Inflorescence:
Type 1. Racemose Inflorescence:
The primary axis of this form of inflorescence does not terminate in a flower, but continues to grow and create blooms on its lateral sides in acropetal succession (i.e., the lower or outer flowers are older than the upper or inner ones). The many types of racemose inflorescence may be divided into three categories. The following are the details:
(i) With the main axis elongated, i.e., (a) raceme; (b) spike; (c) spikelets; (d) catkin and (e) spadix.
(ii) With the main axis shortened, i.e., (i) corymb and (ii) umbel.
(iii) With the main axis flattened, i.e., capitulum or head.
(i) Main Axis Elongated:
(a) Raceme:
In such circumstances, the main axis stays extended and bears a slew of stalked blooms throughout its length. For example, radish (Raphanus sativus), mustard (Brassica campestris), and other plants, the lower or older blooms have longer stalks than the top or younger ones.
The inflorescence is known as a compound raceme or panicle when the main axis of the raceme is branched and the blooms are carried on the lateral branches, as in neem (Azadirachta indica), gul-mohar (Delonix regia), and so on.
The peduncle is the primary axis of the inflorescence, as well as any later axes that may be present. The pedicel is the stem of each individual flower in the inflorescence.
(b) Spike:
The primary axis of this form of racemose inflorescence remains elongated, and the lower flowers are older, i.e., open earlier than the higher ones, as in racemes, but the blooms are sessile, i.e., without a pedicel or stalk, as in amaranth (Amaranthus spp. ), latjira (Achyranthes aspera), and so on.
(c) Spikelets:
Each spikelet can have one to numerous blooms (florets) linked to the rachilla, which is the centre stalk. Spikeletes are grouped in a spike inflorescence, which is made up of several to many spikelets that are placed on a main axis called the rachis in various ways. Some are found in complex spikes (e.g., wheat—Triticum aestivum), whereas others are found in racemes (e.g., in Festuca).
The typical structure of a spikelet is as follows: At the base of the spikelet, there are two sterile glumes: the lower, outer glume is termed the first, and the higher, inner glume is called the second. A sequence of florets sits just above the glumes, partially enclosing them.
A lemma and palea are found at the base of each floret. The lemma is the floret’s bottom, outer bract. The inferior palea, also known as the lemma, usually has a long awn that extends from the mid-rib at the apex or rear.
The floral portions of the lemma are carried in the axil. Between the lemma and the rachilla is the palea (also known as superior palea), which has two longitudinal ridges (keels or nerves). Flowers and glumes are placed in two rows on the spikelet. Spikeletes are grasses, wheat, barley, oats, sorghum, sugarcane, bamboo, and other members of the Poaceae (Gramineae) or Grass family.
(d) Catkin:
Mulberry (Moras alba), birch (Betula spp. ), oak (Quercus spp.), and other plants have modified spikes with a long, drooping axis producing unisexual blooms.
(e) Spadix:
This is a kind of spike inflorescence with a fleshy axis that is encircled by one or more enormous, typically vividly coloured bracts called spathes, as seen in members of the Araceae, Musaceae, and Palmaceae families. Only monocotyledonous plants have this inflorescence.[Agrilearner]
(ii) Main Axis Shortened:
(a) Corymb:
The primary axis of this inflorescence stays comparably short, and the lower flowers have considerably longer stalks or pedicels than the higher flowers, bringing all of the flowers to almost the same level, as in candytuft (Iberis amara).
(b) Umbel:
The principal axis of this inflorescence is rather short, and it bears at its terminal a collection of flowers with pedicels or stalks of more or less similar lengths, causing the blooms to appear to spread out from a single point. A whorl of bracts creating an involucre is always present in this inflorescence, and each individual bloom emerges from the axil of a bract.
In coriander (Coriandrum sativum), fennel, carrot, and other plants, the umbel is branched and known as umbel of umbels (complex umbel), and the branches yield flowers. The umbel is sometimes unbranched and referred to as a single umbel, as in Brahmi (Centella asiatica). The Apiaceae (Umbelliferae) family has this type of inflorescence (umbel).
(iii) Main Axis Flattened:
Capitulum or Head:
The main axis or receptacle becomes repressed and virtually flat in this form of inflorescence, and the blooms (also known as florets) are sessile (without a stem) and packed together on the flat surface of the receptacle. On the receptacle, the florets are arranged in a centripetal pattern, with the outer flowers being older and opening earlier than the inner ones.
The florets (individual blooms) are bracteate. In addition, a set of bracts grouped in two or three whorls surrounds the whole inflorescence.
The flowers (florets) are usually of two kinds:
(i) Ray florets (marginal strap-shaped flowers) and
(ii) Disc florets (central tubular flowers).
Only tubular florets in Ageratum or ray or strap-shaped florets in Sonchus are examples of capitulums (heads) that contain only one kind of floret. Asteraceae (Compositae) plants have a capitulum or head, such as sunflowers (Helianthus annuus), marigolds (Tagetes indica), and safflowers (Safflowers) (Carthamus tinctorius). Zinnia, Cosmos, Tridax, Vernonia, and other similar plants It’s also present in Acacia and the Mimosaceae family’s sensitive plant (Mimosa pudica).
The capitulum inflorescence has long been regarded as the most ideal. The following are the reasons:
Individual flowers are tiny and clumped together in heads, so they stand out more to attract pollinators like insects and flies.
At the same time, there is a significant reduction in the amount of material used in the corolla and other floral elements.
A single insect may pollinate several flowers in a short period of time without having to travel from one to the other.
Type 2. Cymose Inflorescence:
The formation of a flower at the apex of the main axis stops its growth, and the lateral axis that generates the terminal bloom likewise culminates in a flower and stops its growth in this type of inflorescence. Pedicellate (stalked) or sessile blooms are also possible (without stalk).
Flowers develop in a basipetal order, with the terminal flower being the oldest and the lateral ones being the youngest. Centrifugal flower opening is the name given to this form of flower opening.
The cymose inflorescence may be of four main types:
(i) Uniparous or monochasial cyme;
(ii) Biparous or dichasial cyme;
(iii) Multiparous or polychasial cyme and
(iv) Cymose capitulum.
(i) Uniparous or Monochasial Cyme:
The main axis terminates in a flower, and only one lateral branch is produced at a time, also terminating in a bloom. Like the primary branch, the lateral and following branches develop just one branch at a time.
There are three forms of uniparous cyme:
(a) Helicoid,
(b) Scorpioid, and
(c) Sympodial
(a) Helicoid Cyme:
The cymose inflorescence is known as helicoid or one-sided cyme when the lateral axes develop sequentially on the same side, producing a type of helix, as in Begonia, Juncus, Hemerocallis, and several Solanaceae members.
(b) Scorpioid Cyme:
The cymose inflorescence is known as scorpioid or alternating-sided cyme when the lateral branches develop on alternate sides, producing a zigzag, as in Gossypium (cotton), Drosera (sundew), Heliotropium, Freesia, and other plants.
(c) Symopodial Cyme:
In monochasial or uniparous cymes, consecutive axes may be curved or zig-zag at initially (as in scorpioid cyme), but because to fast development, they become straight, generating a centre or pseudoaxis. Sympodial cyme is a form of inflorescence seen in several members of the Solanaceae family (e.g., Solanum nigrum).[Agrilearner]
(ii) Biparous or Dichasial Cyme:
The peduncle of this form of inflorescence bears a terminal flower and then stops developing. The peduncle produces two lateral younger blooms or two lateral branches, each of which ends in a flower, at the same time.
The oldest flower is in the centre of the three blossoms. Jasmine, teak, Ixora, Saponaria, and other plants have lateral and following branches that act similarly. True cyme or compound dichasium are other names for this substance.
(iii) Multiparous or Polychasial Cyme:
The primary axis of this form of cymose inflorescence culminates in a bloom while simultaneously producing a number of lateral flowers around it. The oldest bloom is in the centre, and the primary floral axis is completed by it (peduncle). This is a straightforward polychasium.
The entire inflorescence resembles an umbel, but it may be recognised from one by the fact that the central flower opens first, as in Ak (Calotropis procera), Hamelia patens, and so on.
(iv) Cymose Capitulum:
Acacia, Mimosa, and Albizzia all have this style of inflorescence. The peduncle is shortened or condensed to a circular disc in such circumstances. Sessile or sub-sessile blooms grow on it. The oldest flowers bloom in the centre of the disc, while the youngest bloom on the perimeter; this arrangement is known as centrifugal. The blooms form a globose head, sometimes known as a glomerule.
Type 3. Compound Inflorescence:
The main axis (peduncle) of this form of inflorescence branches once or twice in a racemose or cymose pattern. It becomes a compound raceme in the first case and a compound cymose inflorescence in the second.
The following are the most common forms of compound inflorescence:
1. Compound Raceme or Panicle:
Delonix regia, Azadirachta indica, Clematis buchaniana, Cassia fistula, and other racemose plants have branching racemes with racemose flowers on the branches.
2. Compound Umbel:
Umbel of umbels is another name for it. The peduncle (main axis) is short and has a large number of branches that form an umbellate cluster. Each of these branches carries an umbellate cluster of flowers. A whorl of leafy bracts is usually seen at the base of branches, as well as at the bases of flowers grouped in an umbellate pattern.
Involucre refers to the first whorl of bracts, whereas involucel refers to the second. Daucus carota (carrot), Foeniculum vulgare (fennel), Coriandrum sativum (coriander), and others are examples of complex umbels.
3. Compound Corymb:
Corymb of corymbs is another name for it. The main axis (peduncle) branches in a corymbose pattern, with blooms grouped in corymbs on each branch. Cauliflower is a good example.
4. Compound Spike:
Spike of spikelets is another name for it. Wheat, barley, sorghum, oats, and other members of the Poaceae (Gramineae) family are classic examples. This kind was previously discussed under the subhead spikelets.
5. Compound Spadix:
Spadix of spadices is another name for it. The main axis (peduncle) remains racemosely branched, with sessile and unisexual blooms on each branch. [Agrilearner]
Type 5. Verticillaster:
A dichasial (biparous) cyme with a cluster of sessile or sub-sessile flowers in the axil of a leaf, generating a false whorl of flowers at the node. The first of the major floral axes gives birth to two lateral branches, each of which bears just one branch on alternating sides, as do the successive branches.
The inflorescence is typical of the Lamiaceae (Labiatae) family. Examples include Ocimum, Coleus, Mentha, Leucas, and others. [Agrilearner]
Type 6. Hypanthodium:
The receptacle develops a hollow hole with an apical aperture guarded by scales in this style of inflorescence. The blossoms are carried on the cavity’s inner wall. The blooms are unisexual, with female flowers developing at the cavity’s base and male flowers developing at the apical orifice. Ficus carica, Ficus glomerata, Ficus benghalensis, Ficus religiosa, and others belong to the Moraceae family’s Ficus genus.
