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– Definition:
Raceme is an unbranched, indeterminate type of inflorescence.
– Flowers have short floral stalks called pedicels along the axis.
– In botany, axis refers to a shoot bearing flowers.
– Oldest flowers grow close to the base in racemes.
– Examples of racemes occur on mustard and radish plants.

– Spike:
– An unbranched, indeterminate inflorescence similar to a raceme.
– Bears sessile flowers attached directly without stalks.
– Examples occur on Malabar nut and chaff flowers.
– Spikelet refers to the ultimate flower cluster unit in grasses and sedges.
– True spikelet comprises florets enclosed by glumes in grasses like rice and wheat.

– Catkin:
– Ament or catkin is similar to a spike or raceme.
– Has conspicuous subtending bracts that conceal flowers until pollination.
– Examples include pussy-willow, alder, and birch.
– Sometimes called amentaceous plants.

– Spadix:
– A form of spike with densely crowded florets along a fleshy axis.
– Enclosed by large, brightly-colored bracts called spathes.
– Female flowers typically grow at the base, male flowers above.
– Characteristic of the family Araceae, e.g., jack-in-the-pulpit and wild calla.

– Examples:
– The inflorescence of Spathoglottis plicata is a typical raceme.
– Tightly packed raceme of Xeronema callistemon with red stamens.
– Every radiating unit in a Cyperus sedge inflorescence is a spikelet.
– Examples of catkins include pussy-willow, alder, and birch.
– Spadix examples include jack-in-the-pulpit and wild calla.

Raceme (Wikipedia)

A raceme (/rˈsm, rə-/) or racemoid is an unbranched, indeterminate type of inflorescence bearing flowers having short floral stalks along the shoots that bear the flowers. The oldest flowers grow close to the base and new flowers are produced as the shoot grows in height, with no predetermined growth limit. Examples of racemes occur on mustard (genus Brassica), radish (genus Raphanus), and orchid (genus Phalaenopsis) plants.

The inflorescence of a Phalaenopsis orchid, is a typical raceme.
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