Developmental morphology of Gigartina pistillata (Gigartinaceae, Rhodophyta)

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Abstract
The vegetative and reproductive development of the type species of the marine red alga Gigartina. G. pistillata (S.G. Gmelin) Stackhouse (Gigartinaceae, Rhodophyta), is described based on material from Brittany, France. Gigartina pistillata possesses a loose primary network of medullary filaments in which every cell is normally connected to each of its neighbouring cells by a single primary or secondary pit connection. Secondary filaments up to three cells long fill the gaps created as the network expands and link terminally to cells below by secondary pit connections. The procarp is derived by successive concavo-convex divisions of a surface cell that is homologous to the apical initial of a vegetative cortical filament, and consists of an inner cortical supporting cell bearing a curved, 3-celled carpogonial branch and a single, lateral cortical filament. After presumed fertilization, the carpogonium fuses with the auxiliary cell effecting transfer of the zygotic nucleus. Small diploid nuclei increase in number and become intermixed with larger haploid nuclei in the cytoplasm of the auxiliary cell. An enveloping tissue composed of short. branched filaments is produced from vegetative cells around the auxiliary cell. Gonimoblast filaments penetrate this tissue, and gonimoblast and vegetative cells fuse or unite by secondary pit connections to form a heterokaryotic placenta. Carposporangial chains derived from both diploid gonimoblast and heterokaryotic placental cells differentiate in clusters, separated by sterile placental tissue and surrounded at the periphery by the enveloping tissue. Tetrasporangial sori are formed progressively at the boundary between cortex and medulla. Cruciately divided tetrasporangia are intercalary and originate from uninucleate cells in both primary and secondary filaments. Sporangia in each tetrasporangial sorus are evidently released as a unit at maturity, leaving an exposed surface which is repaired by means of the proliferation of subcortical cells.

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