Conceptacles

Conceptacles are specialised cavities of marine and freshwater algae that contain the reproductive organs. They are situated in the receptacle and open by a small ostiole. Conceptacles are present in Corallinaceae, and Hildenbrandiales, as well as the brown Fucales. In the Fucales there is no haploid phase in the reproductive cycle and therefore no alternation of generations. The thallus is a sporophyte. The diploid plants produce male (antheridia) and female (oogonia) gametangia by meiosis. The gametes are released into the surrounding water; after fusion, the zygote settles and begins growth.

Two taxa, the corallines and Hildenbrandiaceae, bear conceptacles, although the striking difference between their formation indicates that the conceptacles are not homologous. Similar structures also exist: cryptostomata are similar to conceptacles but differ having only hairs and are sterile.Caecostomata, are found only in Fucus distichus, in these the ostiole becomes blocked during development.

Conceptacles can measure from 40–150 μm in diameter in some Hildenbrandiales. Some conceptacles are pits within the thallus, whereas others form raised domes protruding above it. Conceptacles open to the environment via one or many pores, depending on the species and the type of conceptacle.

This is a central pillar constituting central calcified cells covered by cellular debris. These sterile cells rise up from the conceptacle floor to form a peak.

Some conceptacles form by the centripetal expansion of a hole near the thallus surface; in such cases, a roof forms by nearby filaments arching over and establishing themselves as short (often 1–9 cells long) filaments that cover the chamber, leaving a central pore through which the spores can escape. If these filaments are vicarious they may produce a beak-like opening.

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