Grasses - my first study, two grasses

I am beginning with the lessons in Agnes Chase's "First Book of Grasses". I became enthused about this in early May in Tucson with the temperatures above 100 degrees. There were lots of nice spring grasses after our abundant rains this winter, but these are all dry and brown now, and I'm not sure how much I can learn from them. I made a trip to a nearby park with a lake and collected two grasses.

My pair are Cynodon dactylon (bermuda grass) and Cenchrus setaceus (crimson fountain grass). Both are disliked invasive species, so I need feel no guilt collecting them.

Grasses have a vernacular all their own (though partially adapted from regular botanical jargon). At least part of my purpose in writing this is to help myself learn these terms, and not just in a textbook way, but by looking at real plants.

Culm -- this is what we would call a "stem" for any other plant. It is the above ground vegetative part of the grass plant. Nodes and internodes are easy enough to see. Each leaf consists of a sheath, blade, and ligule. The ligule is small and found inside at the junction of sheath and blade. It is inconspicuous on Cynodon, but very hairy on Cenchrus. Some grasses have auricles at this same junction, which are small extensions of the blade that wrap around the joint. There is none on either of my grasses.

Both of my plants are perennial. I infer this by the roots, which are wiry rhizomes in both cases. Actually, a rhizome is an underground stem, not a root. A stolon is an above ground stem that roots at the nodes producing new plants. Both of my plants are caespitose, i.e. growing in tufts. So much for the easy part.

The game now is to look for the spikelets that I have read so much about. I start with Cynodon, and before trying to sort of the tiny spikelets, it is worth talking about the inflorescence as a whole. Cynodon is novel, with a palmate collection of spikes. Cenchrus looks like a spike to me, but my book describes it as a panicle, so some definitions are in order.

Spike -- the spikelets are sessile along a single stem called the rachis. Sessile meaning that the spiklets are "mounted" directly on the rachis without some kind of stem.

Raceme -- take a spike and give each spikelet a tiny stem, and you have a raceme. The tiny stem is called a pedicel. These are said to be uncommon in grasses, though they may make up part of a panicle.

Panicle -- anything more complex than a single rachis with spikelets mounted is a panicle. It is divided into branches, and the branches may be subdivided. Many things that look like spikes turn out to be panicles when you examine them closely.

Cynodon spikelet -- these are imbricate (overlapping) along each branch of the digitate inflorescence. I guess I would call each branch a spike. The spikelets are very tiny. There are two small glumes then a single floret. The lemma dominates and is a boat shaped thing. Inside is the smaller palea. There is no stem (rachilla) that I can see.

Cenchrus spikelet -- here again the spikelets are solitary. They are surrounded by a collection of large feathery structures which my book calls sterile branches. I use tweezers to remove these branches, and sure enough I find one tiny glume, then a fairly large one, then a big lemma and then a somewhat smaller palea.

At this stage of my learning, I am pleased just to identify the parts of the spikelet. I consider it a victory to find the spikelet hidden away inside the circle of feathery/bristly structures that guard it on Cenchrus. Native species of Cenchrus are called "sandbur" because these guarding structures are annoying burs. I would call the Cenchrus structure a raceme, because there are tiny pedicels.

Postscript - some words

Two ranked - what does that mean? The writer had a clear picture in mind, but it could mean any number of things to us. Harrington offers a diagram on page 39 of his book showing florets in what might be called an "alternate" arrangement. First one on the left, then further up the branch one on the right.

Subtend - this means one thing in mathematics, another in botany. In botany, it refers to an object that is beneath or close to another. Typically a bract that is beneath a flower. Another definition is (of a bract) that extends under (a flower) so as to support or enfold it.

One peril with words like this is that different authors may use them in their own way (especially when their definition is not well established) so each author must be "calibrated" by the reader. Be prepared.