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Full text of "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 



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. Miller 3 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, Theophrastus 7 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. Miller 9 gives as a possible derivation its contraction from 
p6os, and that from the Greek verb pw "to flow," because certain 
products of the plant were formerly used to check hemorrhages. 
Boehmer 10 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). Paxton 11 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 Aeo>' 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; d 1 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. thick 1 (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 Gray 1 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 Nuttall 8 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 

1 Deam, 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, Knowlton 1 found only the pros- 
trate vine. In New Jersey, Stone 2 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, Lowe 3 found the vinelike 
form throughout the state climbing over trees, while the shrub was 
found only on sandy upland soil. In Alabama, Mohr 4 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 Nebraska 6 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 Turner 6 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. 
Mohr 1 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 4 M;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