Genera of Poaceae. Phalaris. This is a genus of big contrasts, from huge perennials like Reed Canary-grass to delicate little annuals like Canary-grass. What they have in common is that the spikelets have 3 florets but only the terminal one is bisexual (other 2 are mere scales)
As an indication of the complexities involved in differences between the species of Phalaris, the genus appears in no fewer than 3 of the generic keys in Stace. The main one is Key G, but Palaris also appears in Keys C & K as well.
Key G concerns grasses with all spikelets bisexual and similar, but within each spikelet only 1 of the florets is fully developed, with 1 or more sterile or scale-like florets. The first question is easy: is it a Setaria with stiff bristles at the base (left) or not (right)? Not.
Another easy one. Is the ligule hairy (left) or not (right). Not hairy My Lord.
Now for the serious business. Count the male or sterile florets below the bisexual floret (X10 and look very carefully for small, scale-like reduced florests). Are there 1 (left) or 2 (right) ?
Start with the 1 reduced floret. Is there an awn from the proximal half of the back of the lemma (left) or not (right) ?
Lastly, is the ligule membranous (left) or absent (right) ?
There is a membranous ligule, so this plant is Phalaris. Now for the part of the key with 2 male or sterile florets below the bisexual floret.
Measure the lengths of the two glumes. Is the lower glume half as long as the upper (left) or are the two roughly equal (right) ? They's nearly equal, so right.
Finally is the panicle diffuse (left) or compact (right) ? Easy. It's compact. Almost spike-like. So it's Phalaris.
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