Poaceae - This family has an alternate name (Gramineae). This is one of the largest angiosperm plant families, and we have 102 genera with native and/or naturalized species in CA.
Vegetative Features of Grasses
The stem of the grass plant is called a culm, and just as in all stems, it has nodes and internodes. The nodes of culms are somewhat swollen (some authors refer to this swelling as "jointed"). Between the nodes, culms are usually hollow.
Annual versus Perennial Grasses; Rhizomes and Stolons
Grasses have fibrous roots. In annual grasses, these roots are found at the base of the main stem or stems or at the lower nodes of the stems, and at the end of one season, the entire plant dies. In some perennial grasses, the plant dies back in winter to a crown of stem tissue from which new stems arise the following season. These new stems which arise from buds are called tillers. Bunchgrassesare perennials which grow in a defined clump, with the tillers arising near the perimeter of the crown each year. Other perennials may form rhizomes and/or stolons – horizontal overwintering stems that are located either above (stolons) or beneath (rhizomes) the soil level. These stems bear fibrous roots all along their length at their nodes. Grasses that bear long rhizomes or stolons can spread rapidly in any direction (picture Bermuda grass) and are called sod-forming grasses.
As in all vascular plants, the leaves emerge at the nodes. In grasses, the leaves are two-ranked, emerging on two sides of the stem. Like many monocotyledons, grass leaves are divided into a sheath and a blade.
The sheathis the lower part of the leaf, and it wraps around the culm, but it is only attached to the culm at its base (where it emerges from the node). In most grasses, the sheath is open at its edges for at least some of its length. In a few groups, the sheath is fused into a tube all the way from the node to the collar region. This complete fusion is rare and is diagnostic of particular groups, in particular the genus Bromus. The blade is the upper part of the leaf, which is not wrapped around the culm. Blades may be flat, folded (conduplicate), or inrolled (involute or convolute).
Several important characters occur at the junction of the sheath and blade:
1) At this junction, facing the culm, there is often a row of hairs or a little membranous flap of tissue. These hairs or tissue are called a ligule. Not all species have a ligule.
2) At this junction, facing away from the culm, is a region called the collar.
3) At either side of the collar, there may be projections called auricles. Having auricles is rare, and therefore their presence is diagnostic of particular groups of grasses.
Grass flowers are very reduced, lacking any noticeable perianth and generally consisting of just one pistil (usually with 2 style branches - sometimes 1 or 3) and either 2 or 3 stamens (there can be as few as 1 or as many as 6). At the base of the ovary of the pistil are structures called lodicules which are the remains of the perianth. Surrounding the flower, one usually finds two bracts. The bract on the side furthest from the flowering axis is called the lemma, and the bract closest to the flowering axis is called the palea (which is sometimes absent, especially in Agrostis). The flower plus these two bracts is called a floret.
Florets are arranged on flowering axes in units called spikelets. Spikelets are the basic unit of the grass inflorescence. Usually, every spikelet has two sterile (non-flowering) bracts at its base (sometimes there is just one or none); these sterile bracts are called glumes. Above the glumes, the spikelet bears one or more florets. The axis of the spikelet is called a rachilla (in the Jepson manual, it is just called the "spikelet axis"). The stalk of the spikelet is called a pedicel.
Grass floret sexuality
Grass florets can be bisexual, unisexual (and then either staminate or pistillate), or sterile. Grasses can bear florets of different sexes on the same plant, even within the same spikelet. It is common in some genera for the bottom floret of a spikelet to be barren and the upper floret to be perfect. In other genera, all the florets of a spikelet may be perfect except the upper florets, which become staminate or barren. When florets of two separate sexes are borne on the same plant, the plant is said to be monoecious. When the sexes are born on two separate plants, the plants are said to be dioecious.
Upon maturity, a grass ovary matures into a caryopsis or grain, which is a hard fruit in which the ovary wall closely adheres to the seed and the entire fruit is dispersed (not just the seed).
Characters of the Spikelet
The characters of the glumes and lemmas of a spikelet are heavily used in the classification of grasses (the palea is used in a few groups), which is why having the use of a hand lens or microscope is a must in learning grasses. The number of veins (called nerves) on the back of these bracts, the presence or absence of hairs or pubescence (and what kind), the presence of an awn (either terminal or dorsal), and the texture (indurate or membranaceous) are all important characters. Keys will also ask about whether a spikelet is compressed in cross-section; if it is compressed, it will ask about whether it is dorsally compressed (the florets flattened back to front) or laterally compressed (the florets flattened side to side creating a crease in the lemma).
One last detail that is often stressed is disarticulation which is how a spikelet falls apart upon maturity to release the fruits. In some grasses, the entire spikelet falls off the inflorescence (including the glumes); in this case the spikelet disarticulates below the glumes. In other grasses, the spikelet disarticulates above the glumes and between each floret (ie. the rachilla falls apart (fragile rachilla), but the glumes remain on the inflorescence axis). In most cases, usually the caryopsis remains attached to its floret (lemma and palea); in a few cases (notably, the dropseeds - Sporobolus), the caryopsis drops out of the floret.
Spikelets are arranged into larger inflorescences, and the terminology of these inflorescences parallels that of other groups. However, in the case of grasses, it is the spikelet that is the basic unit of the inflorescence, rather than an individual flower. Therefore, a spike is an unbranched inflorescence bearing sessile spikelets, a raceme is an unbranched inflorescence bearing pediceled spikelets, and a panicle is a branched inflorescence. Spikes and racemes can bear spikelets on one side (one-sided or unilateral) or two sides (two-sided or bilateral). Panicles can be open and spreading or narrow and and compact. The branches of a panicle can themselves be spikes, racemes, or panicles. In the key to genera in the Jepson Manual, you have to distinguish between branched inflorescences in which the primary branches do not have additional secondary branches vs. those that do; only the latter are referred to as “panicle-like.” The main axis of the inflorescence is the rachis.
Interesting Accessory Structures found on some Inflorescences
In some inflorescences, there are other interesting structures. In Pennisetum and Setaria, one finds modified inflorescence branches resembling bristles; in Cenchrum, there are modified branches that form flat prickly burs that surround the spikelet; in Hordeum there are sterile spikelets with modified glumes that help disperse the fertile spikelet; and in Poa bulbosa, bulblets form rather than caryopses.
How to use the Poaceae key in the Jepson Manual
The Poaceae key to genera in the second edition of the Jepson Manual includes six groups, which, with the exception of group 1, are based on the overall shape of the inflorescence and the number of florets per spikelet. The use of groups is similar to the key to genera for Asteraceae, except that, in the case of Poaceae, the key does not start with a key to groups. Instead, the groups are interspersed among couplets that lead you to distinctive species and genera; thus Group 1 doesn’t appear until Couplet 12, and Group 2 until Couplet 36. Nonetheless, many of the larger and more common genera are included in one of the groups, so it is important to become familiar with them. The following is a summary of the six groups.
Group 1. Robust perennial grasses, 1.5-7 meters tall, the culms usually very tough or woody (like bamboo), the inflorescences sometimes large and feathery.
Groups 2-6 are grasses usually less than 1.5 meters tall
Group 2. Genera in which the spikelets are dorsally compressed, occasionally cylindrical, bearing 2 florets, the lower sterile or staminate, often reduced to just a sterile lemma, and upper floret bisexual. The textures of the glume and the fertile lemma are noticeably dissimilar, with one leathery and the other membranous (which is which varies among the genera) and the spikelets break apart below the glumes, falling singly or in pairs with segments of the rachis attached.
Group 3. Genera with unbranched inflorescences: spikes (spikelets sessile on rachis) or racemes (spikelets stalked).
Group 4. Genera with inflorescences that either: 1) have branches that all originate from a single point (like an umbel) or 2) have branches that are spike-like (the spikelets appearing to be sessile). In either case, the main branches are not branched again.
Group 5. Inflorescence a panicle with the branches clearly visible secondary branching; spikelets with only one floret.
Group 6. Inflorescence a panicle with the branches clearly visible secondary branching; spikelets with more than one floret per spikelet.
You will now dissect the following grasses with your T.A., and then you will key them, using the Jepson Manual keys.
Avena "oats" Inflorescence type: ____________________ Number of glumes/spikelet: ______
Number of florets: _______ Number of veins on the first glume: _______ on 2nd: ______
Number of veins on the lemmas: ________ Are there awns? ________ Where? ________
Are there hairs on the spikelet? ________ Where? _______ Ligule type: ________
Festuca perennis "rye grass" Inflorescence type: ____________________ Number of glumes/spikelet: _______
Number of florets: ________ Number of veins on the glume: _______
Number of veins on the lemmas: _______ Are there awns? _________ Where? _______ Ligule type: __________ Group number: __________
Cynodon dactylon "Bermuda grass" Inflorescence type: ___________________ Number of glumes/spikelet: _________
Number of florets: ________ Number of veins on the 1st glume: ______ on 2nd: _______
Number of veins on the lemma: _________ Are there awns? _________ Ligule type: __________
Group number: ____________
Polypogon “beard grass” Inflorescence type: ____________________ Number of glumes/spikelet: __________
Number of florets: ________ Number of veins on the 1st glume: ______ on 2nd: _______
Are there awns? _______ Where? ___________________________________________
Number of veins on the lemmas: _________ Ligule type: _____________ Group number: _____________
Other genera/species that we may have on display:
Usually we have two species on display, B. diandrus"rip gut brome" and B. hordeaceus "soft chess," both non-native annual grasses. This is a common genus in CA; we have both native and non-native species (annuals and perennials). What is the inflorescence type? _____________ Are there more than two florets per spikelet? ___________ Are there awns? From where do the awns emerge? ___________________ Look carefully at the sheath; is it closed or open? ______________
Another common genus. Annual bluegrass (Poa annua) is one of our most common turf grasses, and it has become naturalized in some areas. Bulbous bluegrass (Poa bulbosa) is becoming increasingly invasive throughout California. We have a number of beautiful native species in this genus as well (many of the natives are perennials). Most of the species have leaf tips that are shaped like the prow of a boat. Some of the species have lemmas with cobwebby hairs. Look at the lemma; where are the cobwebby hairs located? _____________________________
This genus is also common, and we have native and non-native species as well as annuals and perennials. It is sometimes difficult to separate from Bromus. However the sheath in this genus is usually open at least part way. We usually have the invasive nonnative perennial Festuca arundinacea "tall fescue," for you to examine. This is one of the most common turf grasses planted in new housing developments in Davis.
The native species of this genus are mostly perennial grasses. This includes the common Elymus glaucus "blue wildrye." Also included in this genus is the invasive annual Elymus caput-medusa "medusa head." The inflorescence type is a spike with one to two spikelets per node. The glumes are lanceolate to awn-like.
This genus is somewhat similar to Elymus, in that the inflorescence is a spike and the glumes are awn-like, but there are 3 spikelets per node. Although we have some native species (annual and perennial), the nonnative annuals are invasive and very common. When the caryopses of this grass are ripe, the entire inflorescence falls apart at the nodes, and the groups of three spikelets are dispersed as a unit. What role do you think the awns and glumes play in dispersal? _________________________________________________ Look at the auricles in Hordeum murinum "wild barley;” they are well-developed in this species.
Aegilops triuncialis "barbed goat grass"
A non-native annual with very interesting looking spikelets on a spike. How many spikelets are there per node of the inflorescence? ___________
Polypogon "beard grass, rabbit-foot grass"
A non-native genus with 5 species in CA. Note the extremely soft inflorescences. The spikelets are tiny. How many florets are there per spikelet? ________ Are there awns? _________ Where? _____________________________
Phalaris aquatica "Harding grass"
A perennial non-native that is poisonous to livestock. Note the large, compact inflorescences. What group would this key to in the Jepson Manual? ___________
Look at the glumes - they look like little boats. There are actually two florets per spikelet - one large bisexual floret above and a tiny reduced one below that looks like a flap of tissue at the base of the upper floret.
Cyperaceae - These are annual or perennial herbs, usually of wet areas. Unlike grasses, sedges usually have three-ranked leaves and three-cornered, solid stems. The structure of sedge inflorescences is much simpler than that of grasses, and there is much less terminology involved in keying sedges. The flowers are just as reduced as in grasses, but unisexual flowers are more common in this family. Each flower is subtended by just one bract. These flowers and bracts are arranged into larger inflorescences called spikelets (which can be very large) - however, the spikelets lack glumes.
Cyperus "umbrella sedge"
This is a an easy genus to recognize, because the inflorescence is subtended by a whorl of bracts. The flowers in this genus are usually bisexual. The ovary can be two or three sided. Look at the ovary of C. eragrostis - how many sides does it have? ______ How many stigmas are there? __________ Does this number differ from the grass pistils that you examined earlier? _______________________
This is also an easy genus to recognize, because the inflorescence consists of just one terminal spikelet. Dissect a flower out of a bract of the spikelet. Note the white bristles at the base of the ovary (don't mistake the filaments for the bristles - the filaments are brownish). These bristles are found in many species of this genus. Is the stem in this genus three-sided? ________
This is a very large genus worldwide, with over 100 CA species (mostly native). Look at the inflorescence in this species. Are the flowers bisexual? _______ Are the staminate and pistillate flowers borne in separate spikelets? Explain: __________________________________________ Dissect out a pistillate flower. Surrounding the lower part of the pistil is an envelope, called a perigynium (1, 2), out of which the stigmas protrude. Perigynia are typical of the genus Carex.
Scirpus / Schoenoplectus / Bulboschoenus "bulrush", "tule"
Important to California Indians, because the stems of several species are used for building materials. Some species get quite large, and the stem is spongy. The inflorescences are panicles, but the can be quite condensed and head-like. The ovary is subtended by bristles, as in Eleocharis.
Juncaceae - Rushes usually have flowers with a perianth consisting of 6 papery, greenish or brownish tepals, and although they are grouped into bracteate inflorescences, there is no special terminology to learn. In California, we have only 2 genera, with Juncus being the most common. These are annual and perennial herbs of wet areas.
Juncus - "rush" and/or Luzula “hairy wood rush” Juncus is a large genus in CA (56 species, most native); Luzula has eight native species.
In some species of Juncus, the bract subtending the inflorescence is round in cross-section and looks like a continuation of the stem; in such cases, the inflorescence, which is terminal, is pushed sideways, so that it looks lateral. In other species, the bract subtending the inflorescence looks very different from the stem. Look at the tepals of an individual flower. Although bracts may subtend the flowers, the flowers are not hidden by the bracts as they are in the sedges and grasses. A few Juncus species have equitant leaves such as those found in the genus Iris, but there are horizontal markings on the leaves (septae) not found in Iris leaves.
Typhaceae - A family with 2 genera in CA, with Typha the most common.
These are the "hotdogs" of the marshes. Examine the inflorescence. Are the flowers bisexual? _______ Which flower type (1, 2) is uppermost in the inflorescence? _____________
In this genus, the inflorescence consists of a series of spherical, unisexual heads. Which head type is uppermost in the inflorescence? _____________
Arecaceae - Another family with an alternate name, Palmae. One of the confusing things about this family is that when you say Arecaceae it can sound like Ericaceae. The members of this family are trees and shrubs. This family is well-developed in tropical and sub-tropical areas of the world, but in California we have only one native species.
Washingtonia filifera "California fan palm"
A native tree of moist areas in the desert. These plants are characterized by an unbranched trunk which bears large leaves at the apex, the bases of which sheath the trunk and leave a "skirt" below the current leaves. The laminas of the leaves are pleated, and they become lobed by tearing along the pleats. The large inflorescences hang in the axils of the leaves, bearing large fleshy, white flowers.