by Michael Rasser

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The coralline algal body (called thallus) is formed by adjacent filaments, which are calcified and repeatedly branched. Consecutive cells within one filament (vertical cell rows in the images) are connected by primary pit connections (red arrow).

Cells of adjacent filaments (horizontal row in the upper and right image) may be connected by secondary pit connections (above – white arrow), or by cell fusions (right – white arrows).

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Most encrusting coralline algae are dorso-ventrally organised. This means, they have a basal (dorsal) part, which is mostly near to the substrat and is responsible for the lateral growth, and an outer (ventral) part, which is responsible for plant thickness. Until recently, the basal part of the thallus was called "hypothallus", the outer one "perithallus". However, things got more difficult: Depending on the thallus organisation, dimerous and monomerous thalli are differentiated.

Monomerous thalli (A, B) consist of basal multilayered core filaments ("hypothallium" of older literature) which can be coaxial (A) (i. e., they are arranged in tiers) or non-coaxial (B) (= plumose). Some derivates of core filaments curve outward to form the peripheral filaments ("perithallium" of older literature).

Dimerous thalli (C) consist of unilayered basal primigenous filaments ("hypothallium" of older literature), from which the postigenous filaments ("perithallium" of older literature) arise dorsally at right angles.

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If you want to know more details about thallus organisation, follow this link.

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