Spore release and dispersal

Once spores have been formed, it stands to reason that they can only effect the next stage in the alternation process if they are released, dispersed, and alight somewhere conducive to germination and gametophyte growth. This presents a huge challenge for propagules that lack vectors which target such sites (cf. flowering plants with insect pollinators). There is scant evidence for productive interactions between animal vectors and fern spores, but very little study of this seems to have been made. Spore-feeding was one nutritional strategy for insects in early terrestrial ecosystems (Habgood et al., 2004), however, and there have been reports of insects feeding on various taxa of ferns (see Srivastava et al., 1997, for review). As the latter paper reports, some lepidopterans do feed on mature spores of bracken, despite their chemical arsenal (Alonso-Amelot et al., 2001), but the consequences for the spores have not been explored. Hemipterans dining on immature spores are clearly of no benefit to the ferns involved (e.g., Balick et al., 1978) and in general it is likely that herbivores are not as helpful to ferns as detritivores. Given that coprolites of early terrestrial detritivores resemble droppings of modern day insect taxa, it seems possible that insects do contribute to dispersal of fern spores, but we must await the necessary experiments. Only one case has been reported of birds possibly specializing on ferns as a major component of their diet. This was the case of the extinct Hawaiian, flightless bird Thambetochen chauliodous ("moa-nalo"), for which evidence was obtained from coprolites showing a high density of fern spores (James and Burney, 1997).

The vast majority of fern and lycophyte spores are undoubtedly dispersed by wind or water. The inadequacies of these agents are considerable. The endangered status of the aquatic quillwort Isoetes sinensis in China, for example, does not reflect low viable spore output, or poor germination, but poor dispersal (Wang et al., 2005). Dispersal in air, as used by most lycophytes and ferns, is generally rather more efficient.

Hornwort Spore Dispersal Mechanism

Figure 2.8 Underside of a fertile frond of Dennstaedtia cicutaris showing the cup-like indusia that first protect the developing sporangia, then allow them to emerge into the dry air such that the mechanism shown in Figure 2.9 is triggered. (Image courtesy of Professor E. G. Cutter.) (SEM, scale bar 1.0 mm.)

Figure 2.8 Underside of a fertile frond of Dennstaedtia cicutaris showing the cup-like indusia that first protect the developing sporangia, then allow them to emerge into the dry air such that the mechanism shown in Figure 2.9 is triggered. (Image courtesy of Professor E. G. Cutter.) (SEM, scale bar 1.0 mm.)

Most taxa have specialized structures that first protect the developing sporangia (Figure 2.8), then ensure an effective launch for spores via hygroscopic movements (Figures 2.9 and 2.10). The process has been well understood for over a century and has even inspired the production of biomimetic microactuators (Borno et al., 2006). For a full and engaging account of "spore shooting" the reader seeking details is recommended to read Chapter 2 in Moran (2004). From the moment they are catapulted into the air, the spores are reliant on air or water currents for transportation.

Discuss The Spore Dispersal Ferns
Figure 2.9 Diagram showing the catapult mechanism that launches spores from fertile fronds of ferns. (From Moran, 2004.)

We might expect ferns on forest floors to face the greatest challenge, but careful study of soil samples taken at distances from hay-scented ferns (Dennstaedtia punctilobula) reveal plentiful spores up to 50 meters from their point of origin (Penrod and McCormick, 1996). The highest spore rain fell within a few meters of the sporing fronds, as observed in research on ferns ranging from other woodland taxa to tree ferns (e.g., Conant, 1978; Peck et al., 1990; Schneller, 1998; Simaan, 2000). Tree-fern spores are those most likely to experience conditions conducive to establishment, and most form a long-lived spore bank (Dyer and Lindsay, 1992). With some notable exceptions, for example, bracken (Lindsay et al., 1995), almost every lycophyte and fern studied seems to generate spores that survive well in soil, and almost every substratum sampled contains spores. This holds true for different vegetation (e.g., del Ramirez-Trejo et al., 2004) and substratum types, including gravel (Ranker et al., 1996) and tree bark (Ranal, 2004). The relatively recent recognition of the importance and potential value of spore banks is encouraging. Researchers are now focusing on conservation methods that take this into account (see Dyer and Lindsay, 1996), including

Ripe Sporagia

Figure 2.10 Ripe sporangium of bracken (Pteridium) which has split open, catapulted out spores and returned to its original position (see Figure 2.8). (LTSEM, scale bar

Figure 2.10 Ripe sporangium of bracken (Pteridium) which has split open, catapulted out spores and returned to its original position (see Figure 2.8). (LTSEM, scale bar

restoration of pre-existing vegetation from the soil diaspore bank after weed (bracken!) control (e.g., Ghorbani et al., 2003).

The lucky spore that arrives directly in, or finds its way out of, the spore bank and into the right conditions for germination leads us to events covered in later chapters in this book. Whether it encounters antheridiogen (see Chapter 5) or not, what type of gametophyte it is (see Chapter 13) and whether its gametes can find a mate (see Chapter 8) will all dictate whether it manages to finish the cycle and complete an alternation of generations.


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  • Pentti
    How spore dispersal in plants?
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  • Donald
    How are spores dispersed in ferns?
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