Western Poison-oak (Toxicodendron diversilobum) is found at elevations below around 1650 m in westernmost North America from British Columbia (Canada) to Baja California (Mexico), including the U.S. states of Washington, Oregon, California, and Nevada. It may grow as a shrub (sometimes tree-like), 0.5 to 4 m tall, or (when externally supported) as a vine more than 30 m long--or any form in between (Gartner 1991a,b). The (typically) 3-leafleted leaves turn bright red in autumn. Poison-oak and its close relatives are well-known for possessing skin-irritating oil (urushiol), which can cause severe allergic reactions in humans.
The taxonomy and nomenclature of North American Toxicodendron has been in flux for over a century, largely due to confusing within-species variation in growth form, leaf and leaflet shape, and other features (e.g., Gillis 1971; Gartner 1991). This has resulted in an abundance of synonyms, but five species are now generally recognized: Common Poison-ivy (T. radicans), Western Poison-ivy (T. rydbergii), Eastern Poison-oak (T. pubescens), Western Poison-oak (T. diversilobum), and Poison-sumac (T. vernix) (Senchina 2006).
Senchina (2008) reviewed the literature on animal and fungal associates of Toxicodendron in North America with a particular eye toward identifying potential biological control agents. Interest in finding new ways to control poison-oak and its relatives may increase in coming years given data suggesting that these plants may become more abundant and more ‘‘toxic’’ in the future, potentially affecting global forest dynamics and human health (Mohan et al. 2006).
Pacific poison oak, Toxicodendron diversilobum (also known as Rhus diversiloba), is an angiosperm of the Anacardiaceae (sumac) family that is confined to western North America and is native to British Columbia, Washington, Oregon, northwestern Nevada, and California, where it grows most prolifically (USDA 2016). It is not a true oak and is closely related to poison ivy (Toxicodendron radicans), its counterpart in eastern North America (NPIN 2007). It is a many stemmed, deciduous shrub or woody vine that can survive and flourish in conditions from part shade to full sun. As a shrub it grows 2-6 ft. tall, and as a vine it climbs trees or other support by adventitious roots and/or wedging stems within grooves or crevices of the support. Vine stems typically grow 10-30 ft., but may be as long as 100 ft. (Gartner 1991). The leaves are bright green (often red in fall) with three or five round to ovate, diversely lobed or toothed leaflets that usually resemble oak leaves (Munz 1974). Leaves are usually glossy and both they and the other parts of the plant are covered with a toxic oil (urushiol) that causes a sometimes severe skin rash, at least in humans, although deer and livestock often browse the plant with impunity (NPIN 2007). Poison oak flowers in the summertime, with clusters of white flowers blooming from leaf axils. The fruits are small green or white berries (Munz 1974). As a perennial, T. diversilobum regrows and expands on its previous branches each year, and it is a hardy plant that is one of the first to regrow after forest fires.
Poison oak thrives in mixed coniferous forests, shaded woodlands, and chaparral, as well as in open fields, growing most thickly near streams or springs where it has unlimited access to water. It is common in most warm-climate woodlands and generally cannot survive at elevations above 5000 feet. Often, poison oak dominates the undergrowth, smothering surrounding trees and ground level vegetation with its sprawling vines. Its hardiness and ability to proliferate, coupled with its danger to humans, contributes to the almost universal perception of poison oak as a pest plant (Howard 1994).
- Gartner, Barbara L. 1991. Relative growth rates of vines and shrubs of western poison oak, Toxicodendron diversilobum (Anacardiaceae). American Journal of Botany 78(10): 1345-1353.
- Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). http://www.fs.fed.us/database/feis/. Accessed: May 17, 2016.
- Munz, Philip A. 1974. A Flora of Southern California. Berkeley, CA: University of California Press.
- NPIN. 2007. Toxicodendron diversilobum (Pacific poison oak). Native Plant Information Network, Lady Bird Johnson Wildflower Center. http://www.wildflower.org/plants/result.php?id_plant=LOCI3. Accessed May 15, 2016.
- USDA, NRCS. 2016. Plant profile for Toxicodendron diversilobum (Pacific poison oak). The PLANTS Database (http://plants.usda.gov, accessed: May 16, 2016). National Plant Data Team, Greensboro, NC 27401-4901 USA.
Occurrence in North America
Columbia [31,45,58]. It occurs west of the Cascade Range in Washington,
Oregon, and California  and is ubiquitous in California west of the Sierra
Nevada and the Mojave Desert .
Regional Distribution in the Western United States
This species can be found in the following regions of the western United States (according to the Bureau of Land Management classification of Physiographic Regions of the western United States):
1 Northern Pacific Border
2 Cascade Mountains
3 Southern Pacific Border
4 Sierra Mountains
Pacific poison-oak is a many-stemmed, deciduous, native shrub or woody vine.
Shrubs are erect with stems from 2 to 6 feet (1-2 m) tall. Vine stems
commonly reach 10 to 30 feet (3-10 m), but may be as long as 100 feet
(30 m) . As a vine, Pacific poison-oak climbs trees or other support by
adventitious roots and/or wedging stems within grooves or crevices of
the support [7,22,62]. The bright green leaves have three (sometimes
five) round to ovate, diversely lobed or toothed leaflets that usually
resemble oak leaves [45,62]. Small flowers occur in leaf axils, with
male and female flowers on separate plants [38,53]. The fruits are
white drupes . Rhizomes are at or just below the soil surface, and
are extensive .
Key Plant Community Associations
Pacific poison-oak occurs in mixed evergreen forests [29,30,32,59], woodlands,
chaparral, [25,26,27], coastal sage scrub , and riparian zones
[25,26,27,39,58]. It is the most widespread shrub in California .
Holland  described a Pacific poison-oak chaparral community type that may be
maintained by frequent fire. Because it is dominated by Pacific poison-oak,
little is known of its community composition.
Many of the plant species commonly associated with Pacific poison-oak were
previously listed under DISTRIBUTION AND OCCURRENCE information. Other
common associates follow, listed by community type.
Associates in mixed evergreen forests include Pacific madrone (Arbutus
menziesii), sugar pine (Pinus lambertiana), bigleaf maple (Acer
macrophyllum), tanoak (Lithocarpus densiflorus), California bay
(Umbellularia californica), and chinquapin (Chrysolepsis chrysophylla)
Woodland associates include valley oak (Quercus lobata), interior live
oak (Q. wislizenii), Monterey pine (Pinus radiata) , Coulter pine
(P. coulteri) , bigcone Douglas-fir (Pseudotsuga macrocarpa) [8,64],
and California walnut (Juglans californica) .
Chaparral associates include toyon (Heteromeles arbutifolia), chamise
(Adenostoma fasciculatum), and California scrub oak (Quercus dumosa).
Coastal sage scrub associates include California sagebrush (Artemisia
californica), coyote brush (Baccharis pilularis), and sugar sumac (Rhus
Pacific poison-oak associates in riparian zones include bigleaf maple,
California sycamore (Plantus racemosa), white alder (Alnus rhombifolia),
, boxelder (Acer negundo), willow (Salix spp.), California
blackberry (Rubus vitifolius), toyon, and wild grape (Vitis spp.) .
Published classifications naming Pacific poison-oak as a dominant part of the
Description and classification of the forests of the upper Illinois
River drainage of southwestern Oregon 
Preliminary plant associations of the Siskiyou Mountain Province 
Coast redwood ecological types of southern Monterey County, California 
Plant communities of Santa Rosa Island, Channel Islands National Park 
Plant association and management guide: Siuslaw National Forest 
Plant association and management guide: Willamette National Forest 
The community composition of California coastal sage scrub 
Plant associations within the Interior Valleys of the Umpqua River
Basin, Oregon 
The vascular plant communities of California 
An introduction to the plant communities of the Santa Ana and San
Jacinto Mountains .
Habitat: Plant Associations
This species is known to occur in association with the following plant community types (as classified by Küchler 1964):
More info for the term: shrub
K001 Spruce - cedar - hemlock forest
K002 Cedar - hemlock - Douglas-fir forest
K005 Mixed conifer forest
K006 Redwood forest
K009 Pine - cypress forest
K010 Ponderosa shrub forest
K026 Oregon oakwoods
K028 Mosaic of K002 and K026
K029 California mixed evergreen forest
K030 California oakwoods
K034 Montane chaparral
K035 Coastal sagebrush
This species is known to occur in the following ecosystem types (as named by the U.S. Forest Service in their Forest and Range Ecosystem [FRES] Type classification):
FRES21 Ponderosa pine
FRES24 Hemlock - Sitka spruce
FRES28 Western hardwoods
FRES34 Chaparral - mountain shrub
particular soil texture or drainage pattern. It occurs on well-drained
slopes and in riparian zones [1,39,64]. It is found at elevations of
less than 5,000 feet (1,524 m) west of the Sierra Nevada, growing on all
aspects . In the Siskiyou Mountains it is found at up to 4,400-foot
(1,340-m) elevations on steep southern exposures .
Habitat: Cover Types
This species is known to occur in association with the following cover types (as classified by the Society of American Foresters):
213 Grand fir
222 Black cottonwood - willow
224 Western hemlock
229 Pacific Douglas-fir
230 Douglas-fir - western hemlock
231 Port Orford-cedar
233 Oregon white oak
234 Douglas-fir - tanoak - Pacific madrone
235 Cottonwood - willow
243 Sierra Nevada mixed conifer
244 Pacific ponderosa pine - Douglas-fir
245 Pacific ponderosa pine
246 California black oak
247 Jeffrey pine
248 Knobcone pine
249 Canyon live oak
250 Blue oak - Digger pine
255 California coast live oak
Fire Management Considerations
Urushiol volatilizes when burned, and human exposure to Pacific poison-oak smoke
is extremely hazardous . The smoke often poisons people who think
they are immune to the plant .
Pacific poison-oak vines are a ladder fuel .
Goats can be used as an alternative to prescribed fire for fire hazard
reduction at urban-wildland interfaces. Near Oakland, California, goats
were put on a Monterey pine-redgum (Eucalyptus camaldensis) forest with
a heavy shrub understory and on an adjacent site where the forest was
managed as a fuelbreak and had less shrub cover in the understory. Goat
utilization of Pacific poison-oak was in the fuelbreak 67 percent, somewhat
lower than utilization of toyon, California blackberry, and coyote brush.
Annual production of Pacific poison-oak biomass before goat browsing in the
fuelbreak was 99 kilograms per hectare; it was 33 kilograms per hectare
afterwards. Total biomass of forage species was significantly (p less than 0.05)
A stocking rate of 600 goats per hectare on the Oakland site broke the
vertical live fuel continuity in the dense shrub stand. Initial goat
browsing to reduce biomass and vertical fuel continuity could be
followed up by prescribed fire .
Broad-scale Impacts of Plant Response to Fire
the plant community. Dense Pacific poison-oak thickets may develop in chaparral
that is control burned several times . Pacific poison-oak may become
locally extinct in Douglas-fir forest, however, that is burned every 4
years for 20 years or more .
Plant Response to Fire
Pacific poison-oak sprouts vigorously from the root crown and/or rhizomes after
fire [13,15,43,46,52]. It sprouts in the first postfire growing season,
and for several years thereafter [13,16,52]. Pacific poison-oak sprouts were
noted the September following the July, 1985, Wheeler Fire on the Los
Padres National Forest, California. The wildfire had spread into a
riparian zone containing Pacific poison-oak; prefire Pacific poison-oak density was
unknown. By postfire year 3, Pacific poison-oak sprouts dominated most burn
plots in the riparian zone .
Westman and others  estimated that Pacific poison-oak fails to sprout when
fire reaction intensity exceeds 200 kcal/sec/sq m. Their estimate was
derived by modelling fire behavior of a backfire set in coastal sage
scrub in the Santa Monica Mountains of California, and observing
sprouting the following year. The coastal sage scrub had not burned for
20 to 22 years.
Pacific poison-oak also establishes from seed after fire, although this response
is not well documented in the literature. Pacific poison-oak seedlings were
observed following site preparation and prescribed burning of an
interior live oak-blue oak woodland in Madera County, California.
Prefire Pacific poison-oak seedling density was 0 percent; seedling density at
postfire year 1 was 42 per 8,712 square feet .
Response of vegetation to prescribed burning in a Jeffrey pine-California
black oak woodland and a deergrass meadow at Cuyamaca State Park,
California provides information on prescribed fire use and postfire
response of many mixed-conifer woodland species including Pacific poison-oak.
Immediate Effect of Fire
Fire top-kills Pacific poison-oak [13,16]. Wirtz  reported that an October,
1953, wildfire in a coastal sage scrub/grassland community near
Berkeley, California, top-killed all Pacific poison-oak present, leaving only
large branches and stumps.
Rhizomes on the soil surface are probably killed by all but
light-severity fire, and shallowly buried rhizomes are probably killed
by moderate to severe fire. More deeply buried rhizomes are probably
Tall shrub, adventitious-bud root crown
Rhizomatous shrub, rhizome in soil
Ground residual colonizer (on-site, initial community)
Secondary colonizer - off-site seed
Pacific poison-oak's primary POSTFIRE REGENERATION STRATEGY is vigorous
sprouting from the root crown and/or rhizomes [16,46,68].
Fire is not required for Pacific poison-oak seed germination. Keeley ,
however, reported a significant (p less than 0.001) increase in germination when
seeds were exposed to charate. Postfire seedlings probably originate
from both soil-stored seed and fresh seed dispersed by birds.
More info for the terms: climax, cover
Facultative Seral Species
Pacific poison-oak is a somewhat shade-tolerant species commonly occurring in
seral woodland and mixed evergreen forest understories [51,56]. It is
considered a climax species on south-slope Douglas-fir forests of the
Willamette Valley foothills, Oregon . In climax oak woodland,
Pacific poison-oak cover may reach 25 to 50 percent .
Pacific poison-oak reproduces vegetatively by sprouting from the rhizomes and
root crown after disturbance such as fire or browsing has removed
topgrowth [15,44,53]. It also reproduces by layering when vine stems
contact the ground .
Pacific poison-oak seeds are dispersed by birds . Seedlings occur both
before and after fire, suggesting that the seeds do not depend upon fire
for scarification. The seeds have a gummy seedcoat which leaches off
very slowly, resulting in delayed germination .
Pacific poison-oak is propagated by stem cuttings .
Growth Form (according to Raunkiær Life-form classification)
More info for the terms: hemicryptophyte, phanerophyte
Life History and Behavior
Pacific poison-oak leaf buds open from February to March, and stems elongate
from March to April . Flowering occurs from March to
June . Leaves drop from late July to early October ,
and fruits disperse in summer and fall [37,53].
National NatureServe Conservation Status
Rounded National Status Rank: N2 - Imperiled
Rounded National Status Rank: N5 - Secure
NatureServe Conservation Status
Rounded Global Status Rank: G5 - Secure
Safety/Medical: The entire Pacific poison-oak plant is covered with oily resin.
Human dermatitis results when skin comes in direct contact with the oil,
either by touching the plant or touching something that has contacted
it, such as clothing or firewood. Urushiol is the poison present in the
oil . Pacific poison-oak does not cause dermatitis in wildlife or
livestock, but pets may react to it . (See FIRE MANAGEMENT.) American
folklore holds that drinking the milk of Pacific poison-oak-fed goats bolsters
the immune system against Pacific poison-oak because the poison is present in
the milk in trace amounts. Drinking the milk probably does not grant
immunity, however. Analysis of milk from does fed a straight Pacific poison-oak
diet for 3 days showed no trace of urushiol. Some urushiol was present
in the does' urine, but most was apparently catabolized .
Control: Pacific poison-oak is controlled by glyphosate, triclopyr, or 2,4,5-T.
Used alone, 2,4-D is ineffective. Goats are an effective biological
Other: Pacific poison-oak vines sometimes kill their support plant by
smothering or breaking it .
Pacific poison-oak blossoms are a source of good honey .
Relevance to Humans and Ecosystems
Value for rehabilitation of disturbed sites
Pacific poison-oak has been recommended for use in restoration projects.
Information on propagation and handling methods to "minimize risks" to
planting crews is available . Having worked on field crews in the
Sierra Nevada foothills, however, this author recommends using native
shrubs other than Pacific poison-oak for restoration.
The federally endangered least Bell's vireo uses Pacific poison-oak for nest
sites in oak woodlands .
Fremont cottonwood (Populus fremontii)/Pacific poison-oak woodlands contribute
to bird diversity and density in California . A rare colony of
ringtail was found inhabiting a Fremont cottonwood/Pacific poison-oak woodland
on the Sacramento River .
California averaged 24.2 in March, 20.6 in May, 10.1 in July, and 6.5 in
September . Pacific poison-oak is relatively high in phosphorus, sulfur,
and calcium as compared to other browse species . The following
mineral content (percentage basis) was reported for the foliage :
Ca P K Mg S
1.00 0.23 1.13 0.59 0.19
Other uses and values
may have application in DNA sequence studies .
Native Americans used the stems to make baskets and the sap to cure
ringworm [15,60]. Chumash Indians used Pacific poison-oak sap to remove warts,
corns, and calluses; to cauterize sores; and to stop bleeding. They
drank a decoction made from Pacific poison-oak roots to treat dysentery .
Importance to Livestock and Wildlife
It is the most important black-tailed deer browse in some areas of
California [5,6]. Birds eat Pacific poison-oak fruits .
Toxicodendron diversilobum, commonly named Pacific poison oak or western poison oak (syn. Rhus diversiloba), is a woody vine or shrub in the Anacardiaceae (sumac) family. It is widely distributed in western North America, inhabiting conifer and mixed broadleaf forests, woodlands, grasslands, and chaparral biomes. It is known for causing itching and allergic rashes in many humans, after contact by touch or smoke inhalation.
T. diversilobum is found in California (also the original name of Los Angeles; Yangna or Iyaanga/poison oak place), the Baja California peninsula, Nevada, Oregon, Washington, and British Columbia. The related T. pubescens (eastern poison oak) is native to the Southeastern United States. T. diversilobum and T. rydbergii (western poison ivy) hybridize in the Columbia River Gorge area.
T. diversilobum is common in various habitats, from mesic riparian zones to xeric chaparral. It thrives in shady and dappled light through full and direct sunlight conditions, at elevations below 5,000 feet (1,500 m). The vining form can climb up large shrub and tree trunks into their canopies. Sometimes it kills the support plant by smothering or breaking it.
The plant often occurs in chaparral and woodlands, coastal sage scrub, grasslands, and oak woodlands; and Douglas-fir (Pseudotsuga menzesii), hemlock–Sitka spruce, Sequoia sempervirens (coast redwood), Pinus ponderosa (Ponderosa pine), and mixed evergreen forests.
T. diversilobum is extremely variable in growth habit and leaf appearance. It grows as a dense 0.5–4 m (1.6–13.1 ft) tall shrub in open sunlight, a treelike vine 10–30 feet (3.0–9.1 m) and may be more than 100 feet (30 m) long with an 8–20 cm (3.1–7.9 in) trunk, as dense thickets in shaded areas, or any form in between  It reproduces by spreading rhizomes and by seeds.
The plant is winter deciduous, so that after cold weather sets in, the stems are leafless and bear only the occasional cluster of berries. Without leaves the stems may sometimes be identified by occasional black marks where its milky sap may have oozed and dried.
The leaves are divided into three (rarely 5, 7, or 9) leaflets, 3.5 to 10 centimetres (1.4 to 3.9 in) long, with scalloped, toothed, or lobed edges. They generally resemble the lobed leaves of a true oak, though tend to be more glossy. Leaves are typically bronze when first unfolding in February to March, bright green in the spring, yellow-green to reddish in the summer, and bright red or pink from late July to October.
Botanist John Howell observed the toxicity of T. diversilobum obscures its merits:
- "In spring, the ivory flowers bloom on the sunny hill or in sheltered glade, in summer its fine green leaves contrast refreshingly with dried and tawny grassland, in autumn its colors flame more brilliantly than in any other native, but one great fault, its poisonous juice, nullifies its every other virtue and renders this beautiful shrub the most disparaged of all within our region."
T. diversilobum leaves and twigs have a surface oil, urushiol, which causes an allergic reaction. It causes contact dermatitis – an immune-mediated skin inflammation – in four-fifths of humans. However, most, if not all, will become sensitized over time with repeated or more concentrated exposure to urushiol.
The active components of urushiol have been determined to be unsaturated congeners of 3-heptadecylcatechol with up to three double bonds in an unbranched C17 side chain. In poison ivy, these components are unique in that they contain a -CH2CH2- group in an unbranched alkyl side chain.
T. diversilobum skin contact first causes itching; then evolves into dermatitis with inflammation, colorless bumps, severe itching, and blistering. In the dormant deciduous seasons the plant can be difficult to recognize, however leafless branches and twigs contact also causes allergic reactions.
Urushiol volatilizes when burned, and human exposure to T. diversilobum smoke is extremely hazardous, from wildfires, controlled burns, or disposal fires. The smoke can poison people who thought they were immune. Branches used to toast food over campfires can cause reactions internally and externally.
Black-tailed deer, mule deer, California ground squirrels, western gray squirrels, and other indigenous fauna feed on the leaves of the plant. It is rich in phosphorus, calcium, and sulfur. Bird species use the berries for food, and utilize the plant structure for shelter. Neither native animals, nor horses, livestock, or canine pets demonstrate reactions to urushiol.
Due to human allergic reactions, T. diversilobum is usually eradicated from gardens and public landscaped areas. It can be a weed in agricultural fields, orchards, and vineyards. It is usually removed by pruning, herbicides, digging out, or a combination.
Californian Native Americans used the plant's stems and shoots to make baskets, the sap to cure ringworm, and as a poultice of fresh leaves applied to rattlesnake bites. The juice or soot was used as a black dye for sedge basket elements, tattoos, and skin darkening.
- Toxicodendron pubescens – Eastern poison oak
- Toxicodendron vernix – Poison sumac
- Toxicodendron radicans – Eastern poison ivy
- Toxicodendron rydbergii – Western poison ivy
- C. Michael Hogan (2008); "Western poison-oak: Toxicodendron diversilobum", GlobalTwitcher, ed. Nicklas Strömberg
- Ron Sullivan (December 7, 2002). "Roots of native names"
- "Toxicodendron diversilobum". Natural Resources Conservation Service PLANTS Database. USDA. Retrieved 2013-09-20.
- U.S. Forest Service: Toxicodendron diversilobum
- Integrated Taxonomic Information System (ITIS): Toxicodendron diversilobum (Western Poison-oak) - Overview
- John Thomas Howell, Frank Almeda, Wilma Follette & Catherine Best (2007). Marin Flora. California Academy of Sciences; California Native Plant Society. p. 264. ISBN 094022870X.
- R. S. Kalish, J. A. Wood & A. LaPorte (1994). "Processing of urushiol (poison ivy) hapten by both endogenous and exogenous pathways for presentation to T cells in vitro". Journal of Clinical Investigation 93 (5): 2039–2047. doi:10.1172/jci117198. PMC 294319. PMID 7910172.
- Mic-ro.com: Contact-Poisonous Plants of the World
- Michael D. Corbett & Stephen Billets (1975). "Characterization of poison oak urushiol". Journal of Pharmaceutical Sciences 64 (10): 1715–1718. doi:10.1002/jps.2600641032.
- John C. Craig, Coy W. Waller, Stephen Billets & Mahmoud A. Elsohly (1978). "New GLC analysis of urushiol congeners in different plant parts of poison ivy, Toxicodendron radicans". Journal of Pharmaceutical Sciences 67 (4): 483–485. doi:10.1002/jps.2600670411.
- Poison Oak/Poison Ivy Information Center
- UC Integrated Pest Management Weed Photo Gallery and information (profile of this plant as an agricultural weed).
- Sunset Western Garden Book [5th edition], (Menlo Park: Sunset Publishing, 1988), p. 506
- Univ. of Michigan, Dearborn – Native American Ethnobotany Database: Toxicodendron diversilobum
- Conrad, C. Eugene. 1987. "Common shrubs of chaparral and associated ecosystems of southern California". Gen. Tech. Rep. PSW-99. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station. 86 p.
- Jan Timbrook (1990). "Ethnobotany of Chumash Indians, California, based on collections by John P. Harrington". Economic Botany 44 (2): 236–253. doi:10.1007/BF02860489. JSTOR 4255231.
Names and Taxonomy
Pacific poison oak
diversilobum (Torr. & Gray) E. Greene (Anacardiacae) . Pacific poison-oak
and western poison-ivy (Toxicodendron rydbergii) hybridize in the Columbia River
Gorge area .
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