grass morphology: inflorescences

Inflorescences

Botanists have developed a set of names to categorize the flowering stems of grasses. The inflorescence may be a single spikelet, a spike, a raceme, or a panicle. Or it might be a combination that doesn't nicely fit any one of those categories, like a spike-like panicle or the inflorescence branches are spike-like or digitate (branching out from a center point like a starfish). So far, this description has probably done nothing but confuse you, so let me go straight to pictures so you can visualize what I'm talking about.

The very simplest inflorescence is a single spikelet. A single spikelet inflorescence is typical for one-spike oatgrass (Danthonia unispicata), although it can have more than one spikelet. The white arrows point to the spikelets.

annual%2520brome_one%2520spikelet_edited

Annual bromes in drought years can also be reduced to a single spikelet This can be confusing because normally the inflorescence would be a panicle with drooping branches, but the plant lacked the resources to produce more than one spikelet and see the seed out to maturity. I advise that you look at the population and select more typical individuals. But if you don't, the field guide has a key that works for these single spikelet inflorescences.

Spikes

In a spike, the spikelets are attached directly to the central stalk (rachis). This is an unbranched inflorescence. There may be one, two, three or more spikelets attached at each node (point of attachment).

In quackgrass (Elymus repens), ryegrass (Lolium) and thickspike wheatgrass (Elymus lanceolatus), there is one spikelet per node and it looks like this:

In Great Basin wildrye (Leymus cinereus) there are 2-7 spikelets per node.

In Leymus, there are usually multiple spikelets at each node. In beardless wildrye (Leymus triticoides), there are 1-3, usually 2 near the middle of the spike.

Racemes

In a raceme, the spikelets are attached to the central stem by a little stalk. Two grasses in our region with racemes are false brome (Brachypodium sylvaticum) and semaphoregrass (Pleuropogon).

false brome (Brachypodium sylvaticum)

nodding semaphoregrass (Pleuropogon refractus).

Panicles

A panicle is a branched inflorescence in which the branches are also branched. Panicles can be dense and spike-like or open and spreading. Sometimes the branches are spike-like or raceme-like, but we will deal with those later. First, since we have already seen spikes, we will look at panicles that are dense and may look to you like they should be a spike. Examples of these include meadow foxtail (Alopecurus), timothy (Phleum), bristlegrass (Setaria), and junegrass (Koeleria). If you bend the inflorescence, you will be able to see that the spikelets are not attached directly to the rachis.

Open panicles

have longer branches. The primary and secondary branches can take on a variety of positions, from upright, to spreading, drooping, or reflexed (pointing downward).