Fruits, seeds and germination in the seagrass Halophila ovalis (Hydrocharitaceae)

Fruits of Halophila ovalis (R. Br.) D. J. Hooker develop on rhizome nodes of the female plants during summer to autumn (February to April) in southern Western Australia. The fruit is protected by a pair of bracts and fruit walls consisting of several enlarged parenchyma cells which contain starch grains. In each fruit there are between one and sixteen seeds with a mean of 7.4 seeds per fruit. The seed has an embryo with a distinct leaf primordium protected by a curved cotyledon, and an enlarged hypocotyl. The hypocotyl consists of numerous uniform parenchymatous cells containing packed starch, protein and lipid as nutrient storage. There is a small zone of meristematic cells at the basal end of the hypocotyl, probably functioning as the radicle initial which elongates in early seed germination. Mature seeds are covered by a thin, two-layered seed cuticle and remains of the compacted pericarp. The surface of the seed covering has a reticular appearance and numerous microsculptures at a higher magnification. During germination the seed covering becomes loose and is discarded, exposing the surface of the hypocotyl. Nutrient stored in the hypocotyl is consumed during germination and appears to assist in the development and emergence of the radicle and leaf primordium. During seedling development, numerous elongated 'hair-like' unicellular structures develop from the surface of the hypocotyl. These may act as an anchor for the young seedling, before the emergence of the adventitous root. The majority of seeds began to germinate after four weeks of culturing at room temperature (20-degrees-C), but germination was not synchronized. The germination rate in culture was 63% after six months, but H. ovalis seedlings rarely reached the three leaf stage.