Information about species of the cashew family (Anacardiaceae)
that grow without cultivation on Mount Desert Island

compiled by the
Champlain Project - 183 Pleasant Street, Apartment 1 - Canton, Maine  04221
info@vfthomas.com

(updated 11 February 2017)


Anacardiaceae - cashew family
Leaves of the cashew family are pinnately compound and grow alternately on the stem. Rhus has many leaflets (an odd number from 11 to 31; i.e., 5–15 pairs of lateral leaflets plus a terminal one), and Toxicodendron has only 3 leaflets. The terminal leaflet of a Toxicodendron leaf has a prominent petiolule (i.e., a leaflet stalk), which is what makes it pinnately compound as opposed to palmately compound, where the terminal leaflet would be sessile (i.e., would have no petiolule).

Mount Desert Island is home to 3 species in 2 genera that grow without cultivation. If you already know which species you have or are interested in learning about, click on the appropriate link below. Otherwise, examine the table below for help with identification.
   Rhus (1 species)
      Rhus typhina - staghorn sumac (common [see note 1 at bottom of page])
   Toxicodendron (2 species)
      Toxicodendron radicans - poison-ivy (rare)
      Toxicodendron rydbergii - western poison-ivy (occasional)


  leaflet number aerial roots
Rhus typhina 11–31 absent
Toxicodendron radicans 3 present
Toxicodendron rydbergii 3 absent






Rhus (sumac)
Rhus (with a lower case “r”) was the ancient Roman name for their sumac plants.

Rhus typhina (staghorn sumac) is a common and easily recognizable species. Of Maine’s three (and New England’s four) species of Rhus, it is the only one growing on Mount Desert Island. A good place to see this species is southeast of the intersection of the Schooner Head Road with the short road connecting the Park Loop Road and Schooner Head overlook.

(click on image to enlarge)


Toxicodendron (poison-ivy). This genus name comes from two words meaning “poison tree”. Toxico- is the combining form of the Latin (second declension neuter) noun toxicum (poison), which came from the Greek verb “toxeuein” (τοξευειν) “to shoot arrows” or “to shoot with a bow”. And the second part of the genus name, -dendron, is a transliteration of the Greek (second declension neuter) noun δενδρον, which means tree. (The Latin word for tree is arbor.) The poison in poison-ivy is urushiol, a compound in the sap that “binds to the skin and ultimately causes an itchy rash, the severity of which depends on the individual’s sensitivity.” (Haines, Ancestral Plants, volume 1, p. 137)
   The distinguishing feature that separates MDI’s two species of Toxicodendron is the presence of aerial rootlets on T. radicans (see photo below left, a close-up of the rootlets from the image further below under the species information for T. radicans) and their absence on T. rydbergii (see photo below right, also a close-up of an image further down, under T. rydbergii). These rootlets enable T. radicans to climb (sometimes many feet high) on rocks and trees. T. rydbergii depends on its own stem for support.
   
(click on images to enlarge)

Toxicodendron radicans (poison-ivy) leaflets grow more or less parallel to the ground. The specific epithet, radicans, is the present participle of the 1st conjugation deponent verb radicor, -ari, -atus, which means “grow roots” or “take root”. As the present participle, it means “rooting”. So, Toxicodendron radicans is “the rooting poison tree”.

(click on image to enlarge)

Toxicodendron rydbergii (western poison-ivy) is the more frequently encounted member of the genus on Mount Desert Island. Good examples can be found along many roadsides and carriage roads. Note the irregular dentition—entire on one side of the leaflet and coarsely and irregularly toothed on the other. This can vary from leaf to leaf on the same plant. The specific epithet, rydbergii, is the possessive form (genitive case) of the name Rydberg, in this case Per Axel Rydberg (1860–1931), the first curator of the New York Botanical Garden herbarium. So, Toxicodendron rydbergii is “the poison tree of Rydberg”. It was at one time the custom to capitalize (the first letter of) a specific epithet if the name it came from was a proper noun, but now the practice is to begin all specific epithets, no matter what their origin, with a lower case letter.

(click on image to enlarge)


Note:
   1. Frequency designations are from the paper “Vascular flora of the Acadia National Park region, Maine” by Craig W. Greene, Linda L. Gregory, Glen H. Mittelhauser, Sally C. Rooney, and Jill E. Weber, published in the spring 2005 issue (vol. 107, No. 930) of Rhodora: Journal of the New England Botanical Club.