Short essay on inflorescence and flower

The shoot of angiosperm plant may have two distinct parts: vegetative and reproductive. Generally, the reproductive part is identified as flower.

The reproductive’ part of the shoot may bear one or many flowers. The group of flowers present on a shoot transforms the shoot into an axis with flowers (flowering axis). Such a flowering axis is known as inflorescence.

INFLORESCENCE:

In a plant, after the full development of the vegetative organs the reproductive phase sets in. The floral buds are developed in the reproductive phase. If the floral bud is single and develops into a flower it is called solitary.

It may either be developed at the leaf axis or at terminal part of the plant. In former case, it is said to be solitary axillary; while in the latter, it is termed as solitary terminal. But in many cases a cluster of flowers develop on an axis, known as floral axis.

A collection of flowers on the floral axis is called inflorescence or anthotaxi. The floral axis bearing the flowers is known as peduncle or rachis and the stalk of individual flower is called pedicel.

The inflorescence consisting of a collection of flowers may be different depending on the mode of arrangement of flowers- pedicel late or sessile; length of the floral axis; and sexuality of the flower. Mainly, there are two types of inflorescences: racemose and cymose.

In some cases, special cymose and mixed racemose and cymose types of inflorescences are marked.

RACEMOSE:

In this case, floral axis never terminates by a flower. The growth of floral axis is continuous or indefinite.

The arrangement of flowers is acropetalous, i.e., the older flowers remain towards the base while younger flowers are found towards the apex. The opening of flowers is centripetal. The basal or marginal older flowers open first which is followed by the upper or centrally placed flowers.

There are various types of racemose inflorescences in which floral axis is elongated or shortened or flattened being concave or convex. The flowers may be pedicel late sessile. Racemose inflorescence is of the following types.

(A) Main axis elongated:

(a) Flowers pedicellate:

(i) Raceme:

The main axis is elongated and the flowers are borne laterally. All the flowers a; pedicel late and borne acropetally. The lower flowers in the rachis have longer pedicel; whereas, the pedicels of upper flowers are gradually shorter. This type is seen in man; plants such as Brassica, Raphanus, Crotalaria, Cleome, etc.

When the main axis is branched and the branches bear the flowers as in raceme it is called compound raceme or panicle. These are found in Caesalpinia, Yucca, Litchi, Andrographis paniculata, etc.

(b) Flowers sessile

(ii) Spike:

It is similar to raceme, excepting that the flowers are sessile. Examples: Achyranthes (Flg.11.1.3), Piper longum, tuberose (polyanthes), Adhatoda vasica, etc.

In case the floral axis is branched and sessile flowers are borne on these branches as in spike then the inflorescence is called compound spike as seen in Amaranthus.

(iii) Spikelet or Locusta:

In the spikelet type of inflorescence, it is subtended by two bracts or glumes at the base. It consists of one too many flowers. Each flower is borne in the axil of a bract called lemma and a bracteole called palea.

In rice, each spikelet is composed of one flower only and whole inflorescence is like a panicle. So it is called panicle of spikelets. In wheat, the multi flowered spikelet on a un branched axis, looking like a compound spike called spike of spikelets, are rseen. Panicle of spikelets is also found in many grasses.

(iv) Strobile:

It is a type of spike consisting of sessile flowers borne on the main axis. Each flower is borne in the axil of a persistent membranous bract, e.g., Humulus lupulus.

(v) Catkin or Amentum:

It is a spike in which the floral axis is thin and weak. So the whole inflorescence is a drooping one (hanging downward). The inflorescence axis is long and pendulous and the flowers are unisexual, e.g., Morus, Acalypha. Sometimes, the pendulous axis is erect as in the male and female catkins of Salix.

(vi) Spadix:

The floral axis is thick and fleshy and the flowers are covered by one or several brightly colored bracts, called spathe, e.g., Colossian esculanta, Amorphophallus titanium.

In Colocasia the female flowers are borne towards the base of the axis. Above the female flowers some sterile flowers are borne followed by the male flowers.

The top of the axis is without flowers and is known as appendix. When the axis of spadix is branched and sessile flowers are borne on these branches as in spadix, it is called Compound spadix, e.g., coconut.

B. Main axis is shortened:

(vii) Corymb:

Here, the main axis is short. The flowers have unequal pedicels. The lower flo have longer pedicels than the upper ones so that all the flowers remain more or less same level, e.g., Iberis, Prunus cerasus. Compound corymb is seen the inflorescence of Pyrus terminalis.

(viii) Umbel:

It is racemose type in which main axis is shortened. The flowers are borne at tip of the axis. The flowers have more or less equal pedicels and appear to form cluster.

This type of umbel is called simple umbel, e.g., Centella asiatica generally the umbel is branched and the branches arise from a single point with whorl of bracts known as involucres.

These branches bear flowers in umbel like man this type of umbel is known as compound umbel, e.g., Coriandrum Foeniculum, Daucus carota and other members of the family Umbelliferae (Apiaceae). The involucres of the secondary umbels are called involucels.

(ix) Capitate:

The rachis is shortened. Large number of sessile flowers grow from the suppress floral axis appearing like a globose structure. Capitate type of inflorescence is seen in many leguminous plants like Mimosa, Acacia, Albizia, etc.

C. With main axis flattened to form receptacle:

(x) Capitulum or head or anthodiu:

The floral axis is very much flattened forming a convex receptacle. The flowers are arranged on the receptacle. A large number of sessile flowers (called florets) are borne on the flattened axis (receptacle). The arrangement of flowers is in centripetal order.

The whole inflorescence is surrounded by whorls of bracts (involucre), e.g., sunflower, marigold, and all members of the family Compositae (Asteraceae).

In some of this type of inflorescence the marginal flowers differ from the central flowers and are known as ray florets and disc florets, respectively. But in Chrysanthemum, the florets are of same type (homomorphic). Compound capitulum consists of several capitula, is found in Echinops echinatus of Composite family.

CYMOSE:

The floral axis terminates with a flower. So, the growth of the floral axis is chi and the inflorescence is limited. The arrangement of flowers is basipetalous, i.e. terminal flower is the oldest and younger flowers are found below it. Where the rat is flattened, the ordor of opening of flowers is centrifugal.

Solitary axillary flowers in Hibiscus and Datura are placed under cymose inflorescence. Cymose is grouped under the following types.

(i) Uniparous or monochasial cyme:

The inflorescence axis terminates with a flower. From the base of the flower 01 one lateral branch comes out which also terminates by a flower. Depending on the branching, it is of following two types.

(a) Scorpioid cyme:

In this case, the axis terminates in a flower. The successive lateral branch develop in a zig-zag-manner on alternate side, each of which also ends in a flower, e.j Ranunculus bulbosus, Heliotropium indicum, etc.

(b) Helicoid cyme:

The floral axis ends in a flower. The successive lateral branches develop on the! Same side forming a helix, e.g., Myositis palustnis, Juncus, Begonia, etc

(ii) Biparous or dichasial cyme:

The main axis terminates in a flower. Two lateral branches develop on the two sides below the terminal flower of the main axis, each one being terminated by flower. The lateral branches may again branch similarly on two sides in biparous manner, e.g., Ixora, Jasminum, Nyctanthes, Clerodendron and Dianthus,

(iii) Multi parous or polychasial cyme:

In this case, the main axis ends in a flower. It produces more than two lateral branches below the base of the terminal flower, each terminating in a flower. Here the middle or central flower opens first, e.g., Calotropis.

Hypanthodium of fig (A) External view (B) Longitudinal section

(ii) Mixed spadix:

The floral axis is elongated like racemose and covered by spathe like bracts as in spadix. But in this case, the individual group of flowers present at each node shows cymose arrangement, e.g., Musa.

Cymose umbel:

In this case, it looks like a simple umbel of racemose inflorescence but on closer observation, it is found that the umbel is formed by the grouping together of a number of smaller cymose inflorescences, e.g., Allium cepa, Crinum, Lantana, etc.

Cymose corymb:

Here, a number of smaller cymose clusters are arranged at the same level in a corymbose manner, e.g., Alstonia scholaris, Holarrhena antidys.entrica, Oldenlardia corymbosa, Ixora parvifolia.

(v) Thyrsus:

A number of cymose clusters borne acropetally (characteristic feature of racemose) on an unlimited axis gives rise to thyrsus type inflorescence, e.g., grape vine.