Protonemal bulbils of Gemmabryum dichotomum (Bryaceae) germinate in the dark and exhibit a burial response

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Abstract
Gemma germination is known to require light in bryophytes. Water-dispersed (hydrochory) species, however, have not been tested for a light response, and one hypothesis is that hydrochory may be ecologically correlated with a burial response (dark germination of stolons capable of surfacing when buried). The eventual burial of floating propagules in muddy soil is likely to be frequent and thereby select for the ability to surface from depth. We found that the widespread often hydrochorous species Gemmabryum dichotomum produces protonemal (chloronemal) bulbils that (1) germinate in total darkness by producing negatively gravitropic or lateral rhizoidal stolons, (2) regerminate with protonemal filaments when subsequently exposed to light, (3) surface from sand burial depths of 4, 8, and 12 mm (grain size < 355 µm) and produce a surface colony of shoots and protonemal bulbils in 6–17 wk depending on burial depth, and (4) can tolerate at least 2 mos. of continuous darkness followed by regermination through protonemata when brought into the light. Prolonged darkness (2 mos.) resulted in delayed regermination, fewer protonemal filaments, and fewer shoots when brought into the light. However, such bulbils produced a greater protonemal spread than control bulbils due to protonemata arising from stolons previously produced in darkness.

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