NEET Biology Morphology of Flowering Plants (Root, Stem And Leaf) Flower

Flower

It can be defined as modified dwarf shoot which is meant for sexual reproduction. It is characteristics feature of angiosperm whease reproductive organs have been aggregated as flowers.

(1) Parts of a typical flower : It comprises a stalk called pedicel which arises in the axis of leaves called bracts. Upon the pedicel there may be one to many small scaly structures called bracteoles. The terminal part of the pedicel is the thalamus or torus. It is a modified and condensed axis of the flower. Modified leaves called floral leaves or floral parts arises from the nodes of the thalamus as successive whorls. A typical flower of an angiosperm consists of four types of floral parts namely calyx, corolla, androecium and gynoecium.

(i) Calyx : It is the outermost whorl composed of sepals.

(ii) Corolla : It is composed of petals and is the second whorl.

(iii) Androecium : It is the third whorl composed of stamens.

(iv) Gyneocium : It is the innermost whorl and is also called pistil. It shows carpels.

(2) General description of a flower : The flowers are termed pedicellate if they possess stalks and sessile if they lack them. The flower may be described as complete if it bears all the floral parts and incomplete, when one or more floral parts are absent. Flowers are called bisexual if they bear both androecium and gynoecium. The unisexual flowers have either androecium or gynoecium. The unisexual flowers may be male flowers or female flowers. The male flower are also called staminate flowers as they have stamens only. The female flowers have only the carpels and hence called pistillate flowers. Flowers with sterile sex organs are described as neutral flowers. According to the distribution of male, female and bisexual flowers, various pattern are recognized.

(i) Monoecious : Presence of male and female flowers on the same plant, e.g., Acalypha, Cocos and Ricinus.

(ii) Dioecious : Presence of male and female flowers on different plants, namely, male plants and female plants. e.g., Cycas, Carica papaya and Vallisneria.

(iii) Polygamous : Presence of unisexual and bisexual flowers on the same plant, e.g., Mangifera and Polygonum.

(3) Symmetry of flower : The number, shape, size and arrangement of floral organs in a flower determines its symmetry. On the basis of symmetry flowers can be of the following types :

(i) Actinomorphic (Regular = Symmetrical) : Actinomorphic flowers can be divided (passing through center) by any vertical plane in to two equal and similar halves. e.g., Mustard, Brinjal, Catharanthus roseus.

(ii) Zygomorphic (Monosymmetrical) : Zygomorphic flowers can be divide into two equal halves by only one verticle division e.g., Pea, Larkspur, Ocimum.

(iii) Asymmetrical (Irregular) : Asymmetrical flowers can not be divided into two equal halves by any vertical division. e.g., Canna, Orchids.

(4) Arrangement of floral organs : On the basis of arrangement of floral organs, three types of flowers are recognized. They are :

(i) Acyclic : Here the thalamus is conical or convex and the floral parts are spirally arranged, e.g., water lily and Magnolia.

(ii) Cyclic : Here the floral organs are arranged in regular whorls at the nodes of the thalamus, e.g., Hibiscus and Datura.

(iii) Hemicyclic (Spirocyclic) : Here some floral parts (sepals and petals) are arranged in regular whorls and the remaining parts (stamens and carpels) are arranged spirally. e.g., Annona  and Polyalthia.

Number of floral parts in whorl is called the merosity. There are two kinds of flowers based on the merosity of the flower. They are isomerous flowers and anisomerous flowers.

• If the number of sepals, petals, stamens and carpels of flower is equal, such flowers are called isomerous flowers.

Dimerous : Two floral parts in each whorl.

Trimerous : Three floral parts in each whorl.

Tetramerous : Four floral parts in each whorl.

Pentamerous : Five floral parts in each whorl.

• A flower with different number of floral parts in each whorl is called anisomerous flower. The sepals, petals, stamens and carpels present at different whorls of a flower vary in their numbers. These are also called heteromerous flowers.

(5) Detailed structure of flower : Angiospermic flowers exhibit many variations in their external morphological characters. Detailed description of a flower helps in its proper identification.

(i) Bract : Bract (hypsophyll) is a small leaf like structure on the peduncle which produces a flower in its axil. The floral buds are usually protected by the bracts. Flower with a bract is described as bracteate and the flower without a bract is known as ebracteate. Bracteoles are small scale like structures present on the pedicel. Bracts are modified into following structures :

(a) Foliaceous bract : Leaf like, expanded green bract is called the foliaceous bract, e.g., Pisum, Lathyrus, Adathoda and Gynandropsis.

(b) Spathe : A large modified bract which encloses spadix inflorescence totally or partially. It may be leathery or woody, e.g., Alocasia, Cocoa, Musa and Typhonium.

(c) Petaloid bract : Brightly coloured petal like bract is known as petaloid bract, e.g., Bougainvillea, Poinsettia and Euphorbia.

(d) Involucre : One or two whorls of green bracts that protect young inflorescence is called involucre, e.g., Coriandrum, Tagetes and Heracleum.

(e) Epicalyx : Whorl of bracteoles present below the calyx or outside the calyx, e.g., Hibiscus rosa sinensis and Malvaviscus arborcus.

(f) Scaly bracts : Reduced, membranous, scale like bracts seen in head inflorescence, e.g., florets in Tridax and Helianthus.

(g) Glumes : The bracts found on the rachilla of spikelet are called glumes. They may be sterile glumes or fertile glumes (lemma), e.g., Oryza sativa.

(ii) Thalamus : The terminal part of the pedicel is called thalamus or torus or receptacle. It is a condensed axis of the flower from which all floral parts arise. Depending upon the position of gynoecium on the thalamus with respect to other parts, flowers are of three kinds – hypogynous, perigynous and epigynous (See details in Embryology Module-II).

In many flowers, the thalamus is condensed and the internodes are not seen clearly. But there are some flowers with elongated, distinct floral internodes as mentioned below :

(a) Anthophore : This is the first elongated internode between the calyx and corolla, e.g., Silene, Pennsylvania and Lychnis.

(b) Androphore : It is the second elongated internode between corolla and androecium, e.g., Gynandropsis.

(c) Gynophore : It is the third elongated internode between androecium and gynoecium e.g., Capparis and Gynandropsis.

(d) Gynandrophore : It is an elongated stalk like part between non essential and essential organs of the flower. It is equivalent to androphore, e.g., Passiflora.

(e) Carpophore : This is a stalk like connection present between two carpels. It is formed due to expansion of the thalamus between the carpels, e.g., Coriandrum and Foeniculum.

(iii) Perianth : The non essential organs, calyx and corolla are together called perianth. The perianth protects the stamens and carpels. In angiospermic flowers, the perianth exists in different forms.

(a) Achlamydeous : Perianth is absent and the flowers appear naked. Mostly the achlamydeous flowers occur in cyathium inflorescence. e.g., Euphorbia, Poinsettia.

(b) Chlamydeous : Perianth is present and the flowers usually appear attractive. The chlamydeous flowers are of two types. They are :

• Monochlamydeous flowers are with perianth in one whorl, e.g., Amaranthus and Ricinus.
• Dichlamydeous flowers are with perianth differentiated into calyx and corolla. They are arranged in two different whorls. The dichlamydeous condition is of two types :
• Homochlamydeous : The two whorl or the perianth (calyx and corolla) are similar in all respects and are not identified by different colours, e.g., Michelia.
• Heterochlamyoeous : The two whorls of the perianth are dissimilar in many respects. The outer whorl consists of small, green sepals and the inner whorl with large variously coloured petals, e.g., Datura and Hibisus. The term “tepals” is used to describe the perianth lobes which appear like petals, e.g., most of the monocots.

(iv) Calyx : It is the outermost whorl of the flower. It consist of sepals. Usually, the sepals are small and green. They protect other floral organs when the flower is in bud condition. The calyx is described as polysepalous when the sepals are free (e.g., Anona, Tomato) and gamosepalous when the sepals are united (e.g., Datura and Hibiscus). If sepals are fused less than half of the length of calyx tube it is called as partite and if the fusion of sepals is very little, just at the base of calyx tube, it is said to be connate. The sepals may be deciduous or persistent. Usually the persistent calyx do not show any growth after fertilization. Such a calyx is termed as marcescent (e.g., Brinjal, Chillis). Sometimes the persistent calyx shows continuous growth even after, fertilization. This type of calyx called acrescent (e.g., Physalis and Shorea).