INTRODUCTION TO FLOWER STRUCTURE
INFLORESCENCE
1. Racemose: Inflorescence with monopodoal branching i.e., the inflorescence axis continues to grow and gives off lateral and axillary flowers.(i) Raceme: Racemose inflorescence having a common axis and stalked flowers arranged in acropetal succession, e.g., Brassica.
(ii) Panicle: Axis of raceme is branched.
(iii) Spike: Inflorescence is similar to raceme but the flowers are sessile: e.g., Achyranthes aspera.
(iv) Spikelet: Very small spike of a compound spike forming a unit; e.g., grasses.
(v) Catkin (or Ament) : A pendulous spike or spike-like inflorescence consisting of small unisexual flowers ; e.g., Morus alba.
(vi) Spadix : A spike with thick and fleshy axis covered by a large spathe ; e.g., Maize.
(vii) Corymb : It is a modified raceme with relatively a short main axis, in which the lower flowers have much elongated pedicels so that the flowers come to the same level ; e.g., Candytuft.
(viii) Umbel : It is a modified raceme, in which the flowers have stalks (pedicels) of nearly equal length and they seem to arise from the same point at the apex of peduncle, e.g., onion.
(ix) Compound umbel : An umbel with branched axis, each unit umbel is called umbellule ; e.g., Coriander.
(x) Capitulum (Head) : A dense concave, convex, spherical or flat-topped inflorescence of numerous sessile flowers clustered on a common disc or receptacle. A capitulum is usually subtended by an involucre of bracts ; e.g., Compositae (Asteraceae) family, e.g., sunflower.
(i) Uniparous (monochasial) : The main axis ending in a flower producing only one lateral branch at a time, each terminating into a flower.
Scorpoid : A uniparous cymose inflorescence where the successive lateral branches develop alternatively on both sides forming a zigzag; e.g., Heliotropium.
Helicoid : A uniparous cymose inflorescence whose successive lateral branches develop towards one side of axis forming a unilateral spiral coil ; e.g., Hamelia patens.
(ii) Biparous (dichasial) : The main axis ending in a flower producing a pair of lateral branches which too, terminate into a flower ; e.g., Ixora, Saponaria.
(iii) Multiparous (polychasial) : The main axis ending in a flower producing more than two lateral branches which too, terminate into a flower ; e.g., Calotropis.
(iv) Glomerule : A very dense single cyme with central flower blooming first.
3. Special types:
(i) Cyathium : A small, cup-shaped, special type of inflorescence found in Euphorbia with single female (pistillate) flower in the centre and several male (staminate) flowers around it.(ii) Verticillaster : A false whorl, composed of a pair of opposed cymes arising in axils of opposite leaves, as in labiatae (Lamiaceae).
(iii) Hypanthodium : A cup-like, fleshy receptacle bearing flowers on the inner wall of the cavity as in Ficus.
(iv) Solitary terminal : Flower borne singly at the apex : e.g., Poppy.
(v) Solitary axillary : Flower borne singly in the axil of a leaf ; e.g., Cotton, Cucurbita.
FLOWER
The following terms are used in connection with the description of flower :Pedicel : Stalk of an individual flower.
Pedicellate : A flower having a pedicel (stalk), e.g., one of the florets of wheat.
Sessile : A flower without a pedicel (stalk). e.g., one of the florets of wheat
Staminate : An unisexual flower with stamens. e.g., pearlmillet, maize.
Pistillate : An unisexual flower with carpels. e.g., maize.
1. Presence of floral whorls :
(i) Complete. A flower having all the four whorls, i.e., calyx, corolla, androecium and gynoecium, e.g., Cotton, Brassica.(ii) Incomplete. A flower lacking one or more whorls, e.g., Wheat.
(iii) Hermaphrodite (bisexual or perfect). A flower with both (stamens) and female (gynoecium) organs, e.g., Cotton, wheat, rice etc..
(iv) Unisexual (Imperfect). A flower having only one reproductive whorl (male or female). The unisexual flowers may be – (i) Staminate. Male flower with only androecium, e.g., maize, pearlmillet or (ii) Pistillate. Female flower with only gynoecium. e.g.,maize.
(v) Neuter. A sterile flower without androecium and gynoecium, e.g., ray florets of Sunflower.
2. Polygamous, Monoecious and Dioecious Plants (distribution of flower types):
(i) Polygamous. Plants having more than two types of flowers (i.e., bisexual, staminate and pistillate), e.g., Mango, Cashewnut, etc.
Polygamomonoecious: staminate, pistillate and perfect flowers are on the same plant.
Flower Structure
What is a flower?
A flower is a functional unit concerned with reproduction. A flower can be pictured as a very short stem (the receptacle) which holds the components of the flower in sequence at the very tip of this stem, so they appear in the centre of the flower, c/a female organs (the gynoecium). Next to them are the male organs (the androecium) and next to them are the petals (corolla) and sepals (calyx).
A complete flower is one with all parts (calyx, corolla, androecium and gynoecium) present. A flower lacking one or more of these parts is said to be incomplete.
A perfect flower is one with both androecium and gynoecium present. If either are lacking, the flower is said to be imperfect.
Gynoecium-(=pistil)-collective term for all the female reproductive organs of a flower comprising one or more free or fused carpels.
Carpel-One of the flower’s female reproductive organs comprising a stigma, a style and an ovary.
- Stigma- The receptive part of the female reproductive organ on which pollen germinates.
- Style-the elongated part of a carpel bearing the stigma, usually at the tip
- Ovary-the hollow basal region of a carpel, consisting one or more ovules.
- Ovules- the structures in the chamber of an ovary containing the egg cell, within the embryosac.The ovules develop into the seeds after fertilization.
Stamen-the male reproductive organ of a flower consisting of an anther and a filament.
- Anthers- usually bilobed, contains the pollen.
- Filament-the stalk bearing the anther
Perianth-the floral envelop usually divisible into an outer whorl (calyx) of sepals and inner whorl of petals (corolla).
Corolla-collective term for all the petals of a flower
Petal- a non-reproductive accessory organ of a flower. This is sterile and usually brightly colored, attracts insect pollinators.
Calyx-collective term for all the sepals of a flower
Sepal- a floral leaf or individual segment of the calyx of a flower, generally green, that forms the outer protective layer of a flower bud.
Receptacle- flat, concave or convex part of the stem from which all parts of a flower arise.
The term polygamomonoecious is used to describe a species population containing plants that are polygamous and plants that are monoecious. Coconut palms provide a good example of a polygamomonoecious species.
Polygamodioecious: staminate, pistillate and perfect flowers are on the different plants. Polygamodioecy ensures pollination in the absence of cross-pollination.
(ii) Monoecious. Plants with unisexual flowers and both staminate and pistillate flowers occur on the same plant, e.g., Maize, Castor, Cucurbits, etc.
(iii) Dioecious. Plants with unisexual flowers. A plant bears either staminate or pistillate flowers, e.g., Date Palm, Betel, Mulberry, etc.
3. Floral Symmetry
The shape, size and arrangement of floral appendages (i.e., calyx, corolla, androecium and gynoecium) around the axis of a flower is called floral symmetry. The axis to which the flower is attached is called mother axis. The side of flower towards mother axis is called posterior side and the side away from it is called anterior side.
On the basis of floral symmetry there may be following three conditions of a flower:
(i) Actinomorphic. A flower with radial symmetry, i.e., the parts of each whorl are similar in size and shape. The flower can be divided into two equal halves along more than one median longitudinal plane, e.g., Hibiscus, Solanum, etc.
(ii) Zygomorphic. A flower with bilateral symmetry, i.e., the parts of one or more whorls are dissimilar. The flower can be divided into two equal halves in only one vertical plane, e.g., Pisum sativum, chickpea etc.
(iii) Asymmetric. A flower which cannot be divided into two equal halves by any vertical plane, e.g., Canna.
4. Arrangement of floral organs
(i) Cyclic. The floral parts are arranged in definite whorls around the axis of flower, e.g., Brassica, Solanum, etc.
(ii) Acyclic. The floral parts are arranged in spirals and not in whorls, e.g., Magnolia.
(iii) Spirocyclic. Some of the floral parts are in whorls and others in spirals (Half cyclic), e.g., Rose, Ranunculus, etc.
5. Position of floral parts on thalamus in respect to ovary
(i) Hypogynous. The upper part of thalamus is slightly swollen and forms a cushion-like disc. The gynoecium is situated at the top of the thalamus (Thus, the ovary is superior). All other floral parts (i.e., calyx, corolla and androecium) arise below the level of ovary. The condition is called hypogyny, e.g., Citrus, Brassica, etc.
(ii) Perigynous. The upper part of thalamus may be of three types - (a) disc-shaped, (b) cup-shaped, or (c) flask-shaped. In disc-shaped thalamus, the ovary of gynoecium lies in the centre while all other floral whorls occur on the periphery but at the same level, e.g., Pea. In cup- shaped thalamus, the ovary of gynoecium arises from the bottom of cup while all other whorls arise at the rim of the cup, e.g., Prunus. In flask-shaped thalamus, ovary of gynoecium is placed at the bottom of the flask while all other floral whorls are attached at the mouth of the flask. The ovary is half-superior. The floral parts arise from around the ovary and not beneath it, e.g., Rose. The condition is called perigyny.
(iii) Epigynous. The upper part of thalamus is cup-shaped, flask-shaped or tubular in which the ovary of gynoecium is completely inserted. The wall of ovary is fused with the thalamus. All other floral whorls are borne on the upper part of thalamus. The ovary is inferior and other floral parts appear to arise above its level. The condition is called epigyny, e.g., Cucurbita.
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