Confusion between Cerastium fontanum subsp. vulgare and Cerastium glomeratum

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J.M. Garg

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Jun 13, 2021, 9:12:52 AM6/13/21
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Both these species appear quite confusing as per details at POWO: Cerastium fontanum subsp. vulgare (syn. of Cerastium holostedides) and Cerastium glomeratum
To get the clear idea let us see the descriptions from Flora of China. 

1. Cerastium fontanum subsp. vulgare (syn. of Cerastium holostedides)   

Herbs short-lived perennial or annual, 15--40 cm tall. Stems caespitose or simple, suberect; sterile stems decumbent; flowering stems pilose and/or glandular pubescent all round. Basal leaves ovate, obovate-lanceolate, ovate-spatulate, or spatulate, 5--13 × 3--10 mm, both surfaces pilose, base attenuate into a petiole; cauline leaves subsessile, ovate, oblong, or narrowly ovate-oblong, 1--3(--4) × 0.3--1(--1.2) cm, both surfaces pilose or pubescent, margin densely ciliate, apex acute. Cyme terminal, spreading, lax, up to 40-flowered; bracts leaflike, ovate, 3--5 mm, both surfaces glandular pubescent. Pedicel slender, 0.5--2.5 cm, densely glandular pubescent, recurved after anthesis. Sepals oblong- or ovate-lanceolate, 5--6 mm, abaxially densely glandular pubescent, margin membranous or narrowly so. Petals shorter than or equaling sepals. Petals obovate or obovate-oblong, shorter than to 2 × as long as sepals, apex 2-lobed. Stamens shorter than petals. Ovary ovoid, ca. 2 mm. Styles 5, linear, slightly longer than ovary. Capsule cylindric, 8--10 mm, ca. 2 × as long as calyx, 10-toothed, teeth usually recurved. Seeds brown, usually 0.4--0.8 mm in diam., tuberculate. Fl. Apr--Jun, fr. May--Jul.

Forests, forest margins, mountain slopes, hilltop grasslands, fields, sandy soils, rock crevices, roadsides; 100--4300 m.

2. Cerastium glomeratum Thuillier

Herbs annual, 10--20 cm tall. (Annual herb with straggling, ascending stems up to 45 cm as per POWO) Stems simple or caespitose, densely villous, distally glandular pubescent. Proximal leaves spatulate; distal leaves obovate-elliptic, 1.5--2.5 × 0.5--1 cm, base attenuate into a short petiole, both surfaces villous, midvein prominent, margin ciliate. Inflorescence of compact, cymose clusters (glomerules); rachis densely glandular pubescent; bracts leaflike, ovate-elliptic, densely pubescent. Pedicel 1--3 mm, densely pubescent. Sepals 5, lanceolate, ca. 4 mm, abaxially densely long glandular pubescent, margin narrowly membranous, apex acute. Petals 5, white, oblong, subequaling or slightly longer than sepals, base pilose, apex 2-lobed. Stamens shorter than sepals. Styles 5. Capsule cylindric, 1.5--2 × as long as calyx, 10-toothed. Seeds brown, compressed triangular, tuberculate. Fl. Mar--Apr, fr. May--Jun.

Forest margins, mountain slope grasslands, sandy riversides; 100--3700 m. 

So the main difference appears to be in cyme terminal, spreading, lax, up to 40-flowered, Sepals oblong- or ovate-lanceolate and Petals shorter than or equaling sepals. Petals obovate or obovate-oblong in Cerastium fontanum subsp. vulgare (syn. of Cerastium holostedidescompared to inflorescence of compact, cymose clusters (glomerules), Sepals 5, lanceolate and Petals 5, white, oblong, subequaling or slightly longer than sepals in Cerastium glomeratum Thuillier.

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With regards,
J.M.Garg

Gurcharan Singh

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Jun 15, 2021, 7:13:49 AM6/15/21
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Garg ji 
Let us compare Indian taxa, best to start with FBI, which recognises 
1. var. glomerata, now recognised as C. glomeratum, rightly so in Flora of India
2. var. trivialis which was recognised as C. fontanum subsp. triviale in Flora of India
3. var. grandiflora, now considered synonym of C. fontanum subsp. grandiflorum (Edgew.) H.Hara Found in Tibet and Nepal
Other two varieties were doubtful. Flora of India rightly recognised two taxa C. glomeratum and C. fontanum subsp. triviale. If we follow POWO, there is no confusion as latter is relegated to synonymy of former. 
However, if we follow World Flora online (and Flora of China) we have to look for all these differences. 
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