The seeds of such aquatic plants as Echinodorus and Sagit-taria readily take up water and swell quickly because they do nof possess hard and impervious outer coatings. They germinate very readily and quickly. In other genera the seeds are firm and hard, germinating only after a long immersion during which bacteria and other organisms have time to weaken or dissolve the waterproof outer coating. The seeds of Potamogetón, for example, require a 'fermentation' process in marshy conditions. In cultivating them, large quantities of seeds are left in small dishes of water where their surface decomposes anaerobically with the emission of foul-smelling gases. Under these conditions many will germinate successfully.
Some aquatic plants develop seeds which germinate immediately after maturing. This group includes the Pontederiaceae such as Eichhornia and Heteranthera. In other species, perhaps the majority of aquatic plants, the seeds require longer or shorter resting periods of 1 to 6 months between maturing and germinating, During this resting period the seeds can survive both dry and cold conditions.
Fully mature seeds usually are collected from the parent plants in the late summer or autumn and are sown at the beginning of spring or late winter (February or March). The seeds of almost every species of aquatic plant can withstand drying, even when the seeds ripen under, water as in Ottelia, Barclaya, and the like.
If seeds are to be over-wintered they should be dried out properly after collection and then put in paper bags and stored in a dry, well ventilated room. Storing in greenhouses with a high humidity very quickly diminishes their viability. As a general rule,
A floating "Islet" of Cerafopfer/s with new plantlets on the margin of the leaf blades. Photo by Dr. D. Sculthorpe.
if the seeds hibernate at average indoor temperatures above 59° F (15° C), they require a very dry medium,
If 'moist' seeds are over-wintered they should be stored at low temperatures, which will inhibit germination. Even a temporary increase of temperature, for instance above 46° F (8° C), may greatly decrease the chances of germination, especially in plants of the genus Nuphar. The seeds of Nuphar must be stored in cold surroundings, either covered with moss or buried in the sand. When dry seeds are stored, germination is inhibited by lack of moisture; with moist seeds it is by lack of warmth.
At the beginning of spring the seeds of aquatic plants from temperate and subtropical areas are sown at temperatures of 59° to 68° F (15° to 20° C), while those from the tropics are sown at temperatures of 77° to 86° F (25° to 30° C),
The seeds of typically submersed plants (Ottelia, Barclaya, Nuphar) are sown in water about 10 cm deep. Amphibious species are sown on the surface of moist sand; only after the seeds have germinated and the first roots have appeared are they covered by a layer of water 12 to 25 mm deep. This is suitable for Bacopa, Lim-nophila, Myriophyllum, Sagittaria. and some species of the genus Echinodorus.
Many plants are provided with only moist sand in their dishes even after germination and are not covered with water (and then only a few centimeters) until the seedlings reach a height of 50 to 90 mm. This procedure is followed in the case of the majority of Echinodorus species, such as E. cordifolius, E. macrophyllus, E, paniculotus, E. tenellus, and others, while it is the only suitable way of ensuring a high yield of such genera and species as Samolus valerandi, Ammania, Ludwigia, and Hygrophila.
Aponogetons are the only plants commonly transplanted after germination. The mature seeds are left in soft water (rain water is best) at temperatures around 54° to 59° F (12° to 15° C) for a period of 1 to 2 months. Then the temperature is raised to about 77° F (25° C) and the seeds are left to germinate. The seedlings, 25 to 40 mm high and with well developed leaves and roots, are then planted in sand on the bottom of a shallow, well lit aquarium and the water level is gradually raised. The seeds of the true species (not hybrids) of Aponogeton are left in cool water, usually for the whole winter and until the spring (which in nurseries and greenhouses may begin as early as late February).
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