Fragilariforma virescens (Ralfs) D.M.Williams & Round (1988b: 265)

Pottiez, Margaux & Garden, Bart Van de Vijver Meise Botanic, 2024, Observations on the lectotype material of Fragilariforma virescens (Fragilariaceae, Bacillariophyceae), Notulae Algarum 321, pp. 1-7 : 3-5

Fragilariforma virescens (Ralfs) D.M.Williams & Round (1988b: 265) Figs 1–43

Basionym: Fragilaria virescens Ralfs (1843: 110 : pl. II: fig. 6)

Syntype localities: “In freshwater pools. Cold Bath, Tunbridge Wells [Kent], Mr. Jenner. Madron and Chyanhâl [sic] Moor near Penzance [Cornwall].” ( Ralfs 1843: 110) ,

Lectotype: slide BM 81303 designated by Williams & Round (1988a: 280). Although not stated at the time, this slide was made from Jenner material from Cold Bath, Tunbridge Wells (herbarium sheet BM 001222540 , coll. date December 1842) as mentioned by Williams (2001, p. 105)

Isolectotype: BR -4832 ( Meise Botanic Garden , Belgium), slide made from original material also from Cold Bath, Tunbridge Wells, Dec. 1842, leg. Mr. Jenner , BR! We therefore conclude that this is the same material as was used by Williams & Round (1988a) to designate the lectotype .

Registration (of isolectotype): http://phycobank.org/104405

Description: Frustules rectangular in girdle view, linked to each other forming long, ribbon-like chains using spatulate linking spines with a narrow base and broad apex. Chains with up to more than 100 cells observed. Cingulum composed of 5 open, ligulate girdle bands perforated by a single row of small, rounded poroids ( Figs 34–37). Valvocopula with a dentated pars interior and a distinct septum at the closed pole ( Figs 35, 36). Valves linear-lanceolate in larger specimens to elliptic-lanceolate in smallest specimens. Valve margins straight to clearly convex. Apices shortly protracted in smaller valves to elongated rostrate in longest valves. Capitate apices so far not observed. Valve dimensions (n=50): valve length 16–38 µm, valve width 6–8 µm, frustule width ca 10 µm. Mantle broad with broad advalvar part bearing long striae and narrow abvalvar hyaline edge. Mantle edge with thick mantle plaques, irregularly scattered along the entire valve length ( Fig. 34). Valve face flat ( Figs 38, 39). Continuous series of marginal linking spines, regularly placed on each virga at the valve face edge ( Fig. 39). At one apex, row of spines interrupted ( Fig. 41) whereas on the other apex, spines present on the apical pore field ( Fig. 40). Occasionally valves with acute separation spines ( Fig. 38). Sternum variable, but usually very narrow. Central area absent. Striae uniseriate, composed of small, rounded areolae, 16–17 in 10 µm. Areolae not discernible in LM. Striae either continuous over the sternum or alternating, often irregularly spaced. Large apical pore fields present on both apices, composed of an irregular pattern of short series of small, rounded poroids, continuing on one apex from the valve face onto the mantle, whereas on the other apex, pore field only present on the valve face. Pore fields not equal in size and shape, giving the valves a heteropolar outlook, only discernible in SEM. One rimoportula per valve, slit-like, present near the smaller apical pore field with circumpolar spines, replacing some of the areolae in the last stria ( Figs 38–41). Internally, sternum well visible. Rimoportula labiate, perpendicular to the apical axis ( Figs 41, 43).

The valve dimensions differ from the reported length and width ranges in Williams & Round (1988a). Despite a scan of the isolectotype slide for the longest and shortest specimens, no valves longer than 38 µm and shorter than 15 µm have been observed among the hundreds of valves in the sample. Williams & Round (1988a) reported a valve length range of 12–120 µm with a valve width of 5–10 µm. A similar width range was also not observed. The reported ranges in Williams & Round (1988) included auxospore sizes to capture the measurements from the entire cell cycle ( Williams 2001). This underlines not only the limitations of type material; frequently, observed specimens in such material are captured at one point in the life cycle, but also may lead to the description of a lot of varieties as part of the life cycle, such as for instance Fragilariforma virescens var. elliptica (Hustedt) Aboal. A better analysis of F. virescens populations in Europe will be necessary to verify whether these longer valves also belong to F. virescens sensu stricto.

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