The taxonomy of poison ivy

580,5

FB

V, 4:2-9

1919-1929

cop, 2

FIELD MUSEUM OF NATURAL HISTORY

PUBLICATION 225
BOTANICAL SERIES VOL. IV, No. 3

THE TAXONOMY OF POISON IVY

WITH A NOTE ON THE ORIGIN OF
THE GENERIC NAME

BY

JAMES B. McNAiR

B. E. DAHLGREN
Acting Curator, Department of Botany

Km-r,M<

CHICAGO, U. S. A.

March 14, 1925

FIELD MUSEUM OF NATURAL HISTORY

PUBLICATION 225
BOTANICAL SERIES VOL. IV, No. 3

THE TAXONOMY OF POISON IVY

WITH A NOTE ON THE ORIGIN OF
THE GENERIC NAME

BY

JAMES B. McNAiR

B. E. DAHLGREN

Acting Curator, Department of Botany
EDITOR

NATURAL
HISTORY

SHI UBRURK
APR 13 1925

DIVERSITY 0

CHICAGO, U. S. A.

March 14, 1925

THE TAXONOMY OF POISON IVY

WITH A NOTE ON THE ORIGIN OF
THE GENERIC NAME

JAMES B. McNAiR

During a chemical investigation of Rhus,1 the writer became inter-
ested in the geographical distribution of "poison ivy" (Rhus Toxicoden-
dron L.). The common name applies to several species. On closer
scrutiny it soon became evident that many of the new species which
have been proposed during recent years were synonymous. Conse-
quently an investigation of the taxonomy of the group seemed desirable.

For this study, material was borrowed from the United States
National Herbarium, Washington, D.C. (U. S.) ; the Academy of Natural
Sciences, Philadelphia, Pennsylvania (A.N.S.); and the Rocky Moun-
tain Herbarium, Laramie, Wyoming (R.Mt.). Single specimens were
borrowed from the Gray Herbarium, Cambridge, Massachusetts (G.H.)
and the Herbarium of the Geological Survey of Canada, Ottawa, Can-
ada (Can.). In the citation of specimens, the abbreviations indicated
have been used. Specimens in the Herbarium of the Field Museum are
noted by the letter F. The writer wishes to express his appreciation for
these loans to W. R. Maxon, F. W. Pennell, Aven Nelson, B. L. Robin-
son, and M. O. Malte. For assistance with the taxonomy and form of
the paper the writer is indebted to Mr. J. Francis Macbride, and to
Dr. B. E. Dahlgren, of the Field Museum for criticism of the manu-
script and suggestions.

ORIGIN OF THE GENERIC NAME

The common English name, "sumach," is similar to the ancient
Arabian ' 'sommaq" and the Byzantian aovpam, 2 from which it is probably
derived. The botanical name for the genus Rhus has a more obscure
origin. Miller3 states that the genus name Rhus, as used by him, is

, James B . Rhus Dermatitis, Its Pathology and Chemotherapy. Chicago :
Sy^L University of Chicago Press, 1923.
^ 2 Hehn, V. Kulturplanzen u. Haustiere, ed. 7. Berlin, 1902.

1 Miller, Phillip. The Gardener's and Botanist's Dictionary. Ed. ix, Vol. 2,
"Rhus," 1804.

55

56 FIELD MUSEUM OF NATURAL HISTORY — BOTANY, VOL. IV.

that of Pliny. Pliny (A.D. 23-79)1 says that rhus is from the Greek
name for these trees, povs, and that povs has no Latin equivalent.
The word povs is also used by Dioscorides as a name for these plants
and together with the ablative rore or a corrupted form roris is employed
by various other ancient writers on medicine and animal husbandry.2'6

Some 350 years before Pliny, Theophrastus7 used the word povs in
describing sumachs, and in the sixth century B.C. the word is found to
have been used similarly by Solon, the Athenian.8

When one investigates the etymology of the word povs, a great
difference of opinion is encountered as to its possible derivation and
meaning. Miller9 gives as a possible derivation its contraction from
p6os, and that from the Greek verb p£w "to flow," because certain
products of the plant were formerly used to check hemorrhages.
Boehmer10 suggests that the name is derived from the red color of the
berries, from the word poos, Latin rufus (red), or from tpvdpov,
Latin rubrum (red). Paxton11 gives as a possible origin the Celtic word
"rhudd," or red, from the prevailing color of the autumn foliage, but
the Celtic tribes were probably too far north to have influenced the
derivation of the word rhus, as the plants mentioned by Theophrastus
and Pliny were native to Asia Minor. Since Pliny says the word rhus
or ros has no Latin equivalent, it is thus likely that its origin is to be
found in Greek or in some kindred language of Asia Minor.

ORIGIN OF THE GENERIC NAME AS APPLIED TO
AMERICAN POISON IVY

The first specimen of the poison ivy group to be classified by botan-
ists was that of Cornutus, which, in 1635," he called Edera trifolia
canadensis.

1 Pliny, Caius Secundus. Natural History. English transl. by John Bostock
and H. T. Riley. 3: 179. London, 1855.

2 Dioscorides. Pedanos Anazarbeus Opera quae extant omnia, p. 21. Frank-
fort, 1498.

3 Taurus, Palladius Rutillius. Martius Mensis or Liber xi.i5.i A.D. 350.

4 Columella, Junius Moderatus. A.D. 50.

5 Largus, Scribonius. Compositiones Medicamentorium, p. 152. A.D. 50.

6 Celsus, Aulus Cornelius. Medicinae libri octo, 6.11. A.D. 50.

7 Theophrastus. Enquiry into plants. English transl. by Sir Arthur Hort.
i: 269. New York, 1916.

8 Photius, Patriarch of Constantinople. &wlov rov varpi&pxov A«£eo>»' avvaymyiiL
Ed. by P. P. Dobree. 2: 491, 1. 21. London, 1822.

9 Loc. cit.

10 Boehmer, Georg Rudolf. Lexicon rei herbariae tripartitum. Lipsiae, 1 802.

11 Paxton, J. Botanical Dictionary. Rev. ed., p. 482, 1868.

12 Cornutus, Jacob. Plantarum Canadensis, etc., pp. 96-98. Paris, 1635.

1925. TAXONOMY OF POISON IVY — McNAiR. 57

In 1719* Tournefort renamed this plant Toxicodendron triphyllon
and at the same time established two genera: Rhus, with unequally
pinnate leaves and a villose fruit with a globular nucleus, and Toxico-
dendron, with ternate leaves, a striated fruit, and compressed nucleus.

Linnaeus, in I753,2 reduced Toxicodendron to Rhus.

Miller, in 1804," again divided the genus into Rhus and Toxicoden-
dron, describing the former genus as having only hermaphrodite flow-
ers and the latter dioecious.

Several modern botanists, including Kuntze,4 Greene,8 and Britton
and Brown,8 have accepted Miller's segregation but not the characters
upon which it was based. For their division Britton and Brown have
the following key:

Fruit densely pubescent, its stems smooth.

Flowers in dense terminal panicles, appearing after the leaves

Rhus

Fruit glabrous, or sparingly pubescent, its stones striate

Toxicodendron

From facts pointed out later in this paper, it is evident that the divi-
sion of the genus on any of the foregoing grounds of leaf, flower, fruit,
and seed structure will no longer hold. The genus cannot be divided,
giving to Toxicodendron the 3-foliate Rhus, as R. Vernix L., a poisonous
species, is 7-13 foliate; nor would Toxicodendron include all those with
dioecious flowers; furthermore one of the Toxicodendron species, R. quer-
cifolia (Michx.) Steud., has densely pubescent fruit and a smooth stone.

KEY TO SPECIES

Seeds smooth; fruit usually distinctly papillose or pubescent; d1 petals

2-3 x i ; leaflets with 3-7 regular lobes i . R. quercifolia

Seeds roughened; fruit usually smooth; cf petals 3-4 x 1-2; leaflets
various, if lobed, irregularly.

Leaflets obtuse or rounded at apex, entire, crenate or bluntly

lobed; Pacific Coast species 2. R. diver siloba

Leaflets acute, sometimes abruptly so, entire or serrate.

Seeds regular in outline, at least not definitely pinched in at
the sides; distribution general, except California

3. R. Toxicodendron

Seeds irregular in outline, rounded-cuneiform, definitely
pinched in at the sides; Lower California
4. R. divaricate

1 Tournefort, Joseph Pitton. Institutiones Rei Herbariae. i : 6 10-1 1 . Paris, 1 700.

2 Linnaeus, Carolus. Species Plantarum. i: 266. 1753.

3 Miller, Phillip. Loc. tit.

4 Kuntze, Otto. Revisio Generum Plantarum. Part i, pp. 153-54. Leipzig, 1891.

5 Greene, Edward Lee. Leaflets of Botanical Observation and Criticism, i: 114-
44. Washington, D.C., 1903-6.

6 Britton, Nathaniel Lord, and Brown, Addison. An Illustrated Flora of the
Northern United States, etc. 2: 484. New York, 1913.

58 FIELD MUSEUM OF NATURAL HISTORY — BOTANY, VOL. IV.

i. Rhus quercifolia (Michx.) Steud., Nom. Bot. ed. i: 690 (1821)

R. Toxicodendron L., var. quercifolium Michx., Fl. Bor.-Am. i:

183 (1803).

Toxicodendron compactum Greene, Leaflets, i: 126 (1905).
T. monticola Greene, loc. cit. 126.
T. quercifolium Greene, loc. cit. 127.

Erect, 3-5 dm. high; leaflets broadly rhombic-ovate, conspicuously
3-7 lobed, permanently somewhat pubescent beneath (or rarely
glabrous at maturity), rather firm in texture and somewhat veiny,
4-10.5 cm. long and .o6-.22 mm. thick1 (between veins); petals on
male flowers 2-3 mm. long and i mm. wide; fruit 4-5 mm. in diam-
eter, at first pubescent, in maturity papillose or pubescent, or less fre-
quently glabrous; seeds smooth, 3.5-4.7 mm. long and 1.91-2.57 mm.
thick (caliper measurement). April-May.

DISTRIBUTION: Woods and barrens, New Jersey, southward and
westward to Texas.

SPECIMENS EXAMINED: ALABAMA: Auburn, 1898, F. S. Earle and
C. F. Baker (R.Mt. 16714; F. 170945). DELAWARE: Laurel, 1874,
A. Commons (U.S. 394275); Laurel, 1908, C. S. Williamson i (A.N.S.).
FLORIDA: Levy Co., 1898, A. 5. Hitchcock (F. 232712); Suwanee Co.,
1898, A. S. Hitchcock (F. 232410). GEORGIA: Lookout Mt., 1898,
A. Ruth (U.S. 345540); Taylor's Ridge, 1900, P. Wilson (U.S. 384663).
LOUISIANA: Alexandria, J. Hale (A.N.S.). MARYLAND: Salisbury,
1878, Chickering (U.S. 43273). NEW JERSEY: Millville, 1909, B. Long
(A.N.S.). NORTH CAROLINA: Wilmington, 1885, G. McCarthy (U.S.
19859). SOUTH CAROLINA: Columbia, 1912, E. B. Bartram (A.N.S.
551397); Manning, 1914, W. Stone 353 (A.N.S. 554265)- VIRGINIA:
Woodlawn, 1899, Wm. Hunter (U.S. 364962).

In New Jersey,2 it is found in sandy ground in the lower middle
district and Cape May peninsula, spreading into the pine barrens as a
rare straggler. In Alabama,3 it occurs throughout the state in dry,

1 The relative thickness of leaves having been employed as a character of spe-
cific importance in Rhus, measurements seemed necessary. Those given in this
paper were made by a micrometer screw caliper (No. 2342 Catalogue "C," Central
Scientific Co., Chicago, made by L. S. Starrett Co., Athol, Massachusetts). This
caliper is graduated to read to o.oi mm. It is provided with a friction head so that
all measurements are made with the same pressure.

2 Stone, Witmer. The Plants of Southern New Jersey with Special Reference to
the Flora of the Pine Barrens. In Annual Report New Jersey State Museum, pp.
536-38. 1911.

3 Mohr, Charles. Plant Life of Alabama. Contributions from the U. S. National
Herbarium. 6: 601. 1901.

1925. TAXONOMY OF POISON IVY — McNAiR. 59

sterile soil, barren hillsides, and pine barrens. In Mississippi,1 it has
been reported from sandy upland soil in Tishomingo Co.; Oxford;
Jackson; Hattiesburg; Montrose.

Linnaeus in 1753, in his summing up of the previous plants of the
genus, does not mention any with oak like leaves. Apparently, it is not
until 1762 that there is any reference to such a plant, when its existence
was noted by Gronovius.2 It remained for Michaux,3 however, in
1803, to give the plant botanical recognition.

On the whole, the species exhibits its essential characteristics with
remarkable uniformity. Three mature plants, however, from Georgia,
North Carolina, and Texas have glabrous leaves. Also, in a very few
instances, specimens were noted with seeds that showed a tendency
toward the tubercled character of those of R. Toxicodendron. These
may be hybrids.

2. Rhus diversiloba T. & G. Fl. N. Amer. i: 218 (1838)

Rhus lobata Hooker, Fl. Bor.-Am. i: 127 (1831).

Toxicodendron diversilobum Greene, Leaflets i : 119 (1905).

T. coriacewn Greene, loc. cit. 120.

T. comarophyllum Greene, loc. cit. 120.

T. isophyllum Greene, loc. cit. 121,

T. oxycarpum Greene, loc. cit. 121.

T. vacicarum Greene, loc. cit. 122.

Suberect and bushy, scrambling over fences, walls, etc., or in woods,
climbing by rootlets to considerable heights, sparingly pubescent or
glabrate, leaves pinnately 3 — (rarely 5)— foliolate; leaflets very obtuse,
entire, crenulate, or irregularly obtusely lobed, the incisions acute;
paler and with some persistent or tardily deciduous pubescence beneath;
panicles axillary, racemose; petals of female flowers 2-3 mm. long and
1-1.5 mm- wide, of male flowers 3-4 mm. long and 1-1.5 mm- wide;
fruit whitish or cream-colored, subglobose, glabrous or nearly so, 4-7
mm. in diameter, sometimes sulcate in age; seeds 4-5.7 mm. long
and 1.84-2.55 thick (caliper measurements); flattened and more or
less irregularly roughened with knoblike protuberances. According to
herbarium specimens, the plant flowers in Santa Catalina Island in
February and March; in California, from April to June; and in Oregon
and Washington, from April to August.

1 Lowe, E. N. Plants of Mississippi. Missisippi State Geol. Survey Bulletin
No. 17, p. 188. 1921.

' Gronovius, John Frederick. Flora Virginica, pp. 45-46. Lugduni Batavorum,
1762.

3 Michaux, Andreas. Flora Boreali- Americana, i: 183. Paris, 1803 (Anno xi).

60 FIELD MUSEUM OF NATURAL HISTORY — BOTANY, VOL. IV.

DISTRIBUTION: Borders of woods, etc., Washington, Oregon, and
California.

SPECIMENS EXAMINED: CALIFORNIA: Big Chico Creek, 1914, A. A.
Heller 11211 (F. 426609); Calaveras Co. 1887, B. H. Smith (A.N.S.);
Chico, 1916, A. A. Heller 12321 (F. 460347); Little Chico Creek, 1896,
Mrs. R. M. Austin 780 (U.S. 285227; 286258); Los Buillos Hills, 1906,
C. 5. Williamson (A.N.S.); Los Gatos, 1904, A. A. Heller 7327 (A.N.S.
510379; F. 215988); Los Gatos, 1889, B. F. Leeds (F. 403353); Mendo-
cino, 1898, H. E. Brown 750 (F. 412997); Mt. Lowe, 1901, C. 5. Wil-
liamson (A.N.S.) ; Oroville, 1913, A. A. Heller 10787 (A.N.S. 558128;
F. 411335); Salinas Valley, 1880, G. R. Vasey 86 (U.S. 19804); San
Jacinto, 1898, /. B. Leiberg (U.S. 342019); Santa Barbara, 1902, A. D.
E. Elmer 3940 (F. 235586); Santa Catalina Island, 1922, E. C. Knopf
485 (F. 516143); 1920, C. F. Millspaugh 4716, 4734 (F. 496253; 496272) ;
1920, L, W. Nuttall 622 (F. 497126; 497127; 493350); 1912, H. H. Smith
5069 (F. 389415); Santa Cruz, 1884, /. Ball, (U. S. 292229); Saratoga
Springs, 1888 (F. 403262); Sierra Valley, J. G. Lemmon 70 (F. 151861);
Tighes, San Diego Co., 1875, E. Palmer 45 (F. 302931); 1878, E. Palmer
(U.S. 19802). OREGON: Bridal Veil, Multnomah Co., 1910, H. H.
Smith 3117 (F. 295650); The Dalles, 1906, /. Lunell (R.Mt. 56238);
Hood River, 1898, T. E. Savage et al. (F. 92203); Multnomah Co.,
1903, E. P. Sheldon S. 12087 (F- 217012); Portland, 1886, Drake and
Dickson (F. 253592); 1890 (F. 253991); 1884, L. F. Henderson 176
(A.N.S. 549008); Salem, 1871, E. Hall (F. 455143); 1921, /. C. Nelson
3837 (A.N.S. 592684). WASHINGTON: Mercer Island, Seattle, 1895
(F. 366941); W. Klickitat Co., May 6-July, 1885, W. N. Suksdorf
(F. 155984; 255875); 1885, W. N. Suksdorf (U.S. 19803).

Clavigero (1798) in his "Historia de la California" mentioned this
plant under the name of "hiedra maligna" and among the Mexicans of
today in California it is still known as "hiedra."

The plant was given botanical individuality when Hooker in 1831!
called it R. lobata after examining a specimen obtained by Douglas at
Fort Vancouver on the Columbia River.

Hooker and Arnott in 18322 considered the specimens obtained
by Captain Beechey at San Francisco and Monterey, California, as
similar to the more northern specimen. Perhaps the most marked char-
acter of this plant, compared with R. Toxicodendron L. as pointed out
by Hooker, is its ovate and obtuse leaflets.

1 Hooker, William Jackson. Flora Boreali- Americana. 1:127. London, 1831.

2 Hooker, W. J., and Arnott, G. A. W. The Botany of Captain Beechey 's Voy-
age. Part III: 137. London, 1832.

1925. TAXONOMY OF POISON IVY — McNAm. 61

In 1839, Torrey and Gray1 noticed that the name R. lobata had been
used by Poiret in 1817. z As Poiret used the name for a species of Rhus
entirely different from the plant of Hooker, Torrey and Gray renamed
the plant R. diversiloba.

After examining many plants in herbaria and in the field, I am
unable to substantiate the observation of Nuttall8 that the female
plant has "almost entire or slightly lobed" leaflets, while the male "has
rather deeply lobed leaflets." There seems to be no apparent difference
between the sexes in this respect.

Like R. Toxicodendron L., it may have leaflets with an entire or
crenate (or crenately lobed) margin. Entire leaflets and crenate leaflets
may occur either on the same plant or on different plants. It also
exhibits, when in good soil, the same tendency to climb trees, etc., by
aerial rootlets, or it may grow as a shrub, and yet no one has made for
it a variety lt radicans," as was done for R. Toxicodendron L.

20. Rhus diversiloba T. & G., forma radicans, f. nov.
Toxicodendron dryophyllum Greene, Leaflets, i: 121 (1905).

SPECIMENS EXAMINED: CALIFORNIA: Little Chico Creek, Butte Co.,
1896, Mrs. R. M. Austin 780 (U.S. 285287); Santa Catalina Island,
"Extensively twining and rooting. Quite different habit from shrub,"
Millspaugh 4734 (F. 496272).

Few collections of this form have been made, but from personal
observation it may be said that it is at least as common throughout
the range of the shrub-form of Rhus diversiloba as the analogous forma
radicans in the range of Rhus Toxicodendron. Although this climbing
form of both species is probably an ecological or vegetative condition
correlated with the vitality of the plant, taxonomic recognition as a
form may be desirable.

3. Rhus toxicodendron L., Sp. PI. i: 266 (1753)

R. Blodgetti Kearney, Bull. Torr. Bot. Club 21: 486 (1894).
R. littoralis Mearns, Proc. Biol. Soc. Wash. 15: 148 (1902).
R. Toxicodendron Rydbergii Garrett, Spring Fl. Wasatch Reg.

ed. 3: 69 (1917).
R. Toxicodendron var. microcarpa Michx., Fl. Bor.-Am. i: 183

(1803).

R. microcarpa Steud., Nom. Bot. ed. 2: 452 (1840).
Toxicodendron divaricatum Greene, Leaflets i: 122 (1905).

x Torrey, John, and Gray, Asa. A Flora of North America, i : 218. New York,
1838.

- Poiret, J. L. M. Dictionnaire de Botanique. Supplement 5: 264. Paris, 1817.

* In Torrey and Gray. Loc. cit.

6z FIELD MUSEUM OF NATURAL HISTORY — BOTANY, VOL. IV.

T. arizonicum Greene, loc. cit. i: 123.

T. aboriginum Greene, loc. cit. i: 125.

T. rhotnboideum Greene, loc. cit. i : 125.

T. rufescens Greene, loc. cit. 2: 46 (1910).

T. Rydbergii Greene, loc. cit. i: 117 (1905).

T. Toxicodendron Britton, Britton and Brown, 111. Fl. ed. 2, 2:

484 (1913).

Rhus rhomboidea Small, Fl. Southeastern U. S. 727, 1334 (1903).
R. Toxicodendron Small, loc. cit.

R. Rydbergii Small, Mem. N. Y. Bot. Gard. i : 268 (1900).
Toxicodendron Negundo Greene, loc. cit. i : 117.
T. longipes Greene, loc. cit. i: 118.
T. hesperium Greene, loc. cit. i: 118.
T. lobadioides Greene, loc. cit. i: 119.
T. pumilum Greene, loc. cit. i: 124.
T. punctattim Greene, loc. cit. i: 125.
T. macrocarpum Greene, loc. cit. i: 117.
T. desertorum Lunell, Am. Mid. Nat. 2: 185 (1912).
T. Pother gilloides Lunell, loc. cit. 186.

Erect or suberect and bushy, or scrambling over fences, walls, etc.,
or in woods even climbing by rootlets to considerable heights, sparingly
pubescent or glabrate; leaflets pinnately 3-foliolate, ovate to rhombic,
mostly acute, entire, serrate or irregularly and coarsely few-toothed,
paler and with some persistent or tardily deciduous pubescence be-
neath, especially along the veins; length of terminal leaflet 3.7-19 cm.;
panicles axillary; petals of female flower 2 mm. long, i mm. wide; of
male 3-4 mm. long, 1-2 mm. wide; fruit whitish or cream-colored, sub-
globose, normally glabrous or nearly so, sometimes pubescent when
young (rarely persistent pubescence), 3-6 mm. in diameter, in age
sometimes sulcate; seeds roughened similarly to those of R. diversiloba,
3-5 mm. long, 1.74-2.49 mm. thick (caliper measurements). According
to herbarium specimens, it flowers in Mexico in February and March
(in one locality June); in Florida and the Bahamas, February and
March; in Texas, April; in Arizona, May to July; in Maine and Nova
Scotia, July; in localities north and west of Virginia, May and June.

DISTRIBUTION: Abundant in hedgerows, thickets, and woods in
Canada, United States, and Mexico between 15° and 50° north latitude
except California.

Rhus Toxicodendron having leaves with serrate margins

SPECIMENS EXAMINED: BAHAMA ISLANDS: Andros, 1910, J. K.

Small and J. J. Carter 8850 (F. 283834).

CANADA: ALBERTA: Rosedale, 1915, M. E. Moodie 1215 (F. 439452).

NEW BRUNSWICK: Woodstock, 1916, Fernald and Long 14016 (A.N.S.

576285). NOVA SCOTIA: Bridgewater, 1921, Fernald and Long (A.N.S.

587814). PORT BEVIS: 1920, Fernald and Long 21792 (A.N.S. 588250).

ONTARIO: Newburgh, 1896, W. R. Baker (F. 85618).

1925. TAXONOMY OF POISON IVY — McNAiR. 63

MEXICO: VICTORIA: 1907, E. Palmer 138165; 228 (F. 217477;
217506; 217578).

UNITED STATES: ARIZONA: Flagstaff, 1898, D. T. Macdougal 28
(F. 697784); Flagstaff, 1898, D. T. Macdougal (U.S. 334125); Fort
Apache, 1901, P. S. Mayerhofi 47 (F. 113386); Grand Canyon, C. F.
Millspaugh 122 (F. 69772); Willow Spring, 1874, /. T. Rothrock 254
(F. 303933). COLORADO: Boulder, 1906, W. W. Robbins (R.Mt. 56840);
Boulder, 1902, F. Tweedy 4946 (R.Mt. 42996); Denver, 1891, A. East-
wood (F. 82260). FLORIDA: Marco, 1898, A. S. Hitchcock (F. 232095).
GEORGIA: Bainbridge, 1895, /. K. Small (F. 180475). ILLINOIS: Edge-
brook, 1906, F. C. Gates 1517 (F. 458944; 159487); Lake Bluff, 1881,
M. E. Hutchinson (F. 101002) ; Leyden, 1905, F. C. Gates 745 (F. 458666) ;
Liana, 1916, F. C. Gates 10016 (F. 472756); Romeo, 1898, L. M. Umbush
(F. 94764); Springfield, 1861 (F. 13974); Starved Rock, 1909, /. M.
Greennian et al. 66 (F. 248726); Waukegan, 1908, F. C. Gates 2506;
2805 (F. 34472i; 344780); Winnebago Co., 1859 (F. 13975; i39?6).
INDIANA: Hanover, 1876, /. M. Coulter (F. 363331); Whiting, 1899,
0. E. Lansing, Jr. (F. 68011). IOWA: Johnson Co., 1895, T. J. Fitz-
patrick (F. 123607). KANSAS: Riley Co., 1895, /. B. Norton (U.S.
352742); 1896 (U.S. 352743); Riley Co., 1896, J. B. Norton 730 (R.Mt.
18888); Syracuse, 1893, C. H. Thompson (U.S. 265734). LOUISIANA:
Alexandria, 1899, C. R. Ball 429 (F. 93465). MAINE: Fairfield, 1916,
Fernald and Long 14017; 14018 (A.N.S. 576286; 578022). MARYLAND:
Savage Sta., 1905, C. S. Williamson (A.N.S. 524661). MICHIGAN:
Hamlin Lake, 1910, R. W. Chancy 210 (F. 296953). MONTANA: Ravalli,
1908, Mrs. J. Clemens (F. 345138). NEBRASKA: Gage Co., W. C. Knight
(R.Mt. 172). NEW JERSEY: Bennett, 1910, B. Long 5170 (A.N.S.);
Folsom, 1910, B. Long 4200 (A.N.S.). NEW MEXICO: Black Range,
0. B. Metcalfe 1088 (U.S. 498281); Kingston, 1904, O. B. Metcalfe 1088
(F. 187484); Magdalena Mts., 1910, J. Herrick and R. Herrick 70
(F. 292657). NEW YORK: Cheming Co., 1896, T. F. Lucy 7703 (F. 3551) ;
Cheming Co., 1896, T. F. Lucy (F. 140325; R.Mt. 21947); Glenwood
Ravine, 1888, C. F. Millspaugh (F. 18506); Troy, 1834 (F. 476882).
NORTH CAROLINA: Tryon, 1918, C. F. Millspaugh 4060 (F. 479441).
NORTH DAKOTA: Grand Forks, 1894, C. A. Egebretson 148 (F. 352080).
OKLAHOMA: Woods Co., 1900, P. /. White (R.Mt. 26792). OREGON:
Deshutes River, 1885, T. Howell (F. 150943). PENNSYLVANIA: Mer-
cersburg, 1845 (A.N.S.); Nottingham Barrens, 1914, F. W. Pennett
1558 (A.N.S.); Perkiomen, 1892, /. B. Brinton (A.N.S.); Philadelphia,
1921, R. R. Dreisbach (F. 531925); Westmoreland, 1877, P. E. Pierron
(F. 154294). SOUTH DAKOTA: 5. A. Skinner (R.Mt. 61824); Ashcroft,
1910, 5. 5. Visher (R.Mt. 69869); Bald Hills, 1910, /. Murdoch, Jr.

64 FIELD MUSEUM OF NATURAL HISTORY — BOTANY, VOL. IV.

4092 (F. 471322); Deadwood, 1913, W. P. Carr 83 (F. 468198); Edge-
mont, 1911, 5. 5. Visher 2569 (F. 386301); Fall River Co., 1911,
5. 5. Visher 2569 (R.Mt. 76332); Harding Co., 1910, 5. 5. Visher 244
(F. 385898); Piedmont, 1895, A. D. Pratt (F. 140405; R.Mt. 9235).
TEXAS: Hempstead, 1872, E. Hall 78 (F. 453952); Kerrville, 1894,
A. A. Helkr 1670 (F. 17075) ; Tannart Co., 1920, A. Ruth 941 (F. 507859).
UTAH: 1875, L. F. Ward 212 (F. 106360); City Creek Canyon, 1880'
M. E. Jones 1932 (F. 252602; 475769); Glenwood, 1875, L. F. Ward
212 (U.S. 153641). VERMONT: Charlotte, 1879, F. H. Hosford (F. 354-
348). VIRGINIA: (opposite Georgetown, D.C.), A. Schott (F. 44175);
Great Falls, 1909, C. S. Williamson (A.N.S.). WISCONSIN: Elkhart
Lake, 1884, /. H. Schuette (F. 351000); Green Bay, 1906, /. H. Schuette
(F. 378276); Milwaukee, I. A. Lapham (A.N.S.); Richland Center,
1912, 0. E. Lansing, Jr. 3408 (F. 323983). WYOMING: Freezeout Hills,
1898, E. Nelson (R.Mt. 12331); Hartville, 1894, A. Nelson (R.Mt. 3733);
Norwood Hill, 1912, E. P. Walker 498 (R.Mt. 75747); Pole Creek,
1894, A. Nelson (R.Mt. 3958); Upper Goose Creek Ditch, 1909, V. Wil-
lits 1 88 (R.Mt. 68884).

Rhus Toxicodendron having leaves with entire margins

SPECIMENS EXAMINED: BERMUDA: Boaz Island, 1912, 5. Brown
1005 (A.N.S. 556483); Paget Marsh, 1905, 5. Brown (A.N.S. 511291);
Tuckers Town, 1908, 5. Brown 499 (A.N.S. 534730).

GREAT BAHAMA: 1905, L. J. K. Brace 3570 (F. 184239); 1905,
Britton 2446 (F. 173562).

CANADA: NOVA SCOTIA: East Bridgewater, 1910, /. Macoun
81289 (F. 295036); East Jordan, 1921, Fernald and Long 24095 (A.N.S.
589438); Yarmouth, 1920, Pease and Long 21785 (A.N.S. 588259).

MEXICO: Cuyameralis de Cuicatlan, 1909, Conzatti 2409 (F. 246942) ;
Morelia, 1909, T. Maria 10 (F. 387369); San Luis Potosi, 1878, Parry
and Palmer 124 (A.N.S.); Sierra del Pajarito, 1855, A. Schott (F. 42195).

JAPAN: Island of Jesso, 1861, Albrecht (F. 53316).

UNITED STATES: ARIZONA: Fort Huachuca, 1890, Palmer 4530,
(U.S. 19847); Lowell, 1884, W. F. Parish 217 (F. 152917); Santa
Catalina Mts., 1894, /. W. Tourney (U.S. 441725; 619140; 664167).
DELAWARE: Ruthby, 1897, A. Commons (A.N.S. 541741). DISTRICT
OF COLUMBIA: Washington, 1893, L. L. J. Boettcher 250 (F. 286579;
R.Mt. 68107). FLORIDA: Alva, 1900, Hitchcock 39 (F. 101118); Palm
Beach, 1908, W. Gardens (F. 224033); Palm Beach, 1895, Hitchcock
(F. 232708). GEORGIA: Milledgeville, S. Boykin (A.N.S.). ILLINOIS:
Joliet, 1904, H. C. Skeels (F. 177229). MAINE: Mt. Desert Island,
1890, J. H. Redfield (A.N.S.); Winn, 1916, Fernald and Long 14014

1925. TAXONOMY OF POISON IVY — McNAiR. 65

(A.N.S. 576283). MASSACHUSETTS: Bolton, 1910, C. H. Knowlton
(A.N.S. 562021); Falmouth, 1904, A. H. Moore 1775 (F. 468774); Fal-
mouth, 1911, Pennell 3176 (A.N.S. 546542); Middleboro, 1900, /. Mur-
doch, Jr. 527 (F. 469864). MISSOURI: Vulcan, 1908, H. H. Smith 441
(F. 240812). MONTANA: Bozeman, 1905, J. W. Blankinship 106
(F. 190151). NEW JERSEY: Cape May Court House, 1911, B. Long
6671 (A.N.S.); Folsom, 1911, B. Long 5936 (A.N.S.); Tomlin, 1911,
B. Long 6822 (A.N.S.). NEW MEXICO: Kingston, 1904, 0. B. Met-
calfe (U.S. 890258). OREGON: Deshutes River, 1885, Howell (F. 366-
339). PENNSYLVANIA: Philadelphia, 1908, 5. S. Van Pelt (A.N.S.).
SOUTH CAROLINA: Manning, 1914, W. Stone 505 (A.N.S. 554417).
TEXAS: Willis, 1908, R. A. Dixon 292 (F. 238230). VIRGINIA: Hacker
Valley, 1908, H. H. Smith 1532 (F. 241984); Little Falls, C. F. Mills-
faugh (F. 24465); Ocean View, 1898, T. H. Kearney, Jr. 1759 (U.S.
346424); Virginia Beach, 1893, N. L. Britton et al. (F. 394587). WASH-
INGTON: Spokane, 1898, T. E. Savage et al. (F. 93077); Spokane, 1912,
G. W. Turesson (R.Mt. 76339); Wenatchee, 1893, K. Whited 241
(U.S. 268197).
Rhus Toxicodendron having leaves with entire and serrate margins on

the same plant

SPECIMENS EXAMINED: BAHAMA ISLANDS: New Providence, 1905,
E. G. Britton 3416 (F. 184117); North Cat Cay, 1904, C. F. Millspaugh
2336 (F. 156302).

CANADA: Five Mile River, N. S., 1920, A. S. Pease and B. Long
21788 (A.N.S. 588113).

MEXICO: St. Diego, 1891, C. V. Hartman 589 (U.S. 306052; F. 49-
631); Tunicachi, 1890, C. V. Hartman 102 (U.S. 306157).

UNITED STATES: ARIZONA: Chiricahua Mts., 1907, /. C. Blumer
1325 (F. 242184). FLORIDA: Lake City, 1898, A. S. Hitchcock (F. 232-
709); Lake City, 1901, L. McCulloch 45 (U.S. 440443); Tampa Bay,
1893, P. H. Rolfs 247 (F. 228840). ILLINOIS: Glencoe, 1877, M. Brass
(F. 103924); Peoria, /. T. Stewart (F. 114587); Stony Island, 1914,
H. H. Smith 6033 (F. 417145). KANSAS: Riley Co., 1895, J- B. Norton
73 (R.Mt. 19505). LOUISIANA: Alexandria, /. Hale (A.N.S.). MAINE:
Monticello, 1916, Fernald and Long 14015 (A.N.S. 576284). MIS-
SOURI: Independence, 1921, B.F. Bush 9365 (F. 504307). NEW JERSEY :
1911, B. Long 6306 (A.N.S.); Farmingdale, 1910, B. Long and S. Brown
96 (A.N.S.); Locust Grove, 1911, B. Long 6492 (A.N.S.). NORTH
DAKOTA: Devil's Lake, 1902, J. Lunell (R.Mt. 39163). PENNSYL-
VANIA: Allentown, 1922, H. W. Pretz 11394 (A.N.S.); Conewago, 1889,
J. K. Small (F. 117542); Grenoble, 1910, B. Long (A.N.S.); Harrisburg,
1888, J. K. Small (F. 177717; 177718); Mifflin Co., J. T. Rothrock

66 FIELD MUSEUM OF NATURAL HISTORY — BOTANY, VOL. IV.

(F. 321404); Pittsburgh, 1884, /. A. Shafer 544 (F. 1 8508) ; Westmore-
land Co., 1876, P. E. Pierron (F. 154296). VIRGINIA: Marion, 1892,
N. L. Britton et al. (F. 394767). VERMONT: Johnson, 1894, A. J. Grout
(F. 428782).

Rhus Toxicodendron L. was probably the first poisonous species
of Rhus discovered in North America. It was perhaps first noticed by
Captain John Smith in 1609, and in 1635 Cornutus gave it botanical
status as Edera trifolia canadensis. Since that time it has been divided
into many species and varieties by many botanists.

It may readily be distinguished from R. diversiloba and R. querci-
folia by its acute leaflets, as those of both the other species are obtuse.
The leaf -margins and seed shapes are also different, as shown in the key.

Tracings of the outlines of leaves may be found on Plates XVIII-
XXIII. These have been taken from all parts of North America between
lower Canada and lower Mexico, with the exception of California
(where, apparently, it does not grow) . Its terminal leaflets in the north
are generally ovate and in some cases nearly circular, while toward the
south, especially in Lower California, Arizona, Florida, Mexico, and the
Bahamas, they seem more likely to assume a lanceolate shape.

The plant may climb trees, etc., by means of aerial rootlets, but
frequently remains suberect and bushy. Soil conditions may have a
great deal to do with its habit, for in barren sandy soils the climbing
form is not known to occur, while in fertile soils the radicant is frequently
met with. For instance, the sand dunes of Indiana have only the low
form, but in the neighborhood of Laporte, Indiana, on more fertile
soil, the climbing form is found. "This species of so wide a range shows
many variations, some of which have been described as species. This
fact led the writer, who is immune to ivy poisoning, to make an inten-
sive study of the form in this State (Indiana). Especial attention was
given to the study of the erect forms; those with thick and nearly smooth
leaves; and those with hairy fruit. Hundreds of plants have been exam-
ined and from these, 78 sheets from 58 counties have been collected.
The study suggests that the low erect forms are branches of under-
ground stems; that the thick-leaved forms are always found in places
exposed to heavy winds and direct sunlight; and that the hairy-fruited
forms are rare and are distributed throughout our area, and have no
other character to distinguish them."1

1Deam, Charles C. Shrubs of Indiana, p. 176. Publications of the Depart-
ment of Conservation, State of Indiana, No. 4. Indianapolis, December, 1924.

1925. TAXONOMY OF POISON IVY — McNAm. 67

In the Sandy River Valley in Maine, Knowlton1 found only the pros-
trate vine. In New Jersey, Stone2 found the shrubby form in sandy
ground in the lower part of the middle district and Cape May penin-
sula spreading into the Pine Barrens as a rare straggler, while the vine-
like form was found in low woods and along fence rows in the northern,
middle, and coast districts and was absent from the Pine Barrens
"except as an incursion." In Mississippi, Lowe3 found the vinelike
form throughout the state climbing over trees, while the shrub was
found only on sandy upland soil. In Alabama, Mohr4 found the vine-
like form in rich, damp woods and bottom lands, while the shrub was
found in dry, sterile soil, barren hillsides, and pine barrens. Peterson
in his Flora of Nebraska6 found the vinelike form commonly in woods
and along fences in Lincoln; while the shrub was an inhabitant of open
woods and prairies, as found in Kearney, Long Pine, Minden, New-
castle, and Valentine.

The plant has female flowers with petals 2 mm. long and i mm.
wide; male flowers with petals 3-4 mm. long and 1.5 mm. wide; fruit
3-5 mm. long with seeds 3-4.5 mm. long and 1.74-2.29 mm. thick.
The seeds have knoblike protuberances which make them readily
distinguishable from those of R. diversiloba and R. quercifolia.

The terminal leaflets vary from 3.7-12 cm. in length and are larger
in the shade than in sunny locations. Dried herbarium leaves show a
difference in thickness between .06 and .18 mm. Dried specimens of
leaves grown in the sun are thicker than those grown in the shade.
Miss Turner6 states that in fresh material no appreciable difference in
thickness occurs between leaves grown in the shade and leaves grown
in the sun. However, she says that sunny leaves have more compact
tissue, consequently, in dried material sunny leaves would be thicker.

In making measurements on material from many parts of North
America (see Tables III, IV) no difference in thickness could be noticed
between leaves from western and eastern grown plants. But, in gen-
eral, lanceolate leaves from the South were thicker than other shapes.

Panicles may be large or small, dense or open, upright or pendulous .

1 Knowlton, Clarence H. Flora of the Sai dy River Valley in Maine. In Rho-
dora, 16: 14 (1914).
1 Loc. cit.

3 Loc. cit,

4 Loc. cit.

6 P. 161. Plainview, Nebraska, 1923.

6 Turner, Helen. The Ecology of Rhus Toxicodendron. Transactions of the
Illinois State Academy of Science. 15: 208-11 (1922).

68 FIELD MUSEUM OF NATURAL HISTORY — BOTANY, VOL. IV.

Leaflets and growing parts of the plant are covered with short,
deciduous hairs mostly disappearing at maturity, but, in general, leaf-
lets of equal maturity have approximately the same amount of pubes-
cence whether from eastern or western America. However, some
specimens from the District of Columbia, New Jersey, and Texas have
leaflets densely pubescent beneath even at maturity.

As appears from the list of specimens examined, there are many
plants of R. Toxicodendron which possess both leaves with serrate and
entire leaf -margins. These may be hybrids, but this point cannot
be decided without breeding experiments. The habit of growth of
R. Toxicodendron has no relation to size of petals or character of leaf-
margin, size and shapes of seed (see Tables III, IV). In this connec-
tion, it should be remembered that R. diversiloba exhibits similar leaf
variations, showing no concomitant differences in flower, seed, or habit.

The subdivision of the species must be made on characters other
than leaf-margins. Of the hundreds of specimens examined, only a
few exhibit variations that appear sufficiently important or constant
to be worthy of recognition. These may be defined as follows:

Plants erect, not developing aerial rootlets; leaflets acute or acutish,
entire or very coarsely few-toothed; mature fruit glabrous; seeds
somewhat kidney-shaped, roughened var. typica

Plants developing aerial rootlets.often climbing 30. f. radicans

Plants erect or suberect, without aerial rootlets

Fruit pubescent; leaflets not deeply lobed. . .36. f. malacotrichocarpum
Fruit glabrous; leaflets deeply lobed, the lobes very acute

3c. var. eximia

30. Rhus Toxicodendron L., forma radicans (L.) comb. nov.

Rhus radicans L., Sp. PI. 266 (1753).

R. Toxicodendron, var. radicans Torr., Fl. N. and Mid. States, i:

323 (1824).
R. Toxicodendron, a. radicans Dippel, Handb. Laubholzk. 2 : 376

(1892).
R, Toxicodendron radicans Schelle, Beissner, Schelle and Zabel,

Handb. Laub. Benen. 286 (1903).

R. floridana Mearns, Proc. Biol. Soc. Wash. 15: 149 (1902).
Toxicodendron vulgare (Mill.) Greene, Leaflets, i: 115 (1905).
T. glabrum (Mill.) Greene, loc. cit. i: 116.
T. pubescens (Mill.) Greene, loc. cit. i: 116.
T. phaseoloides Greene, loc. cit. i: 123.
T. laetevirens Greene, loc. cit. i: 123.
T. goniocarpum Greene, loc. cit. i: 125.
T. radicans, a. normale O. Ktze; Rev. Gen. i: 154 (1891).

1925. TAXONOMY OF POISON IVY — McNAiR. 69

This is designated as a form only because it has occupied so promi-
nent a place in literature, although its characteristic — a more or less
climbing habit — scarcely entitles it to special taxonomic recognition.

j6. Rhus Toxicodendron, f. malacotrichocarpum
A. H. Moore, Rhodora, n: 163 (1909).

DISTRIBUTION: Occasional throughout the eastern range of the

species.

Y
a

al

ir

n-

S.
9;

4. Rhus divaricate (Greene) comb. nov.

Toxicodendron divaricatum Greene, Leaflets, i: 122 (1905).
This species has entire leaves, but with a seed quite different in
shape from typical R. Toxicodendron (see Plate XXIV). When further
collections of this plant are made, it may prove to be only a variety of
R. Toxicodendron. At present, however, in view of the rather marked
difference in seed characters, as exhibited in the single specimen known,
it seems best to retain the plant as a separate species.

68 FIELD MUSEUM OF NATURAL HISTORY — BOTANY, VOL. IV.

Leaflets and growing parts of the plant are covered with short,
deciduous hairs mostly disappearing at maturity, but, in general, leaf-
lets of equal maturity have approximately the same amount of pubes-
cence whether from eastern or western America. However, some
specimens from the District of Columbia, New Jersey, and Texas have
leaflets densely pubescent beneath even at maturity.

As appears from the list of specimens examined, there are many
plants of R. Toxicodendron which possess both leaves with serrate and

A CORRECTION
Rhus Greenei, nom. nov.
Toxicodendron divaricatum Greene, Leaflets, I: 122 (1905).

Rhus divaricate (Greene) McNair, Field Mus. Pub. Bot. IV: 69
(1925), not R. divaricata Eckl. & Zeyh. Enum. PI. Afr 146
(1834-37)-

In my paper on the taxonomy of poison ivy, Toxicodendron
divaricatum Greene was transferred to Rhus as Rhus divaricata. I was
unaware at the time that the name had already been used for an
entirely different and presumably valid African species. Toxicodendron
divaricatum Greene must, therefore, be renamed.
April 6, 1925.

(1892).
R. Toxicodendron radicans Schelle, Beissner, Schelle and Zabel,

Handb. Laub. Benen. 286 (1903).

R. floridana Mearns, Proc. Biol. Soc. Wash. 15: 149 (1902).
Toxicodendron vulgare (Mill.) Greene, Leaflets, i: 115 (1905).
T. glabrum (Mill.) Greene, loc. cit. i: 116.
T. pubescens (Mill.) Greene, loc. cit. i: 116.
T. phaseoloides Greene, loc. cit. i: 123.
T. laetevirens Greene, loc. cit. i: 123.
T. goniocarpum Greene, loc. cit. i: 125.
T. radicans, a. normale O. Ktze; Rev. Gen. i: 154 (1891).

1925. TAXONOMY OF POISON IVY — McNAiR. 69

This is designated as a form only because it has occupied so promi-
nent a place in literature, although its characteristic — a more or less
climbing habit — scarcely entitles it to special taxonomic recognition.

36. Rhus Toxicodendron, f. malacotrichocarpum
A. H. Moore, Rhodora, u: 163 (1909).

DISTRIBUTION: Occasional throughout the eastern range of the
species.

SPECIMENS EXAMINED: MAINE: Bristol, Chamberlain and Dins-
more 832 (G.H.); Wells, York Co., Fernald and Long 14018 (A.N.S.).
NEW JERSEY: Cape May Court House, Cape May Co., B. Long 6671
(A.N.S.); Delair, Camden Co., B. Long 6306 (A.N.S.); Tomlin, Glou-
cester Co., B. Long 6822 (A.N.S.). PENNSYLVANIA: Grenoble, Bucks
Co., B. Long 4581 (A.N.S.). BERMUDA: Boaz Island, S. Brown and
N. L. Britton 1005 (A.N.S.).

3C. Rhus Toxicodendron, var. eximia (Greene) comb. nov.

Toxicodendron eximium Greene, Leaflets, i: 123 (1905).

T. biternatum Greene, loc. cit. i: 124.

Rhus eximia Standl., Contrib. U.S.Nat. Herb. 23: 668 (1923).

This variety apparently varies greatly from R. Toxicodendron only
in its leaf-shape. It has a leaf shaped somewhat like the leaf of a
Norway maple (see Plate XXIV), although some plants have unlobed
serrate leaves in addition. The petals, fruits, and seeds are similar
in shape and size to those of R. Toxicodendron.

It is found in Mexico in Durango, Morelia, Nuevo Leon, and Tam-
aulipas, and in the United States in Texas.

SPECIMENS EXAMINED: TEXAS: Eagle Nest, V. Havard (U.S.
156164). MEXICO: Durango, 1896, E. Palmer 106 (U.S. 305009;
F. 51217); San Augustin, Morelia, 1910, Arsene (F. 417262).

*

4. Rhus divaricate (Greene) comb. nov.

Toxicodendron divaricatum Greene, Leaflets, i: 122 (1905).
This species has entire leaves, but with a seed quite different in
shape from typical R. Toxicodendron (see Plate XXIV). When further
collections of this plant are made, it may prove to be only a variety of
R. Toxicodendron. At present, however, in view of the rather marked
difference in seed characters, as exhibited in the single specimen known,
it seems best to retain the plant as a separate species.

70 FIELD MUSEUM OF NATURAL HISTORY — BOTANY, VOL. IV.

SPECIMEN EXAMINED: LOWER CALIFORNIA: Calmalli, 1898, C. A.
Purpus (U.S. 383431, type).

THE QUESTION OF HYBRIDS

Without positive data based on breeding experiments, it may be
idle to discuss the question of hybrids. However, it may be noted that
the ranges of R. quercifolia and R. Toxicodendron overlap and, as these
plants are closely related and insect-pollinated, hybrids possibly occur.
Mohr1 states that in Alabama the two plants have different flowering
periods, quercifolia blooming in April and Toxicodendron in May, and
that he has never met with forms intergrading between them. In North
Carolina there also appears to be a difference in the flowering period
according to herbarium specimens.

The ranges of R. diver siloba and R. Toxicodendron overlap in Ore-
gon and Washington, as was observed by Howell. According to her-
barium specimens examined by the author, their flowering periods also
overlap and they are insect-pollinated. No forms suggestive of this
cross have been observed.

1 Loc. cit.

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PLATE XIV.

Fig. i. Seeds of Rhus quercifolia: a, b, d, e views from side, c view from below.

Fig. 2. Seeds of Rhus Toxicodendron: a, b, d, e views from side, c view from below.

Fig. 3. Seeds of Rhus diversiloba: a, b, d, e views from side, c view from below.

Fig. 4. Seeds of Rhus divaricata: a view from side, b view from below.

PLATES XV -XXIV.

The originals of the tracings reproduced, made from specimens cited in this
paper, are deposited in the herbarium of the Field Museum, and bear the author's
notations indicating the source of each.

FIELD MUSEUM OF NATURAL HISTORY.

BOTANY VOL. IV, PLATE XIV.

<9

4
a b

Carl F Gronemann.

RHUS SEEDS (x 4M;x).

THE LIBRARY OF IHt
APR 13 1925

UNIVERSITY OF ILLINOIS

FIELD MUSEUM OF NATURAL HISTORY.

BOTANY VOL. IV, PLATE XV.

RHUS QUERCIFOLIA LEAF TRACINGS (x Mi

THE UBRURK OF THE

APR 13 1925

UNIVERSITY OF ILLINOIS

FIELD MUSEUM OF NATURAL HISTORY.

BOTANY VOL. IV, PLATE XVI.

RHUS DIVERSILOBA LEAF TRACINGS (x Vs).

THE LIBRARY OF IHt
APR 13 1925

UNIVERSITY OF ILLINOIS

FIELD MUSEUM OF NATURAL HISTORY.

BOTANY VOL. IV, PLATE XVII.

RHUS DIVERSILOBA LEAF TRACINGS (x V6).

THE LIBRARr OF THF

A PR 13 1925

UNIVERSITY OF ILLINOIS

FIELD MUSEUM OF NATURAL HISTORY.

BOTANY VOL. IV, PLATE XVIII.

RHUS TOXICODENDRON LEAF TRACINGS (x Mi).

THE LIBRURT OF THE

APR 13 1925

'IIU'VERSITY Of ILLINOIS

FIELD MUSEUM OF NATURAL HISTORY.

BOTANY VOL. IV, PLATE XIX.

RHUS TOXICODENDRON LEAF TRACINGS (x Mi).

THE LIBRART flF THF

APR 13 1925

UNIVERSITY O

FIELD MUSEUM OF NATURAL HISTORY.

BOTANY VOL. IV, PLATE XX.

RHUS TOXICODENDRON LEAF TRACINGS (x

THE LIBRARY OF THF
APR 13 1925

UNIVERSITY OF ILLINOIS

FIELD MUSEUM OF NATURAL HISTORY.

BOTANY VOL. IV, PLATE XXI.

RHUS TOXICODENDRON LEAF TRACINGS (x Mi).

THE LIBRARY Sir THt
APR 13 1925

UNIVERSITY OF !LLINO»

FIELD MUSEUM OF NATURAL HISTORY.

BOTANY VOL. IV, PLATE XXII.

RHUS TOXICODENDRON LEAF TRACINGS (x Mi).

FIELD MUSEUM OF NATURAL HISTORY.

BOTANY VOL. IV, PLATE XXIII.

RHUS TOXICODENDRON LEAF TRACINGS (x Mi).

THE LIBRARY OF THE

APR 13 1925

Of ILLINOIS

FIELD MUSEUM OF NATURAL HISTORY.

BOTANY VOL. IV, PLATE XXIV.

RHUS TOXICODENDRON VAR. EXIMIA (UPPER SIX) LEAF TRACINGS
RHUS DIVARICATA (LOWEST) LEAF TRACING (x %).

I HE IIBHAKT Of IK?

A PR IS 1925
UNIVERSITY Of IUINOI&

THE UBRART OF THF
APS 13 1925

UNIVERSITY 9F ILLINOIS