Hildenbrandia  Hildenbrandia is a genus of thalloid red alga comprising 26 species. The slow-growing, non-mineralized thalli take a crustose form. "Hildenbrandia" reproduces by means of conceptacles and produces tetraspores. "Hildenbrandia" cells are around 3–5 μm in diameter and the filaments are around 50–75 μm in height. The thallus comprises two layers; the hypothallus, which attaches to the rock, and the perithallus, a pseudoparenchymous layer comprising vertical filaments, which unlike coralline red algae is not further differentiated. "Hildenbrandia" comprises orderly layers of vertical oblong cells with thick vegetative cell walls, occasionally connected by secondary pit connections with pit plugs in the septal pores. It grows at its margins, away from the centre, and is able to quickly repair any gaps arising by regenerating from a basal layer of cells. As plants become more mature, they become multi-layered and strongly pigmented near their centres, whilst their single-layered margins begin to grow more slowly. Multi-layered areas may develop in the margins; these will detach and float away as gemmae to form new colonies, leaving a single layer of cells beneath them once they separate from the host plant. Newly settled gemmae form rhizoids. Conceptacles develop in a haphazard manner; cells in conceptacle regions deform one another and become less regularly shaped as they grow larger. In a similar fashion to the coralline algae, the outer layer of the thallus is shed seasonally, presumably to avoid colonization by epiphytes. The freshwater species "H. rivularis" and "H. angularis" seems to form a clade, and require an alkaline pH and hard water, preferring clean water. Unlike most other freshwater red algae (which prefer running water), "H. rivularis" prefers still water, particularly shady lakes or ponds. "H. rubra" and other marine species are found in brackish waters, but freshwater / gemma-bearing species cannot tolerate even moderate salinities. The genus is often found in a symbiotic partnership with fungi. "Hildenbrandia" has a remarkable tolerance to stresses including extreme temperatures, desiccation, and Ultra-violet light; it can be up and photosynthesizing near full capacity just minutes after being cooled to −17 °C or subjected to extreme salinities.