In swamps are found plants which can survive in loose soil which is covered, more or less, by water; many of these plants grow also in drier conditions. Podocarpus dacrydioides may form swamp forest and Cordyline australis is also common under varying conditions. The New Zealand flax, Phormium tenax, raupo, Typha muelleri and Carex secta are most commonly found. Genera common to bogs include sedges, some ferns (Gleichenia), the rush (Hypolaena), Dracophyllum, Hebe, Dacrydium, and Leptospermum. In the bogs on the mountains and lowlands further south are plants of cushion bog type in which representatives of the genera Oreobolus, Phyllachne, Gaimardia, and Donatia are found. Most striking is the mangrove (Avicennia resinifera) a tree of twisted and gnarled stems, which grows in salty estuaries from North Cape to about 38° S. In ponds and slow-moving rivers are Potamogeton spp., water-milfoils (Myriophyllum), and water chickweed (Montia fontana). In addition there are many fresh-water algae, a very large group of plants, living in many different situations including scums on ponds or thread-like forms, one-celled plants, and those forms of blue-green algae which belong to the thermal regions.