Flora of the Canadian Arctic Archipelago
English: Milky whitlow grass,
French: Drave laiteuse.
Brassicaceae (Cruciferae), Draba family.
Published in Mém. Soc. Imp. Naturalistes Moscou 5: 104. 1817.
Synonymy. Draba wahlenbergii Hartm., Handb. Skand. Fl. 249. 1820.
Draba pseudopilosa Pohle, Izv. Imp. Bot. Sada Petra Velikago 14: 469. 1914.
Draba lactea Adams f. heterotricha Ekman, Svensk Bot. Tidskr., 26: 433. 1932.
Draba allenii Fernald, Rhodora 36: 289. 1934.
Draba boecheri Gjærev. and Ryvarden, Kongel. Norske Vidensk. Selsk. Skr. (Trondheim) 1977, 4: 23. 1978.
Vegetative morphology. Plants 5–15(–20) cm high; perennial herbs; caespitose (forming small, loose mats). Taproot present. Ground level or underground stems vertical. Horizontal stems at ground level, branching extensively to shape plant habit as mats (of rosettes). Caudex present (branching loosely, close to ground level in older plants). Aerial stems erect. Aerial stem trichomes spreading (or not applicable). Leaves mainly basal, or basal in a rosette; alternate, or whorled; marcescent. Petioles absent. Leaf blades simple. Leaf blade bases attenuate and truncate. Leaves not grass-like. Blades 8–12 mm long, (1.5–)2.5–3.5 mm wide, oblanceolate or obovate (or linear oblong), appearing single-veined (especially in older marcescent leaves) or with inconspicuous veins (younger leaves). Blade adaxial surface glabrous (usually) or glabrescent or hairy (with sparse hairs), hairs branched (irregularly, if applicable (CAN 267682)), hairs sparse (if present), hairs white, or translucent. Blade abaxial surface glabrous (usually) or glabrescent or hairy, hairs sparse (if applicable), hairs white, hairs irregularly branched, hairs spreading. Blade margins entire, with non-glandular hairs (with long hairs); apices acute.
Reproductive morphology. Flowering stems without leaves. Flowering stem hairs branched (irregularly, if applicable); white or translucent. Inflorescences racemose (often compact in flower but elongating in fruit); diffuse; lanceolate; elongating as the fruit matures. Pedicels glabrous. Flowers per inflorescence 3–9; small (relatively large for a draba); radially symmetrical (actinomorphic). Sepals conventional; 4; free; 0.9–1.2 mm long; (2–)2.5–3 mm wide; green and purple (tinged, sometimes with a membranous margin); herbaceous. Calyx glabrous (mainly), or hairy (with a few sparse hairs towards the apex). Calyx hairs pilose; non-glandular; white or translucent. Petals conventional; free; 4; white; without contrasting markings; obovate, or obtriangular; unlobed, or slightly lobed or undulating; (3–)3.5–4(–4.5) mm long; 2.2–2.7 mm wide (at top, tapering to 0.3–0.4 mm at the base). Stamens 6; stamen filaments markedly unequal in length; stamen filaments glabrous; free of the corolla. Anthers yellow; sub-globose; 0.5–0.6 mm long. Ovary superior; carpels 2; syncarpous. Ovaries ovate; glabrous. Styles 1; 0.2–0.3 mm long (appearing stout); straight. Stigmas per ovary 1. Placentation parietal. Ovules per ovary (16–)20–24(–28). Fruit stalked; stalk (3–)5–12 mm long; dry; a silique; ellipsoid, or ovoid; green at maturity (a drab, yellowish green); (6–)8–9(–10) mm long; 2.5–3.5 mm wide; glabrous; surface appearing veinless; distinctly flattened; dehiscent; shedding the outer walls to expose a thin inner wall, with the seeds attached at the margins on either side. Styles persisting in fruit 0.3–0.5 mm long. Seeds (16–)20–24(–28); 1–1.2 mm long; brown; surfaces smooth (at 10×), rugose (at 40×).
Chromosome information. 2n = 32 and 48.
2n (4x) = 32. Zhukova and Tikhonova (1971, Chukotka, as D. pseudopilosa); Zhukova and Petrovsky (1984, northeastern Asia, as D. fladnizensis and D. pseudopilosa); Grundt et al. (2004a and 2004b, western Alaska, Greenland, tetraploid in cytometry);
2n (6x) = 48. Jørgensen et al. (1958, Greenland); Böcher (1966a, Greenland and Svalbard); Knaben (1966, Norway); Knaben and Engelskjøn (1967, Norway); Zhukova (1968, northeastern Asia, as D. pseudopilosa); Zhukova and Tikhonova (1971, 1973, northeastern Asia, as D. pseudopilosa); Zhukova et al. (1973, northern and northeastern Asia, as D. pseudopilosa); Mulligan (1974, Alaska, two counts, northern Canada, seven counts); Packer and McPherson (1974, northern Alaska); Engelskjøn (1979, northern Norway); Löve and Löve (1982, Arctic Canada); Zhukova and Petrovsky (1980, western Chukotka, as D. pseudopilosa, 1984, western Chukotka, as D. lactea and D. pseudopilosa); Brochmann et al. (1993, Svalbard); Grundt (unpublished Alaska, Canada, Greenland, Svalbard, Norway, Siberia, hexaploid in cytometry).
Ploidy levels recorded 4x and 6x.
Ecology and habitat. Substrates: wet meadows, hummocks, around the margins of ponds, along streams, river terraces, lakeshores, tundra, slopes, ridges, cliffs; imperfectly drained moist areas (seepage swales), seepage slopes; rocks, gravel, sand, silt, clay, till, moss; with low organic content, with high organic content, peat; calcareous (broken limestone (CAN 234113)). Sandstone, basal scree in a gully (CAN 26766682); eroded peat hummocks (CAN 267691).
North American distribution. Alaska, Yukon, Northwest Territories Islands, continental Northwest Territories, Nunavut Islands, continental Nunavut, northern Quebec, Labrador. Range in the Canadian Arctic Archipelago widespread. Common. High Arctic. Arctic islands: Baffin, Devon, Ellesmere, Axel Heiberg, Parry islands (Bathurst, Melville, and Prince Patrick), Cornwallis, Banks, Victoria, Prince of Wales, Somerset, King William, Southampton (Bylot, Coburg, Ellef Ringnes Island, Prince Charles, and Jenny Lind islands, Boothia and Melville peninsulas).
Northern hemisphere distribution. Circumpolar. KaninPechora, Svalbard Franz Joseph Land, Polar Ural Novaya Zemlya, YamalGydan, Taimyr Severnaya Zemlya, AnabarOlenyok, Kharaulakh, YanaKolyma, West Chukotka, Wrangel Island, East Chukotka, West Alaska, North Alaska Yukon, Central Canada, Labrador Hudson Bay, Ellesmere Land Peary Land, West Greenland, East Greenland.
General notes. This species is mainly hexaploid, but tetraploids are also known (Grundt et al. 2005b). The species has been hypothesised as originating from a cross between D. fladnizensis and D. nivalis, but molecular data (including cloning of ITS sequences) rather points towards only one parent, close to the present-day diploid amphi-Beringian species D. palanderiana Kjellm. (Grundt et al. 2005). Natural hybrids between D. lactea and D. fladnizensis or D. nivalis have never been proved, and artificial hybrids are sterile (Brochmann et al. 1993).
Populations collected from Alaska with tetraploid counts have been found to cluster together with hexaploid lactea populations in a UPGMA analysis based on RAPDs (Random Amplified Polymorphic DNAs) (Grundt et al. 2005). There is reason to believe that tetraploid and hexaploid populations contain the same diploid ancestal genomes, and that the hexaploid populations simply have an extra copy of one of them. Thus, both should be contained within the same species (Grundt, personal communication, 2001).
Brochmann et al. (1992a-d) investigated this species as one of three morphologically variable Nordic species and their possible progenitors, using enzyme electrophoresis and restriction site analysis of chloroplast DNA (cDNA) and nuclear ribosomal RNA (rDNA) genes, and found the electrophoretic data showed high levels of fixed heterozygosity in the three polyploids, showing that they are all genetic alloploids. Electrophoretic and rDNA and pollen exine sculpturing data suggest that D. lactea is a polyphyletic polyploid and likely to have formed numerous times in different areas.
Scheen et al. (2002) studied the delimitation of D. lactea and D. fladnizensis morphologically, enzymatically, and using DNA (RAPD) to test the hypothesis that the origin of D. lactea (reported as a hexaploid) is possibly derived from the diploids D. fladnizensis, D. nivalis and (or) D. subcapitata. In extensive analyses they found that the intrapopulational isozyme variation was low or absent, the diploids were almost invariably homozygous, and D. lactea was a highly fixed heterozygous. Multivariate analyses of the RAPD data revealed four very distinct groups of multilocus phenotypes. These groups also differed in several morphological characters and corresponded to the four tentative species. The species were less differentiated in isozyme loci, in particular the diploids D. fladnizensis and D. subcapitata, but D. lactea was clearly distinguished from D. fladnizensis based on all three data sets. Contrary to earlier suggestions, the hexaploid D. lactea was genetically more similar to D. subcapitata than to D. nivalis.
Illustrations. • Close-up of plant. Short plants growing between the markers. Nunavut, Baffin Island, Iqaluit. Aiken and Mallory 02–005. Scale bar in cm. • Close-up of flowering plant. Plant growing in moss mats, a comparatively moist habitat for plants in the "fladnizensis group". Note the comparatively large white flowers, i.e. 3–5 mm long petals verses 2–3 mm in the other taxa in the group. The flowering stems are usually leafless and glabrous, but are occasionally somewhat pubescent at the base. The emerging siliques are glabrous. Svalbard. 1997. Photograph by Hanne H. Grundt. No voucher. • Fruiting plant. Note narrowly elliptic and glabrous siliques and leaves grey-coloured from a dense pubescence. Alaska, Seward Peninsula. 1998. Hanne H. Grundt. Coll. 28. Voucher at O. Photograph by Hanne H. Grundt. • Close-up of leaves. Leaves with margins that have a ciliate fringe of hairs and some stellate hairs on the surface. The number of stelllate hairs on the leaves is highly variable. Aiken and Mallory 02–005. CAN. • Close-up of plant. Drawing by Mrs. S. Bergh and Mrs. L. Barstad based on a collection from Svalbard, Oscar II Land, Kapp Boheman, 1 km aust for signalet [1 km E. of cairn]. 30 August, 1924. J. Lid. (eller: Kapp Boheman, vik inn for ytste neset. 30 August, 1924. J. Loid (as D. wahlenbergii). 0 209747). O 209724 (SJEKK HERB) With permission of the Botanical Museum, University of Oslo, Norway. • Arctic Island Distribution.
This publication is available on the internet (posted May 2011) and on CD-ROM (published in 2007). These versions are identical in content, except that the errata page for CD-ROM is accessible on the main index page of the web version.
Recommended citation for the web-based version of this publication: Aiken, S.G., Dallwitz, M.J., Consaul, L.L., McJannet, C.L., Boles, R.L., Argus, G.W., Gillett, J.M., Scott, P.J., Elven, R., LeBlanc, M.C., Gillespie, L.J., Brysting, A.K., Solstad, H., and Harris, J.G. 2007. Flora of the Canadian Arctic Archipelago: Descriptions, Illustrations, Identification, and Information Retrieval. NRC Research Press, National Research Council of Canada, Ottawa. http://nature.ca/aaflora/data, accessed on DATE.
Recommended citation for the CD-ROM version of this publication: Aiken, S.G., Dallwitz, M.J., Consaul, L.L., McJannet, C.L., Boles, R.L., Argus, G.W., Gillett, J.M., Scott, P.J., Elven, R., LeBlanc, M.C., Gillespie, L.J., Brysting, A.K., Solstad, H., and Harris, J.G. 2007. Flora of the Canadian Arctic Archipelago: Descriptions, Illustrations, Identification, and Information Retrieval. [CD-ROM] NRC Research Press, National Research Council of Canada, Ottawa..