raceme [ rey-SEEM, ruh–SEEM] noun: a flower cluster with separate flowers attached by short equal stalks at equal distances along a central stem (flowers at base develop first). Racemes can be simple or compound.
When you see grasses forming wispy or fluffy heads in the summer, you probably don’t immediately think, “Oh what pretty flowers!” But flowers, they are, their arrangement often described as racemes. If you look closely, you will note that these tiny creamy or beige racemes soon turn to seeds, which eventually ripen and drop or sail away on the breezes to ensure future generations of their parent plant.
Among many other examples of plants with racemes are perennial Baptisia australis (blue wild indigo), winter annual or biennial. Brassica rapa (rape seed or mustard flower), spring ephemeral Claytonia virginica ((Virginia) spring beauty), deciduous shrub Clethra alnifolia (sweet pepperbush, summersweet), wildflower Convallaria pseudomajalis (American Lily-of-the-valley), and woody vine Wisteria frutescens (American wisteria).
Summer-blooming perennial Phlox paniculata (fall or garden phlox) is an example of a plant with compound racemes, which are branched and form a panicle. In fact, its epithet “paniculata” refers to the plant bearing flowers in panicles. Spikelets of some grasses, like Sorghastrum nutans (Indian grass), are also borne in panicles, which persist in winter.
The racemes of the short-lived perennial Lobelia cardinalis (cardinal flower) and ground cover Persicaria virginiana (Virginia knotweed, jumpseed) may sometimes be referred to as spikes. Superficially, both resemble spikes, whose flowers are sessile–attached directly to the rachis. Cardinal flower’s tightly-packed racemes can obscure its small flower stalks or pedicels and Virginia knotweed’s pedicels hide behind protective sheaths. Ornamental perennial grass Schizachyrium scoparium (little bluestem) is a spicate raceme with repeating pairs of sessile spikelets and spikelets with pedicels.
Phlox paniculata, Lobelia cardinalis, Persicaria virginiana, Schizachyrium scoparium.
Racemes, panicles and spikes are types of inflorescences. They are grouped together by the arrangement of their flowers on the stalk and by the sequence of anthesis (opening of the flower bud). Because their youngest buds are at the top of the main axis, they are called racemose or indeterminate–they can continue to flower and fruit until external factors like frost stop their growth.
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Reference
Allred KW. 1982. Describing the Grass Inflorescence. Journal of Range Management, 35(5):672-675. doi: 10.2307/3898662.