Spore dispersal of Selaginella denticulata, S. helvetica, and S. selaginoides, and the significance of heterospory in Selaginellacae

Abstract. Little is known about the functional significance of heterospory in land plants, nor about how species ensure that mega- and microspores, which have strongly different wind dispersal, are dispersed over similar distances to ensure cross-fertilization. We studied active ejection distances and heights of megaspores of Selaginella denticulata, S. helvetica, and S. selaginoides. Under laboratory conditions, the maximum horizontal and vertical ejection distances of megaspores from the sporangia were 24 cm and 22 cm for S. denticulata, 65 cm and 45 cm for S. helvetica, and 120 cm and 76 cm for S. selaginoides. The function of this active spore ejection thus should not only be seen in the ability of horizontal dispersal but also as a means to bring the megaspores into the wind currents. Under laboratory conditions, microspores of all three species showed a maximum horizontal ejection distance of about 10 cm. However, experiments in the open showed that under natural conditions with wind, microspore dispersal occurs over much longer distances and that the megaspores are partly also wind assisted and reach similar distances. In addition, we observed synaptospory, in which microspores electrostatically adhered to megaspores and were dispersed with them. Our results suggest that the active ejection of the megaspores is a means of ensuring similar dispersal distances of micro- and megaspores and thus cross-fertilization. Very little is known about the breeding system in Selaginella, but we surmise that both self-fertilization and outcrossing occur.