(Holboell's rockcress)Select an option below for more information on this species
is a tall biennial or short-lived perennial herb in the Mustard family (Brassicaceae), with small white (occasionally purple) flowers and long siliques
at maturity. Growing up to 1 m tall from a taproot, plants are typically found in warm, rocky soils, in the subalpine or open sites along riverbanks, slopes and terraces. The stems are usually solitary, hairy at the base and glabrous
above. The basal leaves form a tight rosette, are oblanceolate
, and are covered with minute stellate
hairs. The stem leaves are opposite, clasping and oblong
. The perfect
flowers are in densely flowered racemes
with small (6 - 10 mm long) white to purple petals. The fruits are siliques
, a long, narrow seedpod 3-8 cm long and 1-1.5 mm wide. The siliques
and strongly appressed to the stem, distinguishing this rockcress from others in Alaska which have erect or spreading fruits.
is biennial or short lived perennial with diploid and triploid forms, the diploid forms flowering the first year and mostly for only one year (Bocher 1954
). It typically flowers in early summer, fruiting shortly after.
Plants are monoecious
and have bisexual flowers. The flowers are pollinated by insects, but fertilization is not necessary for seed production. Arabis holboellii
can reproduce via apomixis
, producing seeds clonally (Sharbel and Mitchell-Olds 2001
). Globally, populations consist of diploids, triploids and aneuploids (individuals with irregular chromosome numbers). Diploid plants can be both sexual and apomictic
. Triploid and aneuploid populations are strictly apomictic
. Seeds are produced in siliques
, which split open to allow dispersal via gravity and water.
, a rust fungus, produces pseudo-flowers on Arabis holboellii
. The fungus is parasitic, preventing Holboell's rockcress from forming flowers and forcing it to turn clusters of leaves into pseudo-flowers which bear the spermatagonia of the fungus (Roy 1993
Holboell's rockcress, in the broad sense, occurs in Greenland and widely across North America with several subspecific varieties. Recently four varieties have been elevated to a species rank (Flora of North America). Boechera retrofracta is the taxon found in Alaska in this new treatment. This species occurs widely in North America and in the eastern interior of the Alaska, but is known from one location in the park, near the Toklat River, on the north side of the Alaska Range.
Details are shown in the Plots & Charts found at right, depicting recent Denali data.
Holboell's rockcress is found at 330 m elevation on the Toklat River in Denali. Throughout Alaska it is most often found on south-facing or exposed river bluffs, steppes and floodplains in the lowlands and uplands.
Details are shown in the Plots & Charts found at right. For more on how to interpret these figures, visit Understanding Data Presented.
This species prefers open dry sandy and gravelly slopes.
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