High Humidity Low Humidity Flat Soil Agar Control Agar Agar
□ Total # Archegonia ■ % Archegonia on Dorsal Surface
Figure 9.7 Effect of relative humidity on archegonia production in gametophytes of Pteridium aquilinum. High humidity, 100%; low humidity, approximately 90%. Results are averages of three replicates for each treatment; the number per replicate is 20 individuals. Bars are means ±1 standard error.
Experiments indicated that gametophytes in high humidity cultures produce a large percentage of archegonia (48.7%) on the dorsal side of the thal-lus, whereas the low humidity cultures produced very few archegonia (1.4%) on the dorsal surface (Figure 9.7). Perhaps of greater significance, the total numbers of archegonia produced increased nearly four-fold in low relative humidity cultures, compared to high humidity cultures, and nearly two-fold relative to typical flat agar cultures (grown in small culture boxes not within terraria). The total number of archegonia produced by low humidity cultures mimicked the results of cultures grown on soil. Since it was necessary to add water to the soil cultures periodically to prevent them from dying (whereas it was not necessary to add water to agar cultures) it may be that soil cultures experienced lower relative humidities at least part of the time.
A unifying theme may explain both the anomalous production of archego-nia on the dorsal surface and the lower total numbers of archegonia produced in standard agar cultures. Both appear to be induced by reduced light and humidity differentials across the gametophyte's two surfaces relative to that experienced by gametophytes in nature. Though relatively simple anatomically and morphologically, fern gametophytes do have complex differentiation patterns including establishment of polarities in development (Raghavan, 1989). It may be that the strong differentials of light and moisture between dorsal and ventral surfaces experienced by gametophytes in natural habitats provide environmental signals critical to establishment of normal polarity. Incomplete establishment of this polarity in agar cultures may disrupt normal differentiation such that archegonia are produced in abnormal positions and overall in smaller numbers.
Additional experiments may elucidate further the environmental signals and physiological responses involved, but it is clear from these experiments that gametophytes grown in standard agar cultures in Petri dishes may yield seriously misleading data relative to sexual development of natural populations of fern gametophytes. Analysis of breeding systems of fern species based on laboratory data should take these discrepancies into consideration.
Every fern and lycophyte population owes its beginnings to the gameto-phyte generation and the simple gametophyte plant contains the same genetic information that produces the highly complex sporophyte plant. We should not be surprised to find that sophisticated differentiation of form and physiology also exists among gametophytes of different taxa. Studies described in this chapter have made it clear that combinations of growth rate, mature form, sexual differentiation, longevity, and differential physiological interactions with environmental parameters lead to functional gametophyte strategies that initiate the sporophyte phase of each species in its preferred habitat. Without knowledge of these gametophyte-level strategies we cannot comprehend habitat compartmen-talization within fern and lycophyte communities or migration, distribution, and evolution of species.
The simplistic "one size fits all" depiction of the cordiform, bisexual fern gametophyte in standard textbooks does a great disservice not only to ferns and lycophytes, but to the disciplines of ecology, genetics, and evolution as well. The thoughtful student is left to conclude, incorrectly, that ferns and lycophytes do not obey principles of variation and selection and can be set aside as inexplicable curiosities in the broader context of plant evolution.
Data derived from laboratory cultures provide important starting points for ecological studies but must not be surrogate to field investigations. Laboratory cultures do not provide information on ecological safe sites, on interaction with competing vegetation, or on conditions necessary for successful sporo-phyte recruitment. Data from culture studies may, in fact, yield misleading data. Absence of field data on recruitment leaves the haunting prospect, as expressed by Peck (1980) in his field studies, that gametophytes we study in the field could be nothing more than "reproductive noise." Determining which gametophytic ecological and reproductive strategies result in sporophytes capable of completing the life cycle remains a challenge that must be accepted.
Both short-term studies of habitat characterization and population demographics, including sexual structure, and long-term studies on sporophyte recruitment and population dynamics of perennial gametophytes need to be conducted on a broad scale. Functional classes of ecological strategies will likely be redefined from such studies. However, we must be cautious in extrapolation. We must not assume a priori that the gametophyte ecology of a given species is representative of another in the same genus or habitat any more so than is the ecology of the sporophyte generation. These studies will challenge the next generation of fern and lycophyte biologists, but they are sure to increase immensely our insight into the occurrence, survival, and evolution of species.
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