2. Central cavity at least 80% of the diameter of stem (the stems therefore very easy to compress when alive and often very flat when pressed); stems with or without branches, often growing in standing water.

2. Central cavity less than 50% of the diameter of stem (the stems therefore not easily compressed when alive and not as severely flattened when pressed); stems usually with regularly whorled branches, usually not in standing water (although the soil may be wet).