A flower is a modified reproductive shoot-tip of determinate growth, bearing either only microsporophylls (stamens) or megasporophylls (carpels) or both, and may or may not be associated with accessory leaves, meant for the production of fruit and seeds.
Parts of Flower:
A flower consists of an axis known as thalamus, on which four different sets or whorls of floral members are inserted viz –
(a) Outermost, i.e., first accessory whorl of sepals = Calyx.
(b) Above the calyx, the second accessory whorl of petals = Corolla.
(c) Above the corolla, first essential whorl of stamens = Androecium.
(d) Next above, second essential whorl of carpels = Gynoecium or Pistil.
The thalamus or axis or torus is the direct prolongation of the pedicel and bears four whorls of floral members. Thalamus is also known as receptacle which is often short and suppressed, usually convex or concave.
Sometimes it is long, showing distinct internodes or it may be dilated into flat or concave structure. Sometimes thalamus presents certain modifications of forms as various outgrowths which are seen between floral whorls; such modification of the thalamus is known as disc. In case of Artabotrys somewhat elongated and conical thalamus noted bearing spirally arranged floral leaves.
- The arrangement of floral leaves or members on the thalamus is usually whorled, hence it is called whorl of sepals, petals, etc.
- Actually, accessory floral leaves are called perianth — it may be differentiated into calyx and corolla or may not. In some flowers, the calyx and corolla cannot be distinguished, in such cases the term ‘perianth’ is used. Again only one set of accessory members in some flower present which can neither be referred to calyx nor corolla – such accessory members is then termed perianth.
It forms the outermost envelope of the floral whorl and is the first accessory member, its respective leaves are called sepals which may remain free or united – the former is known as polysepalous calyx, the latter as gamosepalous calyx.
Sepals are mostly green and are always sessile. The number of sepals in a calyx whorl may vary from one to many.
It is the second accessory member of the floral whorl. Its constituent leaves are known as petals, which are delicate and coloured or white, rarely greenish.
Petals may be free or united forming polypetalous or gamopetalous corolla respectively. The petal may be sometimes stalked, the stalk of petal is called claw ; the expanded portion of the petal is called limb.
The number of petals in a corolla varies from one to many. Above mentioned two whorls (calyx and corolla) are termed accessory whorls.
The third member and the first essential whorl is the androecium ; its individual members are known as stamens.
In an androecium, the number of stamens may be one to many. The stamens may remain free or variously united or attached to other whorls.
A stamen has a slender stalk called filament, which bears at its tip anther lobes (microsporangia) containing pollen grains i.e. microspores. The filament may be short, slender or flat, or in some cases absent—then the stamen is called sessile.
GYNOECIUM OR PISTIL —
It is the topmost or centrally placed floral member and forms the second essential whorl; its individual members are known as carpels.
In a gynoecium the number of carpel varies from one to many. The gynoecium consists of a swollen basal structure — the ovary, the long filiform structure called style and the knobbed tip of the style called stigma.
The carpels may remain free or united, the former is known as apocarpous gynoecium, the latter as syncarpous gynoecium.
Functions of Floral Members:
(1) Its function is mainly protection.
(2) When coloured, it acts as flag apparatus for insect pollination. Green calyx is photosynthetic in function.
(1) The function of corolla is to attract insects i.e. the function of flag apparatus.
(2) In some cases the bases of petals act as secreting of nectar owing to the presence of glands.
(3) Secondary function of corolla is protection of the essential members.
The function of which is the production of microspores i.e. pollen grains within anther lobes.
The function of which is the production of (a) megaspore, (b) fruits, and finally (c) seed or seeds.
Different Terminologies :
A. COMPLETE AND INCOMPLETE FLOWERS
A flower is said to be complete when all four floral whorls are present, absence of any of the floral whorls renders the flower incomplete, e.g. flowers of Beta sp. is incomplete as corolla whorl is absent; unisexual flowers of Cucurbita sp. are incomplete due to the absence of one the members of essential organs.
B. ACTINOMORPHIC (REGULAR) AND ZYGOMORPHIC (IRREGULAR) FLOWERS
When sepals, petals, stamens, etc. are equal and more or less equidistant so that the flower may be cut into two equal halves through any vertical plane — the flower is called regular or actinomorphic.
When either one sepal or petal is larger or smaller or all the floral members are unequal so that the flower may be cut into two equal halves only through one vertical plane, the flower is then called medianly or symmetrically zygomorphic.
In some cases, e.g. in species of Canna, Maranta, etc. flowers can not even be cut into two equal halves along any plane also, such flowers are called asymmetrically zygomorphic. Both types i.e. symmetrical and asymmetrical zygomorphic flowers, in general are called irregular flowers.
C. BISEXUAL AND UNISEXUAL FLOWERS
When both androecium and gynoecium are present in the same individual flower, the flower is called bisexual or hermaphrodite, e.g. flowers of Cruciferae, Malvaceae, Acanthaceae, etc. ;
when either androecium or gynoecium is present in the same flower i.e. one of the essential members is absent the flower is called unisexual or diclinous, e.g. flowers of Euphorbiaceae, Cucurbitaceae, etc.
D. CYCLIC, ACYCLIC AND SPIROCYCLIC FLOWERS
A flower is said to be cyclic in which all the floral leaves i.e. sepals, petals, stamens, and carpels are arranged in the thalamus in whorled i.e. verticillate manner. Cyclic flower with long axis (receptacle) is met with in many members of the family Capparidaceae. Almost all the flowers of angiosperms are cyclic.
Acyclic flowers are rather rare, occur in very few families of angiosperms, viz. in species of Paeonia (Paeoniaceae), Eupomatia (Eupomatiaceae), Austrobaileya (Austrobaileyaceae), and Calycanthus (Calycanthaceae) of the order Ranales. In this type, all the floral leaves are arranged spirally on the thalamus.
Spirocyclic flowers are also less frequent than cyclic ones but occur in many primitive flowers, e.g. species of Nymphaea (Nymphaeaceae), Magnolia (Magnoliaceae), Ranalisma (Alismataceae), Illicium (Illiciaceae) etc. In this type, of the four sets of floral leaves, either (a) one set of floral leaves is arranged spirally and three other sets are arranged in whorl, or (b) three sets are spirally arranged and one in cyclic manner, or (C) two sets are spiral and other two are cyclic.
E. NEUTER AND NUDE FLOWERS
In neuter flowers both androecium and gynoecium are absent. In nude flowers either androecium or gynoecium, or both are present but calyx and corolla are absent. Example of neuter or sterile flowers are ray florets of many flowers of the family Compositae.
F. PISTILLATE AND STAMINATE FLOWERS
Pistillate stands for unisexual female flowers and staminate for unisexual male flowers. e.g. Cucurbita maxima.
G. DIOECIOUS, MONOECIOUS, POLYGAMOUS ETC.
Plants bearing flowers of one sex only i.e. either male or female flowers are called dioecious, e.g. Gelonium multiflorum (Euphorbiaceae), Cephalandra indica, Trichosanthes dioica (Cucurbitaceae), Borassus flabellifer (Palmae) etc.
Plants bearing unisexual flowers of both sexes on the same plant are called monoecious, e.g. Zea mays (Gramineae), Cucurbita maxima (Cucurbitaceae), Croton bonplandianum and Phyllanthus sp. of the family Euphorbiaceae etc.
Polygamous is the term applied when plants bear unisexual flowers of both sexes i.e. male and female flowers in addition to bisexual flowers, e.g. Litchi chinensis, Sapindus sp. etc. of the family Sapindaceae.
H. PERIANTH, TEPALS ETC.
In a true sense calyx and corolla are also designated as perianth. In dicotyledonous flowers, perianth is differentiated into outer green calyx and inner non-green corolla.
If any one of the accessory floral leaves are present it is also designated as perianth. If the perianth is sepal-like, it is called then sepaloid perianth and if petal-like, it is petaloid perianth.
When the components i.e. sepals and petals or calyx and corolla whorls respectively are alike in size, form and colouration, they are called tepals, e.g. Liriodendron tulipifera (Magnoliaceae), Lilium sp. (Liliaceae), and many flowers belonging to monocotyledons.