web-based key to algae: planktonic colony

sheath diffuse, without a definite outline			*Click on the thumbnail for a larger image*
	Mucilage sheath may only be visible with the aid of Indian Ink: cells crescent-shaped or twisted		*Kirchneriella:* forms coenobia of 2,4,8..32 cells
	Sheath may be indistinct, but cells are clearly linked in pairs by dichotomous branching threads radiating from the centre; chloroplast a parietal cup + 1 pyrenoid..		*Dictyosphaerium*
Sheath of very dense mucilage, almost obscuring cells			Botryococcus: cells rounded or elongate; colony contains yellow oil droplets which can be squeezed out under a coverslip, sometimes forms green to yellow-orange blooms.

Sheath with a firm outline
	layered (lamellate);
	cells spherical/elliptical, chloroplast parietal cup + pyrenoid		*Gloeocystis:sheath may* be concentrically lamellate
	with concentric lamellations; cells spherical, chloroplast stellate (see second image)		*Asterococcus*: concentric lamellations not always present, chloroplast form may be difficult to see also
	Sheath plain, not layered:
	sheath sac-like, cells with pseudocilia extending through the sheath		[Pseudocilium = flexible, but not motile, projecting fibre]
	(a) colony normally attached at 1 end		Apiocystis
	(b) colony planktonic		Tetraspora?
	no pseudocilia, spherical cells
		in groups of 4, 8, 16 within a spherical sheath	? Sphaerocystis: (coenobium)
		in groups of 4 or 8 in tight tetrads or in a wreath-shape	Coenochloris or *Coenococcus:* no illustration available; considerable scope for confusion between these (and Sphaerocystis), and some doubt whether they are different.
		in groups of 2-4 cells in mucilage sheath, with semicircular mother-cell wall remnants lying between; chloroplast a parietal cup, 1 pyrenoid	Schizochlamydella:
			Similar genus: Westella - cells in 4's, sometimes arranged quadrately, linked by mother-cell wall remnants
	no pseudocilia, oval cells
		Cells apparently oval or kidney-shaped, in pairs, linked together: actually desmid semicells	*Cosmocladium:* a colonial desmid; the cells reminiscent of Cosmarium
		Cells ranging from spherical to oval or spindle-shaped (depending on species), in coenobia of 2,4,8,...64, within a mucilage sheath composed of gelatinised mother-cell wall	Oocystis : chloroplast(s) parietal plate or trough, with or without a pyrenoid
		Cells solitary or in coenobia enclosed by mother-cell wall; cells somewhat kidney-shaped, with one side curved and the opposite nearly straight	*Oonephris*: chloroplast stellate, but often shape not readily distinguishable
		Coenobia of 4 cells in a flat, quadrate arrangement; cells various shapes	Crucigenia
		Coenobia in a flat, quadrate arrangement; cells oval, or tapering to one end, often with a space in the centre. Cells in groups of 4 enclosed by mucilage sheath, sometimes connected to 3 other coenobia by mother cell-wall remains, all lying in same plane	Crucigeniella
	cells elongated, cigar-shaped
		cells pointed, banana-shaped in groups of 4 which all lie parallel and aligned.	*Quadrigula*
		cells as Quadrigula, with bundles of cells parallel but not aligned.	*Pseudoquadrigula*
	cells elongated, spindle-shaped or tapering
		Grouped in 2's or 4's with alternate cells pointing in opposite directions, linked by mother-cell wall remnants, in a diffuse mucilage sheath	Dichotomococcus
			Similar genera: Quadricoccus (cells more oval) Forming coenobia linked in a wreath-like formation: Komarekia Crucigenia (some species) Coronastrum

	Genera not included: Botryosphaerella, Ecballocystis,Elaktothrix, Granulocystopsis, Hormotila, Juranyella, Palmodictyon, Paulschulzia, Schizochlamys, Sphaerobotrys

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