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Life history stage ratios of Iridaea cordate and factors controlling these ratios in intertidal populations Green, Lesley Gail


Two populations of Iridaea cordata (Turner) Bory in Vancouver Harbour, Vancouver, British Columbia were examined. Both of the populations alternate between diploid dominance in the winter and haploid dominance in the summer. There was a significant difference between the ratio of diploid to haploid plants at the two sites. The alternation in dominance observed in the population as a whole was also seen in three different size classes (<5cm, 5-15cm and >20 cm) examined. Clearance experiments done in the field indicate that it can take up to 1 year for Iridaea to recolonize a disturbed area. When recolonization does occur the ratio of diploid to haploid plants mirrors the ratio at the time of clearing, not the ratio at the time of regrowth. This suggests that the spores settle when the rocks are cleared but remain dormant until conditions allow for growth and formation of upright plants. The influence of apomeiosis on the number of diploid plants is probably negligible, since only 2% of cultured tetraspores showed evidence of this phenomenon. Some physical characteristics of Iridaea, bearing on reproduction were also explored. Tetrasporophytes and gametophytes have significantly different surface areas, with the gametophytes being larger, and they have significantly different densities of reproductive structures, with the tetrasporophytes having more. As a result, both plant types have statistically the same net number of reproductive structures per plant. There was little difference with respect to surface area or density of reproductive structures between the high and low intertidal plants of either ploidy.

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