A turion is a specialised overwintering bud produced by aquatic herbs, especially in the genera Potamogeton

Potamogeton

Potamogeton is a genus of aquatic, mostly freshwater, plants of the family Potamogetonaceae. Most are known by the common name pondweed, although many unrelated plants may be called pondweed, such as Canadian pondweed...

, Aldrovanda

Aldrovanda

Aldrovanda is a genus of carnivorous plants encompassing one extant species and numerous extinct taxa. The genus is named in honor of the Italian naturalist Ulisse Aldrovandi, the founder of the Botanical Garden of Bologna, Orto Botanico dell'Università di Bologna.The extinct species are known...

and Utricularia. Turion are produced in response to unfavourable conditions such as decreasing day-length or reducing temperature. They are derived from modified shoot apices. They are often rich in starch

Starch

Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...

 and sugar

Sugar

Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

s enabling them to act as storage organs. Although they are hardy (frost

Frost

Frost is the solid deposition of water vapor from saturated air. It is formed when solid surfaces are cooled to below the dew point of the adjacent air as well as below the freezing point of water. Frost crystals' size differ depending on time and water vapour available. Frost is also usually...

 resistant), it is probable that their principal adaptation is their ability to sink to the bottom of a pond or lake when the water freezes. Because water expands anomalously at lower temperatures, water at 4 °C (39.2 °F) is denser than colder water and stays at the bottom, and it is in this water that the turions over-winter before rising again in the spring. Some turions are also drought

Drought

A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...

 resistant allowing them to survive in evanescent pools.

The presence of humic acid

Humic acid

Humic acid is a principal component of humic substances, which are the major organic constituents of soil , peat, coal, many upland streams, dystrophic lakes, and ocean water. It is produced by biodegradation of dead organic matter...

s may speed up the formation of turions.