Family Poaceae – SUBFAMILY ARUNDINOIDEAE

  • mostly cool season grasses (except tribe Aristideae) with a predominantly hairy ligule. The leaf anatomy of the C3 grasses in this subfamily is somewhat similar to C4 anatomy (and the embryo in the grass seed is large like other C4 grasses). Silica cells are present but apparently not distinctive, and either are the microhairs on the leaves. The base chromosome number is x=12 (11 in Aristideae), which is similar to bamboos and rices.

Tribe Arundineae – stout reed-like grasses, stems usually over 2 m tall, leaf blades usually over 2 cm wide, inflorescences dense but diffuse plumose panicles.

  • Phragmites: cool-temperate regions of the northern and southern hemisphere, rachilla with long hairs that serve as a mechanism for dispersal.
  • Arundo: similar to Phragmites but of tropical to subtropical regions, rachilla glabrous and the long hairs emanating from the lower lemma surface.
  • Cortaderia (pampas grass): very similar to Arundo but dioecious and the long hairs emanating from just the lemmas of the pistillate inflorescence. Cortaderia is mentioned here because it is so well known either in cultivation or as an aggressive colonizer in warm temperate to subtropical regions of the USA.

Tribe Danthonieae – large glumes like Aveneae but with hairy ligule, several florets per spikelet, awn flattened and twisted and emanating from between two well developed teeth of the lemma.

  • Danthonia: awns always well developed, flattened, twisted, and emanating from a distinctly bifid lemma apex, the ligule is hairy.

Tribe Aristideae (At one time considered part of the subfamily Chloridoideae mainly because this tribe has C4 photosynthesis. However, recent genetic information strongly suggests that Aristideae is most closely related to Danthonieae and Arundineae. The C4 photosynthetic pathway and anatomy of Aristideae is distinct from that of Chloridoideae, and apparently evolved independently - see below). – tripartite awn, exactly 1 floret per spikelet, hairy ligule, contracted panicle. Aristideae is somewhat similar to Stipeae in having one floret per spikelet, a lemma that bears a long awn from the tip, and large glumes. Stipeae, however, never have a tripartite awn or conspicuously hairy ligules. Also, the first glume in Aristideae is much shorter than the second, whereas in Stipeae both glumes are approximately the same size - this is a good trait for distinguishing among these tribes when the florets have all disarticulated and dispersed.

  • Aristida: the tripartite awn is the main distinction, but as this is the only genus of the tribe in Montana, the characteristics of the genus follow those of the tribe.

Aristideae Type C4 photosynthesis: Malate is the C4 compound (like Panicoideae). The large bundle sheath cells occur in two rings, both of which are derived from the same tissue as the mesophyll. This double ring of bundle sheath cells is referred to as a ‘double parenchyma sheath’.

  • 1. Photosynthesis (the C3 cycle) occurs in the inner parenchyma (bundle) sheath, rarely in the outer parenchyma sheath
  • 2. The mesophyll cells are densely packed and radiate outward from the bundle sheath, and furthermore they are compartmentalized the same as in Chloridoideae
  • 3. A layer of cells normally sheathing the vascular bundle and derived from the same cells as the vascular tissue is absent (such a layer is found in all other C4 grasses)
  • 4. The genera of Aristideae are most common in arid areas in and around the latitudes of the Tropics of Cancer and Capricorn (same as the genera of Chloridoideae

Family Poaceae - SUBFAMILIES BAMBUSOIDEAE AND ORYZOIDEAE

  • Both bamboos and rices are C3 grasses with a predominantly membranous ligule, or reduced to a thickened rim of tissue, sometimes absent, top of leaf sheath often bearing thick bristles (these arise from the sheath rather than blades). Leaf anatomy is unique in having ‘arm’ and ‘fusoid’ cells, which serve to store and efficiently transport photosynthate (the silica cells apparently take on the form of these distinctive cell types. Though these subfamilies are primarily tropical, they are mainly confined to forest understory or riparian to aquatic habitats. The base chromosome number is mostly x=12 (like Arundinoideae).

Subfamily Bambusoideae - we will not study tribes of this subfamily in this class. For the purpose of this class, we will learn only of the distinctive pseudopetiole that is unique to the bamboo subfamily. The pseudopetiole serves as a distinct character for readily distinguishing woody and herbaceous bamboos from all other grasses. No particular genera will be studied.

Subfamily Oryzoideae - tribe Oryzeae – aquatic grasses, glumes reduced to a cupule, one floret per spikelet

  • Leersia: similar to Oryza but with small florets (a few mm long) and with sharp backwardly-oriented hairs that serve as hooks to vegetatively disperse the plant.
  • Oryza: ovate florets with large tough paleas, flowers perfect, includes cultivated rice.
  • Zizania: long narrow (cylindrical) florets, the paleas much smaller than lemmas, plants monoecious, the pistillate above the staminate, includes wild rice.

Bambusoideae and Oryzoideae are considered the earliest branching lineages in the grass family lineage. The evidence for this is that the closest relatives of the Poaceae, the southern hemisphere temperate graminoid families Restionaceae, Joinvilliaceae, and Flagellariaceae have: 1) membranous ligules, 2) C3 photosynthesis, 3) well developed petals and flower parts in 3’s, 4) small embryo relative to size of seed, 5) leaves with typical microhair, and 6) a base chromosome number of x=12. Of all the grass subfamilies, only Bambusoideae and Oryzoideae come close to this set of characters.

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