In this type of inflorescence, the main axis does not end in a flower, but it grows continuously and develops flowers on its lateral sides in acropetal succession (i.e the lower or outer flowers are older than the upper or inner ones). The various forms of racemose inflorescence may be described under three heads. They are as follows-
- With the main axis elongated- raceme, spike, spikelets, catkin, spadix.
- With the main axis shortened- corymb, umbel.
- With the main axis flattened- capitulum or head.
The elongated main axis bears pedicellate flowers in an acropetal order. Example- Dog Flower (Antirrhinum), Radish (Raphanus Sativus), Turnip, Dwarf Gulmohar (Caesalpinia), Mustard (Brassica Campestris), Larkspur. When the main axis of raceme is branched and the lateral branches bear the flowers, the inflorescence is known as compound raceme or panicle. Example- Cassia Fistula, Delonix Regia, Margosa, Neem (Azadirachta Indica).
The elongated main axis bears sessile bisexual flowers in an acropetal manner. Example- Bottle Brush (Callistemon), Latjira (Achyranthes Aspera), Adhatoda, Spinach, Amaranthus.
It is small and a few-flowered spike which is covered by two scales called glumes. Spikelet is basic inflorescence in the grass family Poaceae.
Catkin or Amentum:
It has an elongated spike possessing unisexual and apetalous flowers. Example- Oak (Quercus), Mulberry (Morus), Willow (Salix), Betula (Birch), Poplar (Populus).
The inflorescence is like a spike which is covered by one to a few large bracts called spathes. The peduncle is thick and fleshy which bears unisexual flowers except in the apical region. The sterile region is called the appendix. It is coloured in aroids. In aroids the spathe is single. It forms a tube in the lower half to cover and protect the flowers. The flowers are sessile, generally male towards the upper side, female towards the lower side and downwardly pointed hair or neuter flowers in the middle. Example- Colocasia, Cobra Plant (Arisaema). In Cobra Plant the male and female flowers are formed in separate spadices. Both the spathe and the appendix are bent to appear like the head of a snake. In Maize, the female inflorescence is a spadix having a number of spathes.
The main axis is shortened, broad and flat. The old flowers at the lower levels on the axis have longer pedicles than the upper ones so that all the flowers reach more or less to the same level. Example- Candytuft (Iberis Amara), Cassia Sophera.
In this inflorescence, the primary axis remains comparatively short, and it bears at its tip a group of flowers which possess pedicels or stalks of more or less equal lengths so that the flowers are seen to spread out from a common point. In this inflorescence, a whorl of bracts forming an involucre is always present, and each individual flower develops from the axil of a bract. Generally, the umbel is branched and is known as umbel of umbels (Compound Umbel), and the branches bear flowers. Example- Coriander (Coriandrum Sativum), Fennel, Carrot. Sometimes, the umbel is unbranched and known as a simple umbel. Example- Brahmi (Centella Asiatica), Androsace. Umbel inflorescence is the basic inflorescence of coriander family Apiaceae or Umbelliferae.
Capitulum or Head:
The peduncle is flattened like a flat, convex or concave disc. It bears small, sessile flowers or florets over its upper surface in centripetal fashion with younger towards the centre and older towards the periphery. The whole inflorescence is covered by one or more whorls of involucre or bracts. Bracts may also occur at the base of individual florets. Florets are of two types, tubular and ligulate. Capitulum can be tubular homogamous (example- Ageratum), ligulate homogamous (example- Sonchus) or heterogamous having tubular flowers on the central platform and ligulate flowers on the periphery (example- Sunflower). Capitulum inflorescence is a typical inflorescence of family Asteraceae or Compositae. Example- Dahila, Cosmos, Zinnia, Tagetes, Aster. In Compound Capitulum, many daughter capitula arise from a flattened receptacle of mother capitulum. Example- Echinops.