What are the basic units of the grass inflorescence that can vary in number and arrangement?

The correct answer focuses on spikelets, which are the basic units of grass inflorescences. Spikelets are small clusters that hold the florets, which are the individual flowers of the grass plant. They can vary significantly in number and arrangement among different grass species, which contributes to the diverse appearances of grass inflorescences. This variability is important for identification and classification in agronomy.

Spikes refer to a different structural component, where florets are directly attached to a central stalk without a substantial pedicel. Nodes are points on a stem where leaves or branches emerge, but they do not pertain to the arrangement of flowers. Florets represent the individual flowers that are found within the spikelets, so, while they are related, they are not the basic unit of the inflorescence itself. Understanding the role of spikelets in the structure of grass can help in studying and identifying various grass species in agronomy.