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Day1 Day 1 Day 1 Day 7 Stirred Agar Stirred Soil Flat Soil Flat Agar Flat Agar

(control)

Treatment

Figure 9.6 Effect of various treatments on the percentage of male gametophytes in laboratory cultures of Pteridium aquilinum. Day 1 Day 7 indicates that a second sowing of spores was made in the same culture 6 days after the first sowing. Approximately the same total number of spores was sown in each replicate of each treatment. Results for the flat agar treatments are an average of six replicates for each treatment; the number per replicate is 24 to 30 individuals. Results for the stirred agar and soil treatments are an average of three replicates per treatment; the number per replicate is 20 individuals. Bars are means ±1 standard error.

the first and second sowings were in adjacent strips (so that they could be analyzed separately), male gametophyte percentages in the second sowing reached 88%, indicating that the increase in percentage of males in the mixed-age populations was due primarily to effects on younger gametophytes derived from the second sowing.

To determine whether a similar effect could result from uneven developmental stages induced by a heterogeneous substrate surface, we compared even-aged populations developed on flat and uneven agar surfaces, and compared these to development on flat and uneven (stirred) soil surfaces in Petri dish cultures. Uneven agar surfaces produced an even higher increase in male gametophytes than did sequential sowings (Figure 9.6). Likewise uneven soil surfaces produced an elevated male-to-female ratio, though not as great as that observed on uneven agar. However, flat soil cultures contained significantly higher male-to-female ratios than did flat agar cultures. Thus it seems probable that uneven-aged populations resulting from either heterogeneous substrates or from sequential spore introduction, both of which might be expected in nature, could produce elevated male-to-female ratios relative to even-aged laboratory cultures.

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