The reproductive phenology of Isoetes lacustris L.: Results of field studies in Scandinavian lakes

The phenologic change of the leaf rosette structure of Isoetes lacustris L. was studied in 26 lakes of temperate, boreal, or subarctic Scandinavia between 59° and 70° n.l. The investigations were carried out during six defined seasons: late winter, spring, early summer, late summer, autumn, and early winter. From 640 plants, gained with the aid of SCUBA, six leaf types were distinguished: immature megasporophylls, mature megasporophylls, immature microsporophylls, mature microsporophylls, and sporophylls that had released their spores and leaves with undeveloped sporangia. Mean numbers per rosette of each leaf type were established in each study lake and study season, resulting in a common pattern embracing all lakes studied.

Megasporophylls are developed throughout the year, whenever the water temperature is about 10 °C. Their share was always more than 30%, excepting winter. Microsporophylls are produced preferentially in spring/early summer when the days are longest; they amount to more than 50% of the rosette leaves during this period, but only to some 10% in the remaining seasons. The spores mature and are released between late summer and early winter. It is concluded that not all spores mature in the year of their birth, and those that do not mature are released in the early summer of the following year, as well as the old empty leaves become detached.