The following points highlight the four essential accessory whorls of a flower. The essential accessory whorls are:- 1. Calyx 2. Corolla 3. Androecium 4. Gynoecium.
Accessory Whorls # 1. Calyx:
The calyx is the outermost accessory set of floral leaves, consisting of sepals. Sepals are usually green inconspicuous bodies. They protect the essential whorls against mechanical injuries and desiccation in the bud stage.
So, their function is mainly protective. Sepals of some plants become coloured like the petals and are called petaloid, e.g. Caesalpinea (B. Krishnachura). In Mussaenda one of the sepals is transformed into a flat leaf-like structure (Fig. 88). In the sunflower family the sepals are modified into hairy bodies called pappus which help in the dispersal of fruits.
If the sepals are free the calyx is called polysepalous, and they are united the calyx is gamosepalous. The gamosepalous calyx is more or less like a tube in which the number of sepals may be easily determined by counting the teeth or lobes. In many cases sepals fall off before the flowers fully open out. They are called caducous sepals, e.g. Poppy; while in others they fall off with other parts after fertilisation.
These are deciduous sepals. In some flowers sepals persist even up to the fruit stage and are called persistent, e.g. brinjal, tomato. Sometimes they not only persist but grow to form a part of the fruit, as in Dillenia (B. Chalte), Physalis (B. Tepari). These are called accrescent sepals.
Accessory Whorls # 2. Corolla:
It is the second accessory whorl of the flower, the individual parts of which are called petals. They make the flowers showy and attractive, as the petals are brightly coloured and often sweetly scented.
The function of corolla is twofold:
Firstly, the protection of the essential whorls in the bud stage; and
Secondly, the attraction of insects for pollination.
The petals of Polyalthia (B. Debdaru), custard apple, etc., are called sepaloid, as they are green and sepal-like in appearance. If the petals are free, the corolla is called polypetalous; and if they are united, it is gamopetalous.
The free petals often have two parts—a narrow stalk called claw and the upper flat blade known as limb. Petals having claws are said to be clawed, as in pink, radish, mustard (Fig. 89, 1). The corolla is regular, if all the petals are similar in size and shape; if they are dissimilar, it is irregular.
Forms of Corolla:
Different forms of corolla are found in flowers. They may be free or united, regular or irregular. It is, by no means, easy to give a precise account of all the forms found in nature.
The following are the principal and more common forms (Fig. 89):
A. Polypetalous Regular:
It consists of four free clawed petals arranged in the form of a cross (Fig. 89, 1). Examples—mustard, radish. It is characteristic of the mustard family (Cruciferae).
It has five clawed petals, the limbs of which lie more or less at right angles to the claws, e.g. pink (Fig. 89, 2).
It is composed of five petals without claws, e.g. roses, tea (Fig. 89, 3).
B. Polypetalous Irregular:
It looks more or less like a butterfly (papilion=butterfly). It consists of five free petals. The largest and the outermost one is called standard or vexillum; two lateral ones are known as wings or alae; and the innermost two, called keel or carina, unite to form at boat-shaped body. Examples—- pea (Fig. 89, 4 & 5), beans, Sesbania (B. Bak). It is characteristic of the pea family (Papilionaceae).
C. Gamopetalous Regular:
Here the petals unite to form a tube-like body having lobes or teeth at the tip, by counting which the number of petals is determined, e.g. disc florets of sunflower, marigold (Fig. 89, 6).
Five petals unite to form a bell-shaped corolla, as in gourd (Fig. 89, 7), Physalis (B. Tepari).
It consists of five petals which unite to form a funnel-shaped corolla gradually spreading towards the tip, e.g. Datura (Fig. 89, 8), Thivetia (B. Kalke).
Here the tube is short and the spreading limbs are arranged more or less at right angles to the tube, giving the corolla wheel-shaped appearance (Fig. 89, 9), e.g. brinjal, Nyctanthes (B. Shieuli).
It is like the rotate one but with a comparatively long tube (Fig. 89, 10), e.g. Vinca (B. Nayantara).
D. Gamopetalous Irregular:
1. Bilabiate or Two-Lipped:
Here the petals by union form two parts with an open wide mouth between them. The lower part is made of three petals and the upper of two (Fig. 89, 11), e.g. Ocimum (B. Tulsi), Leonurus (B. Drone).
It is like the bilabiate one but the mouth is closed, as the two lips are placed very near each other, e.g. snapdragon (Fig. 89, 12), Lindenbergia (B. Basanti).
3. Ligulate or Strap-Shaped:
The five petals are united in such a way as to form a short tube at the base, but they spread out above like a strap, e.g. ray-florets of sunflower, marigold.
Appendages of the Corolla:
The corolla is often provided with spurs or long projections, as in Balsam (B. Dopati), and outgrowths, called corona, which enhance the beauty of the flowers. Corona is present in passion-flower (B. Jhumko) as a beautiful whorl of outgrowth between the corolla and the androecium.
Nectaries, the honey-secreting glands, are sometimes present at the base of the petals. These are natural extra-devices for attracting insects for pollination.
Aestivation is the mode of arrangement of sepals and petals in the bud stage in respect to one another (Fig. 90).
It is of the following types:
(i) Valvate, when the members either touch or lie side by side very closely but do not overlap, as in Calotropis (B. Akanda), Artabotrys (B. Kantali champa).
(ii) Twisted or contorted, when the members are arranged in such a way that each of them is overlapped at one margin and overlaps the next member by the other margin, as in China-rose.
(iii) Imbricate. It is modification of twisted arrangement where one member is completely external, i.e. overlaps by both the margins, one member is completely internal, i.e. is overlapped at both the margins and the rest remain arranged as in twisted aestivation, e.g. Cassia (B. Kalkasunde).
(iv) Vexillary: Here out of five petals the largest is the outermost. It includes two laterals which, in their turn, enclose the innermost two. It is characteristic of papilionaceous corolla like pea, Sesbania (B. Bak).
The accessory whorls calyx and corolla are often referred to as perianth. In monocotyledons the perianth is not differentiated into two sets, as in tube-rose, Crinum (B. Sukhadarsan), etc. If the lobes are free, the perianth is polyphyllous and, if united, it is gamophyllous. In grasses the perianth is represented by a pair of small scales called lodicules.
Accessory Whorls # 3. Androecium:
Androecium is the third set and the first essential whorl of the flower. It is composed of stamens or microsporophyll’s which are male organs. A stamen has usually two parts, viz. a slender stalk called filament and a swollen body at the tip of the filament known as anther.
The anther has usually two lobes on the two sides; each lobe, in its turn, has two bag-like bodies, called pollen-sacs or microsporangia, which contain innumerable small unicellular pollen grains or microspores.
In China-rose one pollen-sac is present in each anther lobe. The prolongation of the filament which holds together or connects the two anther lobes is called connective. Vascular bundle runs through the connective to convey food to the growing anther.
All stamens do not always bear fertile anthers. In Cassia sophera (B. Kalkasunde) four out of ten stamens are sterile. They are called staminodes. In Canna (B. Sarbajaya) staminodes are coloured and petal-like in appearance. Outgrowths on the stamens, called staminal corona, are found in Calotropis (B. Akanda).
Attachment of the Filaments to the Anthers:
The stamens are of the following types according to the modes of fixation of anthers to the filaments:
(i) Basifixed or innate, when the filament is attached just to the base of the anther, as in mustard (Fig. 91).
(ii) Adnate, where due to the presence of a well-marked connective the filament seems to run throughout the whole length of the anther, e.g. Michelia (B. Champaka).
(iii) Dorsifixed, when the filament is attached to the back of the anther, as in passion-flower, Sesbania (B. Bak).
(iv) Versatile, when the filament is attached at the back of the anther by a sharp point, so that the anther may freely swing in air, e.g. grasses, paddy.
Dehiscence of the Anthers:
With the maturity of the anthers the two pollen-sacs coalesce due to the fact that the wall between them breaks up. Pollen grains then remain scattered in the cavity.
The anther dehisces or bursts to liberate the pollen grains, so that they may be carried by different agencies to the stigmas of flowers. Dehiscence takes place mostly by longitudinal splitting, as in Datura, by pores formed at the apices of anthers, as in brinjal, or by a number of valves (valvular), as in cinnamon.
Relative Length of the Stamens:
The stamens of a flower may be of equal or unequal length. Ipomoea (B. Kalmi) has five stamens, all of which are of unequal length. Flowers of mustard family have usually six stamens in two whorls.
Two of the outer whorl are short and four of the inner whorl have long filaments. They are called tetradynamous stamens (Fig. 91). Leonurus (B. Drone), Ocimum (B. Tulsi), etc., have four stamens, of which two are long and two short. They are called didynamous stamens.
Union of the Stamens:
Stamens may unite amongst themselves or with other whorls. The union of similar members is called cohesion; and that of dissimilar members is spoken of as adhesion.
Cohesion of the Stamens:
As the stamens have two main parts —filaments and anthers, two conditions in cohesion are possible:
(i) Stamens may unite by filaments, anthers remaining free; or
(ii) They may be united by anthers, filaments remaining free (Fig. 92).
The union by filaments results in the formation of bundle or adelphy. If one such bundle is formed by union of filaments, the stamens are called monadelphous, as in China- rose, cotton.
If two bundles are formed, they are diadelphous, as in pea, Sesbania (B. Bak); and when more than two bundles are formed, they are called polyadelphous, as in silk-cotton. On the other hand, when they unite by the anthers, filaments remaining free, the stamens are called syngenesious, as in gourd, sunflower.
Adhesion of the Stamens:
The stamens may unite with the next lower whorl, i.e. corolla or perianth, or with the next upper whorl, the pistil. When they unite with petals, stamens are called epipetalous.
Almost all the flowers with gamopetalous corolla have epipetalous stamens. If they adhere to perianth lobes, stamens are called epiphyllous, as in tube-rose. The stamens are called gynandrous, when they are attached to the pistil e.g. Calotropis (B. Akanda).
The pollen-sacs or microsporangia of the anthers contain some special cells called pollen-mother cells or microspore-mother cells. Each of them undergoes reduction division to produce a tetrad of pollen grains or microspores. These are small dusty unicellular bodies usually round or oval in shape. A pollen grain has protoplasm with a single nucleus surrounded by two coats.
The inner coat, known as intine, is thin, delicate and made of cellulose; and the outer coat, called exine, is thick, cutinised and often provided with warty or spiny projections. Some un-thickened areas or weak spots, called germ pores, are present here and there on the exine.
The uninucleate pollen grains generally become bi-nucleate before liberation from the sacs. In some flowers the pollen grains, instead of remaining loose, are aggregated together into masses called pollinia, as in Calotropis (B. Akanda), orchids.
Accessory Whorls # 4. Gynoecium or Pistil:
Gynoecium or pistil is the last and innermost set of floral leaves occupying the central position on the thalamus. It is composed of carpels or megasporophylls which are the female organs of the flowers.
A carpel has usually three parts:
(i) The basal swollen portion is the ovary,
(ii) The slender cylindrical region is the style which usually ends in the receptive spot
The ovary contains one or more egg-shaped bodies known as ovules, which are nothing but immature seeds. Pistil may be made of one, two, three or many carpels. Accordingly they are monocarpellary, bicarpellary, tricarpellary or polycarpellary. The monocarpellary pistil is called simple, while others are termed as compound.
Foliar nature of a carpel is evident in a simple pistil which may be compared to a simple leaf. If the leaf is folded along the mid-rib, the two margins meet forming a hollow cavity, which resembles a simple pistil.
The line where the two margins meet is known as ventral suture and that corresponding to the mid-rib is the dorsal suture. Along the ventral suture soft cushion-like bodies or ridges, called placentae, are formed to which the ovules remain attached.
The compound pistil is called apocarpous, when the carpels are free. Here all the carpels remain inserted on the thalamus, each having its own ovary, style and stigma, as in Michelia (B. Champa), Artabotrys (B. Kantali champa).
But if the carpels are united, the pistil is known as syncarpous where the common ovary and style are usually formed, carpels usually remaining free only in the region of stigmas, e.g. China-rose (Fig. 98), cotton, Datura.
The style normally is the slender cylindrical continuation of the ovary. In Ocimum (B. Tulsi) family the ovary is four-lobed and the style originates from the base of the ovary or from the thalamus. This type of style is called gynobasic (Fig. 93). Usually styles are deciduous, but silky persistent ones are found in maize.
The stigma is the terminal receptive spot. It may be flat and sticky due to secretion of stigmatic fluid, and feathery in case of monocotyledons. These are devices for facilitating reception of pollen grains.
The ovary may be unilocular [uni = one, loculus = chamber) or multilocular (multi = many, loculus = chamber). The ovules or mega sporangia remain attached to the cushion-like parenchymatous protuberances of the ovary, called placenta, and the mode of arrangement of the placentae is known as placentation.
Types of Placentation (Fig. 94):
Marginal placentation is characteristic of the pea family, where the pistil is simple, and placenta bearing the ovules lies at the ventral suture.
Here the pistil is polycarpellary and the placentae are distributed at the regions where the margins of the carpels meet. The ovary is unilocular and the placentae groups correspond to the number of carpels present, e.g. papaw, gourd.
In axile placentation also the pistil is polycarpellary. The margins of the carpels, instead of meeting, bend downwards to form an axis at the centre. Placentae bearing ovules are distributed around the axis. So the ovary becomes multilocular, the number of loculi being equal to the number of carpels present, e.g. orange, China-rose, Datura.
4. Free Central:
Here the placentae are present around the axis, as in axile placentation, but due to the absorption of the partition walls the multilocular ovary is rendered unilocular. Sometimes remnants of the partition walls may be noticed. Example—pink
It is also like axile having many chambers but placentae with ovules are present all over the inner surface of the partition walls, as in water-lily.
Here the ovary is unilocular, and the placenta bearing a single ovule develops directly on the thalamus at the base of the ovary, e.g. Sunflower, Dahlia.
The ovule or mega-sporangium develops into the seed after fertilisation. Each ovule is an egg-shaped body, having a stalk called funiculus, by means of which it remains attached to the placenta.
The main soft body of the ovule is known as nucellus which remains surrounded by two coats called integuments. The integuments, outer and inner, almost invest the ovule, only leaving a small opening, known as micropyle, at the tip. In some ovules only one integument is present.
The broad basal region, where the integuments meet the nucellus, is known as chalaza. An oval body, failed embryo-sac, is embedded in the nucellus towards the micropylar end. The embryo-‘sac has usually eight nuclei. Three of them aggregate at the micropylar end foiling the egg-apparatus.
The middle one of the three is the egg or female gamete, and the other two are called help cells or synergids. Three collecting at the chalazal end are known as antipodal cells; and the remaining two, called polar nuclei, either lie side by side or fuse up at the central region, forming what is known as definitive nucleus (Fig. 95). The embryo- sac is the most important part of the ovule.
Formation of the Ovule and the Embryo –Sac:
The ovule originates as a small rounded protuberance of soft tissue (nucellus) on the placenta. Gradually collar-like outgrowths or integuments begin to develop which cover the nucellus, leaving an opening, the micropyle, at the tip.
Before complete formation of the integuments a hypodermal cell at the apex of the nucellus becomes conspicuous. This is the megaspore-mother cell. It undergoes reduction division to produce four cells in a row, what is known as linear tetrad.
Three of them generally disintegrate and the innermost one survives. The nucleus of the functional mega- spore now divides by mitosis into two, which move to the two ends of the megaspore. Each of them again divides twice. So that four nuclei are present at each end.
Three of them at the micropylar end form the egg-apparatus with the central prominent egg and the two synergids. Three aggregated at the chalazal end constitute the antipodal cells. The remaining two, often called polar nuclei, move to the centre of the megaspore to produce the definitive nucleus (Fig. 96).
This is an outline of the process of normal type of development of the embryo-sac in the angiosperms.
Forms of Ovules:
Ovules are of the following types (Fig. 97):
(1) Orthotropous or straight, when the ovule is erect with the funiculus, micropyle and chalaza in the same straight line. Here micropyle lies towards the apex and chalaza towards the base.
(2) Anatropous or inverted, when the ovule is inverted due to overgrowth of the funiculus, so that micropyle lies at the base near the funiculus and chalaza at the other end. This type is more common.
(3) Amphitropous or transverse, where the main body of the ovule is placed more or less at right angles to the funiculus, e.g. Lemna (B. Kshudi-pana).
(4) Campylotropous or curved, where the main body of the ovule is curved or bent becoming horse-shoe-shaped and is arranged more or less at right angles to the funiculus, e.g. mustard.
The parts present in a flower and their arrangement with reference to one another may be represented by diagrams called floral diagrams. They have a number of concentric rings corresponding to the floral sets present, where united and free parts can be shown. The position of the floral axis is represented by a ‘dot’. The best idea about a floral diagram may be had by cutting a floral bud crosswise.
Floral formula represents the parts of a flower with their respective position. Some signs and symbols are used here. K stands for calyx, C for corolla, P for perianth, A for androecium and G for gynoecium. Union is shown by putting the letters within brackets, and adhesion, by lines above the whorls concerned.
Members of the same set present in two whorls are represented by putting ‘+’ (plus) sign between them, co stands for many members in a whorl. The superior or inferior ovary is indicated by putting horizontal lines below or above the number representing carpels.