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PHYCOLOGICAL SOCIETY OF AMERICA
Gracilariopsis carposporophyte
The first picture is a close up of a female alga with a red, bulbous carposporophyte, and two while bulbous parasites growing on it. The picture on the right is a really-close up of a cross section though a carposporophyte.
As described in the gametophyte page, a spermatium fertilizes the egg to form the zygote nucleus inside the carpogonium. Several cells then fuse with the carpogonium to form one large fusion cell that contains the zygote nucleus and nuclei from the female alga. The fusion cell grows to form the carposporophyte, with rows of carpospores on its outer margin (the darkly pigmented cells in the cross-section photo). The cortical cells of the female alga form a protective wall (pericarp) around the developing carposporophyte. When the carpospores are mature, they are released from the carposporophyte through a small hole (ostiole) and if they are extremely lucky, they will develop into a tetrasporophyte.
--Ben Hale 1999.