Saxifrage Family
Saxifrage Family
Ecological and economic importance
The saxifrages, currants, and gooseberries are about 40 genera and about 850 species of plants that make up the family Saxifragaceae. These plants occur in all parts of the world, but are most diverse and prominent in arctic, boreal, and montane habitats of North America and Eurasia. The largest genera in the family are the saxifrages (Saxifraga spp.), of which there are about 300 species, most of which occur in the tundras of alpine and arctic environments, and the currants and gooseberries (Ribes ), with about 150 species in boreal and temperate habitats.
Most species in the saxifrage family are perennial herbs, while others are woody shrubs or small trees. Their leaves are usually simple, small, with a toothed margin or tip, and can be arranged alternately or oppositely on the stem. The flowers are perfect (that is, bisexual), containing both female and male reproductive structures. There are usually five sepals and five petals, and usually twice as many stamens as petals. The pistil usually has two (but as many as four) carpels, each with its own stigma and style, producing a distinctive, split unit with outward-curving stig-matic tips. The fruit is a dry capsule, containing many small seeds, or in the case of Ribes, a many-seeded berry. The stems of the shrub-sized currants and gooseberries (Ribes spp.) are often armed with spines and prickles.
Species in North America
Species of the saxifrage family are prominent in certain habitats in North America. The genera are described below, with particular reference to species occurring in North America.
The most diverse group is the saxifrages. The swamp saxifrage (Saxifraga pensylvanica ) occurs in wet meadows, bogs, and moist woods over much of eastern North America, while the early saxifrage (S. virginiensis ) occurs in dry forests and rocky habitats. Most species, however, are alpine or arctic in their distribution. Relatively widespread species that occur in both alpine and arctic tundras include the
purple mountain saxifrage (S. oppositifolia ), golden saxifrage (S. aizodes ), spider-plant (S. flagellaris ), prickly saxifrage (S. tricuspidata ), snow saxifrage (S. nivalis ), and bulblet saxifrage (S. cernua ).
The lace flowers or foam-flowers occur in moist woods and include Tiarella cordifolia of eastern North America and T. trifoliata and T. unifoliata of western North America.
Miterworts occur in moist woods and bogs. Mitella diphylla and M. nuda occur in the east, while M. pentandra is in western North America.
Several species of grass-of-parnassus occur in cool, open, wet places, including the widespread northern grass-of-parnassus (Parnassia palustris ).
Currants and gooseberries are shrubs that occur extensively in boreal and temperate habitats. The bristly black currant (Ribes lacustre ), northern black currant (R. hudsonianum ), skunk currant (R. glandulosum ), and northern red currant (R. triste ) all occur widely in boreal and montane habitats. More temperate species include wild black currant (R. americanum ), gooseberry (Ribes hirtellum ), swamp currant (R. lacustre ), and golden currant (R. odoratum ).
The hydrangea (Hydrangea arborescens ) is another native shrub in the saxifrage family that occurs in southeastern North America.
Ecological and economic importance
Species in the saxifrage family are important components of certain natural habitats, especially in alpine and arctic tundras, where as many as 7-10 species of Saxifraga can occur in the same local habitat.
Many species in the saxifrage family are grown as ornamentals in horticulture. Various species of native and Eurasian Saxifraga are commonly grown in rock gardens. Some other species native to temperate North America are also sometimes grown in horticulture, including bishop’s cap (Mitella spp.), coral bells or alum root (Heuchera spp.), and lace flower or foam-flower (Tiarella spp.). Currants and gooseberries that flower prominently are also grown as ornamental shrubs in gardens, including Ribes alpinum, R. americanum, R. speciosum, and other species. Hydrangeas are also cultivated as flowering shrubs, including the Eurasian species, Hydrangea paniculata and H. macrophylla.
The fruits of currants and gooseberries are important agricultural crops in some areas, particularly in Europe and Asia. Currants and gooseberries are not, however, widely grown in North America, because they are an alternate host for white pine blister rust (Cronartium ribicola ), an important, introduced fungal pathogen of white pine (Pinus strobus ) and other five-needled pines, which are economically important species of trees.
The most common species of currants and gooseberries in cultivation are the red-fruited currant (Ribes rubrum ); there is also a white-fruited variety of this species) of Europe, the black-fruited currant (R. nigrum ) of Eurasia, and the gooseberry (R. grossularia ) of Eurasia, which can have red, yellow, green, or white fruits, depending on the variety. Native North American species with abundant, edible fruits include the wild black currant (Ribes americanum ) and wild gooseberry (Ribes hirtellum ). The species of Ribes that are known as currants have smooth fruits and stems, and their flowers and fruits occur in elongate inflorescences known as racemes. The gooseberries have prickly or spiny stems and fruits, and their flowers and fruits occur in a solitary fashion. Most currants and gooseberries are dried as a means of preservation, or are used to make jams, jellies, pies, and wine.
KEY TERMS
Alternate host —Many pathogens and parasites must infect two or more different species in order to complete their reproductive cycle. If one of those alternate hosts can be eliminated from the ecosystem, then disease transmission can be interrupted, and the other host can be productive and healthy.
Boreal —This refers to the conifer-dominated forest that occurs in the sub-Arctic, and gives way to tundra at more northern latitudes.
Montane —This refers to the conifer-dominated forest that occurs below the alpine tundra on mountains.
Perfect —In the botanical sense, this refers to flowers that are bisexual, containing both male and female reproductive parts.
Raceme —An elongate inflorescence, consisting of individual flowers arranged along a linear axis, with the oldest ones being closest to the bottom.
Tundra —This is a treeless ecosystem that occurs at high latitude in the Arctic and Antarctic, and at high altitude on mountains.
Resources
BOOKS
Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics: A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.
Lopez, Barry, Debra Twartney. eds. Home Ground: Language for an American Landscape. Philadelphia: Trinity University Press, September 2006.
Bill Freedman