Peduncle is unbranched and grows indefinitely.
Flowers are many and are arranged acropetally.
Flowers are bracteates and sessile. e.g., Achyranthus, Pupalia, Polyanthus (Lily).
It is of different types.
Compound spike - peduncle is branched.
The branches bear bracteates, sessile flowers as in spike. e.g., Amaranthus spinosus.
Spikelet - In this the inflorescence is a secondary spike. Its ultimate branches are called spikelets. Each spikelet consists of one or more flowers.