The Poaceae
Members of the Poaceae, or grass family, have...
The Poaceae is one of the monocot families. Members have parallel leaf veins and embryos with a single cotyledon (characteristics of monocots). The only reason we are leaving off "monocots have perianth parts that occur in multiples of three" is that flowers of grasses have perianths that are extremely reduced and modified.
Grain crops come from domesticated members of the Poaceae. Bamboo comes from large and rigid members of the Poaceae. Identifying species in the Poaceae requires you to recognize...
An individual flower of the Poaceae has no obvious perianth. The flowers usually consist of one pistil with two plumose stigmas and three stamens. Small scales, called lodicules, at the base of the flower are thought to be reduced remnants of the perianth.
Flowers of grass are sandwiched between two scale-like bracts: the lemma and the palea. The palea is on the adaxial side of the flower. The lemma is on the abaxial side of the flower. The lemma is usually larger than the palea, and it tends to wrap around both the flower and the palea. The entire unit (flower, palea, and lemma) is called a floret.
Florets are arranged in "spikelets". A spikelet is the end of a shoot with:
A glume is just a sterile bract (i.e., it does not enclose a flower) at the base of a spikelet.
Identification of grasses is often based on whether lemmas or glumes have awns, whether they have teeth, the length of the awns and teeth, the size and shape of the glumes and lemmas, etc.
The cross-sectional shape of the spikelet may also be important. Spikelets may be somewhat cylindrical in cross-section. Alternatively, they may be dorsally compressed or laterally compressed. In laterally compressed spikelets, the glumes are are on the edges of the flattened spikelet:
A dorsally compressed spikelet is flattened the other direction: the glumes are on the flat faces of the spikelet.
You may also have to know how the spikelet falls apart at maturity? In some species, the entire spikelet falls off the plant as a unit ("spikelets breaking below the glumes"). In others, the florets fall out of the spikelet individually, leaving the glumes on the plant ("spikelets breaking apart above the glumes").
Spikelets are arranged on the plant in different ways in different species. This is shown on the next page.
Check your understanding:
Look again at the florets below:
The inflorescence of a grass is described with respect to how spikelets are arranged on a plant, not how individual flowers are arranged.
The arrangement may be described as spike-like, panicle-like, raceme-like, etc.
Spikelets may be sessile on the sides of major inforescecence branches. They may have short pedicels, or they may be attached singly at the ends of slender branches.
A grass leaf has a sheath, which wraps around the stem, and a blade. At the place the two meet (the collar) there may be a ligule on the adaxial surface (a flap of tissue that sticks up and continues to hug the stem) and/or auricles (two little lobes or appendages stick out or forward at the base of the blade).
Shapes, sizes, and presence or absence of ligules and auricles is often important in identifying grasses. A concise glossary page that shows a wide variety of ligules and auricles is available on the web through this link
Check your understanding:
The collar regions of two grass leaves are shown below. These are from different species.
Grasses can be annuals or perennials. Perenial grasses can be rhizomatous, stoloniferous, or bunch grasses.
Rhizomatous grasses spread by underground horizontal stems (rhizomes).
Stoloniferous grasses spread by aboveground horizontal stems (stolons).
Bunch grasses put up new upright stems next to the old ones year after year, forming a mound or tussock or ...bunch.
In the photo below, the grasses in the left panel are annual grasses. The plant in the right panel is a perennial grass that puts up new leaves and flowering stems from the same root year after year.
Annual grasses may grow large, given the right conditions. (Corn, or Zea mays, is a large annual grass.) Other species may remain relatively short, like the Bromus tectorum (cheatgrass) above.
Perennial grasses may be even taller, or they may be short and prostrate. Bamboo is an example of a large perennial grass. Arundo donax (giant reed) is another example of a large perennial grass. (Arundo donax has become an invasive problem in California, choking waterways.) In contrast, many grass species used for turf are perennial grasses that spread by rhizomes or stolons. Cynodon dactylon (bermuda grass) is one of the latter. It has also become naturalized in California.
Check your understanding:
Below are photos of four grasses:
Look at the brown grasses in the picture below:
This keying exercise will be a guided exercise. The plant you will be identifying will be this one:
These plants were growing at an elevation of 3800 feet in chaparral in the San Gabriel Mountains. One was collected on May 30, 2020.
Links to images of the plant are given below. They will open in new windows or tabs and can be enlarged.
Plant
Inflorescence
Spikelets
Spikelets at different stages of maturity on plant
Spikelets at different stages of maturity (detached)
Spikelet - dissected
Floret - spread
Stem and leaves
Stem width
Leaf - blade width
Leaf - magnification
Leaf - collar region (adaxial view with possible damage)
Look at the "Plant" photo (link above) and identify the plant's growth form and/or life history characteristics:
Note: Higher resolution images of these photos are linked above, if you need to see them.
Here is another view of spikelets at different stages of maturity:
Answer two more questions, then we will run through the key.
Now you are ready to begin working through the key. You already know that this plant is in the Poaceae. In real life, you could go straight to the key to genera in the Poaceae. For practice, though, start at the Key to Families in the Jepson eFlora.
The Poaceae is a very large and variable family. Different species will have characteristics that lead you through the key in different directions. When you got to Asteraceae, you saw:
POACEAE {G2,5,6,8}
The "{G2,5,6,8}" means that "Poaceae" also appears in the keys for Groups 2, 5, 6, and 8 in the Key to Families.
Moving on to the Key to genera in the Poaceae ("Key to Poaceae"), you find that this is a very large key with groups (subkeys).
For this key you will need more information. We will go through it step-by-step with occasional illustrations.
The next choice asks you if "Spikelets [are] enclosed in a bristly to spiny, bur-like involcre". To answer that one, it might help to know what a "bristly to spiny, bur-like involucre" looks like. Fortunately, there is only one genus in this key that has such an involucre (Cenchrus). You can look at images of the involcre of Cenchrus that have been uploaded to the Jepson eFlora by following this link.
...AND HERE WE GET TO AN APPARENT ERROR IN THE KEY. It happens. Those hairs on the lemma body look very long to me. Choose 26'
...and this is taking too long, so go to Group 6 in the key to genera in the Poaceae
And in the key to species in this genus, you will need to know what a callus is.
From the Jepson eFlora glossary, a callus is "In some Poaceae, enlarged or projected hard base of floret; sometimes hairy or sharp-pointed."
