of them is external and for each of the remaining petals, one margin is internal and the other margin external. E.g., Cassia, gulmohur.
Part # 3. Androecium:
The androecium is the male reproductive part of a flower. It is the third whorl in a flower that arises inner to the corolla. It is made up of stamens or microsporophylls.
Parts of a Stamen:
Each stamen is composed of a slender stalk called filament and at the tip of it, is the anther. Each anther consists of two lobes, which are connected by a tissue called connective. Each anther lobe has four pollen sacs placed longitudinally.
Each pollen sac represents a microsporangium and it contains numerous pollen grains or microspores.
A stamen is therefore a microsporophyll bearing four microsporangia (tetrasporangiate).
Monothecous and Dithecous Conditions:
Anthers with a single anther lobe bearing a single locule are said to be monothecous. E.g., members of Malvaceae.
Anthers with two anther lobes are said to be dithecous. E.g., Vinca.
Cohesion of Stamens:
Fusion of stamens among themselves is called cohesion.
It is of the following types:
(I) Adelphy,
(II) Syngeny, and
(III) Synandry.
(I) Adelphy:
It is a condition where filaments are variously fused, but the anthers are free.
Based on the numbers of bundles of stamens formed by fusion, adelphous condition is of three types:
(i) Monadelphous:
In this type, the filaments of all the stamens are fused to form a single bundle. E.g., Hibiscus.
(ii) Diadelphous:
In this type, the filaments unite to form two bundles. In members of Papilionaceae, nine stamens form one bundle and the tenth remains free as the second bundle. E.g., pea (Pisum).
(iii) Polyadelphous:
In this type the filaments are fused to form two or more than two (many) bundles. E.g., Bombax malabarica (silk-cotton tree).
(II) Syngeny:
It is a condition where all the anthers of a flower are united but the filaments are free. This condition is termed as syngenesious. E.g., sunflower (Helianthus).
(III) Synandry:
In this type, the stamens are united all along their length i.e., there is fusion of anthers and filaments. E.g., members of Cucurbitaceae.
Adhesion of Stamens:
Union of the stamens with other members like petals, tepals or gynoecium is called as adhesion.
Epipetaly:
It is a condition where the stamens are partially or completely attached to the petals. E.g., brinjal.
Part # 4. Gynoecium or Pistil:
The gynoecium or pistil is the fourth whorl of a flower. It is composed of one or more carpels or megasporophylls which bear megasporangia or ovules. Megaspores are produced inside the megasporangium.
Parts of a Carpel:
A typical carpel has three parts – ovary, style and stigma. The ovary is the lower swollen basal part which continues as a slender elongated style. The style terminates in a stigma on which the pollen grains are deposited during pollination.
Concept of a Carpel:
In the course of evolution, the leaf like carpel or megasporophyll of gymnosperms folded along its midrib and formed a chamber by the fusion of the margins. This formed the angiospermic carpel. The midrib is called dorsal suture and the marginal line along which the carpel fuses, is called ventral suture. A special tissue called placenta develops along the ventral suture. Ovules develop from this placental tissue and remain in the ovary. The cavity enclosed by the ovary wall is called locule.
Simple and Compound Pistil:
The pistil may be a simple pistil or a compound pistil.
A simple pistil is made up of only one carpel. Such a pistil is described as monocarpellary. E.g., bean, pea, Crotalaria.
A compound pistil is made up of two or more carpels. E.g., Hibiscus and rose.
Types of Gynoecium – Apocarpous and Syncarpous Gynoecium:
In a compound pistil, the carpels may be completely free from one another. Such a pistil is described as apocarpous (apo = free). E.g., Michelia and Artabotrys.
In a compound pistil, the carpels are united together. Such a pistil is said to be syncarpous (syn = together). E.g., brinjal and Hibiscus.
Types of Gynoecium Based on the Number of Carpels:
(i) Monocarpellary:
Here the pistil is made up of one carpel. Such a pistil is a simple pistil. E.g., Crotalaria, bean, pea.
(ii) Bicarpellary:
A pistil with two carpels is said to be bicarpellary. E.g., tomato and brinjal.
(iii) Tricarpellary:
A pistil with three carpels is said to be tricarpellary. E.g., Ricinus.
(iv) Multicarpellary:
A pistil with many carpels is said to be multicarpellary. E.g., orange and lemon.
Nature of the Ovary with Reference to Locule:
The cavity enclosed by the ovary wall is called a locule. The number of locules in the ovary correspond to the number of carpels in the gynoecium.
Unilocular:
The ovary is said to be unilocular, if it has only one locule. E.g., pea.
Bilocular:
The ovary is said to be bilocular, if it has two locules. E.g., tomato.
Trilocuiar:
The ovary is said to be trilocular, if it has three locules. E.g., Ricinus.
Multilocular:
The ovary is said to be multilocular, if it has many locules. E.g., orange and lemon.
Placentation:
Placenta is a tissue which develops along the inner wall of the ovary. The ovule or ovules remain attached to the placenta. The placentae frequently develop on the margins of the carpels, either along their whole line of union, called suture or at their base or apex. Placentae may also develop on a direct prolongation of the thalamus at the base of the carpel. The manner in which the placentae are distributed in the cavity of the ovary is known as placentation.
Types of Placentation:
1. Marginal Placentation:
It is seen in a simple pistil. The placenta forms a ridge along the ventral suture. The ovules are borne on this forming two rows. The ovary is superior and unilocular. E.g., garden pea (Pisum), Crotalaria.
2. Axile Placentation:
In this type, the ovary consists of two or more carpels. The ovary is divided into as many chambers as there are carpels fused together. The carpels are united in such a way that the ventral sutures of all the carpels are placed along the axis. The placenta arises on this common axis and hence it is called axile. E.g., lemon, orange, lady’s finger.
3. Parietal Placentation:
In this type, the ovary is unilocular but syncarpous. The placenta arises on the ventral suture which is the line of fusion of the adjacent carpels. In members of the family Cruciferae like mustard, radish the placentation is parietal though the ovary is two chambered. The ovary is unilocular at first, but soon a false partition wall called replum, develops across the ovary dividing it into two chambers. E.g., mustard (Brassica).
4. Basal Placentation:
In this type, the ovary is syncarpous and unilocular. The placenta develops directly on the thalamus. A single ovule is present at the base of the ovary. E.g., sunflower.
Style and Stigma:
Style:
Depending on its position, it may be-
Terminal:
Style arises from the top of the ovary. E.g., Hibiscus.
Lateral:
Style arises from the side of the ovary. E.g., Ficus.
Gynobasic:
Style arises from the depressed centre of the four – lobed ovary. E.g., Ocimum.
Stylopodium:
Disc like swelling is present at the base of the style. E.g., Coriandrum.
Stigma:
Depending on its shape it may be-
Discoid:
Disc shaped.
Plumose:
Feather like.
Dumb-bell shaped:
Hourglass shaped.
Bifid:
Two forked.
Linear:
Long and narrow.
Sticky:
Provided with sticky liquid.
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