The reproductive biology of Hydrocharis morsus-ranae. II. Seed and seedling morphology
This study of seed and seedling morphology in the aquatic monocotyledon Hydrocharis morsus-ranae L. (Hydrocharitaceae) represents part of a continuing study of sexual reproduction in this species. Scanning electron microscope studies of the seeds showed them to have testas covered with hollow spiraliform tubercles. Germination occurs when the radicle and cotyledon of the embryo from the exalbuminous seed elongate, splitting the tuberculate testa. The buoyant embryo then rises to the surface with the first foliage leaves emerging from a highly modified cotyledonary sheath. This makes early growth in the seedling look strictly hypocotyledonary. Both the radicle and cotyledon discontinue growth by the two-leaf stage in the seedlings. The young seedlings undergo several growth phases. At first they look Lemna-like in habit and then, subsequently, very similar to germinating turions of the same species. Several methods are discussed to help distinguish between germinating turions and seedlings. During a subsequent field study of a H. morsus-ranae population, only two germinating seedlings were discovered. Several reasons for the absence of seedlings in this population are discussed.