Poa L., Sp. Pl. 1: 67-70. 1753.

Soreng, Robert J. & Peterson, Paul M., 2012, Revision of Poa L. (Poaceae, Pooideae, Poeae, Poinae) in Mexico: new records, re-evaluation of P. ruprechtii, and two new species, P. palmeri and P. wendtii, PhytoKeys 15, pp. 1-104 : 2-3

Poa L., Sp. Pl. 1: 67-70. 1753.

Paneion Lunell, Amer. Midl. Naturalist 4: 221. 1915; Panicularia Heist. ex Fabr., Enum. 207. 1759; Poagris Raf., Fl. Tellur. 1: 18. 1837. Lectotype: Poa pratensis , designated by Nash 1913: 252.

Anthochloa Nees & Meyen, Reise Erde 2: 14. 1834. Type: Anthochloa lepidula Nees & Meyen. [≡ Poa lepidula (Nees & Meyen.) Soreng & L.J.Gillespie].

Aphanelytrum (Hack.) Hack., Oesterr. Bot. Z. 52: 12. 1902.; Brachyelytrum subgen. Aphanelytrum Hack., Nat. Pflanzenfam. 1: 42. 1897. Type: Anthochloa procumbens (Hack.) Hack.

Austrofestuca (Tzvelev) E.B.Alexeev, Bjull. Moskovsk. Obač. Isp. Prir., Otd. Biol. 81(5): 55. 1976; Festuca subgen. Austrofestuca Tzvelev, Bot. Žurn. (Moscow & Leningrad) 56(9): 1257. 1971. Type: Anthochloa littoralis (Labill.) E.B.Alexeev [≡ Poa billarierei St.-Yves].

Dasypoa Pilg., Bot. Jahrb. Syst. 25(5): 716. 1898. Type: Dasypoa tenuis Pilg. [= Poa scaberula Hook.f.].

Dissanthelium Trin., Linnaea 10(3): 305. 1836. Type: Dissanthelium supinum Trin. [= Poa calycina (J. Presl) Kunth].

Eremopoa Roshev., Fl. URSS 2: 429, 756. 1934. Type: Eremopoa persica (Trin.) Roshev. [≡ Poa persica Trin.].

Graminastrum E.H.L.Krause, Beih. Bot. Centralbl. 32(2): 348. 1914. Type: Graminastrum macusaniense E.H.L.Krause [≡ Poa macusaniense (E.H.L.Krause) Refulio].

Neuropoa Clayton, Kew Bull. 40(4): 728. 1985. Type: Neuropoa fax (Willis & Court) Clayton [≡ Poa fax Willis & Court].

Ochlopoa (Asch. & Graebn.) H.Scholz, Ber. Inst. Lanschafts- Pflanzenokologie Univ. Hohenheim Beih. 16: 58. 2003.; Poa sect. Ochlopoa Asch. & Graebn., Syn. Mitteleur. Fl. 2: 387. 1900. Type: Ochlopoa annua (L.) H.Scholz [≡ Poa annua L.].

Oreopoa Gand., Fl. Eur. 26: 186. 1891. nom. nud., based on: Poa alpina L.

Parodiochloa C.E.Hubb., Bull. Brit. Mus. (Nat. Hist.), Bot. 8: 395. 1981. Type: Parodiochloa flabellata (Lam.) C.E.Hubb. [≡ Poa flabellata (Lam.) Raspail].

Phalaridium Nees & Meyen, Gramineae 29. 1841. Type: Phalaridium peruvianum Nees & Meyen [≡ Poa serpaiana Refulio].

Stenochloa Nutt., Proc. Acad. Nat. Sci. Philadelphia 4: 25. 1848. Type: Stenochloa californica Nutt. [≡ Poa thomasii Refulio].

Tovarochloa T.D.Macfarl. & But, Brittonia 34(4): 478. 1982. Type: Tovarochloa peruviana T.D. Macfarl. & But [≡ Poa apiculata Refulio].

Tzvelevia E.B.Alexeev, Bjull. Moskovsk. Obač. Isp. Prir., Otd. Biol. 90(5): 103. 1985. Type: Tzvelevia kerguelensis (Hook.f.) E.B.Alexeev [≡ Poa kerguelensis (Hook.f.) Hook.f.].

Description.

The following combination of characteristics distinguish Poa from other grass genera in Mexico, and in most of the world. The plants are annual or perennial, and rarely exceed 1 m in culm length. The culms are simple, hollow, and unbranched above the base. Plants sometimes have rhizomes, but only infrequently are weakly stoloniferous. The leaves are unusual in that the upper culm leaf sheath margins are fused over 5 to 100%; the blades are abaxially keeled and typically have only two channels on the adaxial surface (sometimes obscure), one on either side of the midvein, that are lined with thin-walled bulliform cells that collapse to fold the blade in dry conditions; and the blade apex is typically prow-shaped to some degree (in these features they are like Glyceria ). The vegetative structures lack long pilose hairs and microhairs. Like all Pooideae , the blades lack Kranz anatomy. Ligules are scareous (white) to hyaline, without apical or lateral cilia (sometimes minutely ciliolate). The inflorescences are simple, paniculate, and terminal. Poa spikelets are unspecialized, usually between 2.5 and 8 mm long, normally with 2 and 5 florets (sometimes 1 in Poa gymnantha ), and disarticulate above the glumes and between the florets. The spikelets and florets are laterally compressed and lack awns. The florets are all alike, but are reduced upward and the uppermost is often rudimentary. The glumes are persistent, typically the lower one is 1 or 3-veined; the upper 3-veined, slightly narrower and thinner than the lemmas, generally slightly shorter than the lower floret (longer in Poa thomasii and Poa calycina ; the lower glume can be quite short in Poa seleri ), and are often somewhat unequal. The rachilla is terete, usually at least 0.4 mm long between florets and is prolonged beyond the uppermost, well developed floret (except in Poa calycina where the internode is 0.2-0.3 mm long and sometimes is not prolonged above the 2nd floret). The lemmas are usually distinctly keeled (except Poa secunda ), and commonly are pubescent with hairs on the lemma and/or the callus. The lemma apex and margins are variously narrowly to broadly scareous-hyaline. The lemmas are typically 5-veined but this varies from 3 ( Poa calycina ) to 9 ( Poa mulleri ). The palea surface is thinly herbaceous (unlike Koeleria , in which it is hyaline). The callus of the lemma is terete or slightly pinched dorsally, blunt, and fairly indistinct in its transition to the lemma (unlike Festuca and Schedonorus , which have a dorsally compressed rachilla and a somewhat thick, annulated, glabrous callus, that is sometimes angled downward). The callus typically has an isolated dorsal tuft of woolly hairs called a web, but the callus is glabrous in some taxa (In Poa secunda the callus lacks a web but sometimes has a line of short hairs <0.2 mm long or crown on the callus around the base of the lemma). The lodicules are two in number, usually ovate to lanceolate with a lateral lobe, without distinct vascularization, glabrous, and the upper portion is hyaline. Anthers are 3 in number (sometimes 1 or 2 in Poa bigelovii ), and within pistillate flowers are typically reduced to staminodes. Ovaries are glabrous with 2 styles that are apical, approximate, plumose, and white. The caryopsis is firm (but known to contain lipid), ventrally sulcate with a short (usually less than 1/7 the grain in length) subbasal hilum, and the embryo is small (less than 1/6 the grain in length).

Chromosome base number is x = 7, of medium to medium-large size.

Distribution.

The genus is Worldwide but is usually not found in tropical countries without high mountains. In Mexico the genus is known from all states except Nayarit, Quintana Roo, Sinaloa, Tabasco, and Yucatan.

Ecology.

Species of Poa occur in cool temperate to frigid regions, cool habitats in warm temperate regions, and in dry to wet habitats.

Discussion.

Poa is a complex genus of more than 500 species with 23 occurring in Mexico. It is the type genus of the family Poaceae , subfamily Pooideae , supertribe Poodae, tribe Poeae, and subtribe Poinae ( Soreng et al. 2011). The above synonymy for the genus is the most current available, however, two species of Aphanelytrum (and one of Festuca related to those) have not yet been formally transferred into Poa . Poa is known for great diversity in breeding systems ( Anton and Connor 1995; Negritto and Anton 2000; Soreng and Keil 2003). In Mexico species may be strictly perfect flowered; gynomonoecious (spikelets with lower florets perfect and upper ones pistillate); possibly sequentially gynomonoecious (some plants shifting to producing more pistillate flowers as the season progresses); dioecious (sexual dimorphism was not evident in the Mexican species); or pistillate only. Apomictic/asexual reproduction by seed is common in some species of the genus, and is sometimes facultative ( Clausen 1961, Kellogg 1987), and other times effectively obligate ( Soreng and VanDevender 1989; Negritto et al. 2008). In Mexico Poa chamaeclinos and Poa gymnantha are strictly pistillate apomicts, and Poa fendleriana is pistillate and apomictic over much of its range. Poa alpina , Poa compressa , Poa pratensis , and Poa secunda are known to be facultatively apomictic elsewhere, but their breeding systems have not been studied in Mexico. Poa strictiramea is predicted to be apomictic in Mexico based on the high frequency of sterile anthers. No viviparous/bulbiferous spikelets were found among Mexican specimens. Other relevant literature on Poa includes: Soreng (1990), Gillespie and Soreng (2005), Gillespie et al. (2007, 2009), Soreng et al. (2009, 2010), Refulio-Rodríguez et al. (2012) -molecular data; Soreng (2005) -chromosome counts; and Tzvelev (1976), Zhu et al. (2006), Soreng (2007), Giussani et al. (2012) -floristic accounts.