Hi Norma -
An excellent question. Let's first look at how botanists view the Toxicodendron genus. Before mid-last century (around 1950's), these plants where part of the Rhus genus along with other non-toxic plants. The plant classification system is based on natural relationships and it was felt that the poisonous members of the Rhus genus should be within their own grouping to better identify the plants and their relationship hence the Toxicodendron genus. You will find even today some refer to these plants as Rhus and not Toxicodendron even though for decades the proper term is Toxicodendron. So now we have plants within the Toxicodendron who have a similar relationship and these are poison ivy, poison oak and poison sumac.
Here's where it gets a bit dicier. There are some botantists who then break down the plants much, much further based on certain plant characteristics. For this more detailed group, there are 7 species of poison ivy, 2 species of poison oak, and 1 species of poison sumac. As we have seen, poison ivy is highly variable in leaflet characteristics. Then there is another group of botanists who believe that poison ivy and poison oak are the same with just some variations so there should only be 1 species to represent the plants. The most accepted (at this point in time) classification is 2 species of poison ivy, 2 species of poison oak, and 1 species of poison sumac. Here is the naming and the general description:
- Toxicodendron radicans - eastern poison ivy. The most common of these plants in the Toxicodendron genus. This plants grows as a vine and will grow up plants, structures and along the ground.
- Toxicodendron rydbergii - Rydbergii poison ivy or western poison ivy. A less common form of poison ivy which grows as a small shrub.
- Toxicodendron diversilobum - Pacific poison oak found in the westernmost US states (California, Nevada, Oregon, Washington) and western Canada. This plant tends to grow as a vine similar to eastern poison ivy.
- Toxicodendron pubescens - Atlantic poison oak found to grow in southeastern US states. This is not as common a plant as eastern poison ivy which does grow in the same region. Atlantic poison oak grows as a small shrub. This plant used to be called Toxicodendron toxicarium but the name was changed to better identify the plant.
- Toxicodendron vernix - Poison sumac. This is a very elusive shrub which grows along or in standing water such as peat bogs. There are a number of sumacs which look similar but very close inspection will show the differences. Most people will never see a poison sumac plant given its preferred habitat.
Now I'll drop discussion of poison sumac as that was not part of your question but I did want it added here since it is part of the Toxicodendron genus.
Poison ivy is described as a shrub or vine with alternating leaves. Each leaf has 3 leaflets with the middle leaflet on a longer stem. The egg-shaped leaflets are irregularly lobed, variably toothed with the middle leaflet generally having a point at the end. The female plant has very small 5-part axilliary yellowish flowers and produces a whitish-greenish fruit. Axilliary means that the flower grows from that point where the leaf joins at the stem (vine) of the plant. Poison ivy leaves are highly variable.
Poison oak is described similarly to poison ivy EXCEPT that the leaflets are quite distinctive in shape. There is far less variability with this plant. The leaflets resemble white oak leaves, are shallowly lobed, and generally has rounded tips.
Interestingly, where poison ivy and poison oak are found in the same region, the plants will hybridize (cross-breed). This is makes proper identification all the more difficult.
What concerns us about both poison ivy and poison oak is the secondary chemical, urushiol oil, contained in resin canals within the leaves, stem, skin of the fruit, and roots of the plant. Urushiol oil is composed of a mixture of catechols. Poison oak contains a mixture of 4 heptadecycatechols (17 carbon side chain) and poison ivy contains a mixture of 4 pentadecycatechols (15 carbon side chain). The reaction we see is determined by the long side-chain. From what I have read, there needs to be at least an 11 carbon side chain to cause the allergic reaction. So while poison ivy and poison oak vary chemically, the response is the same - an overblown immune system response. A note: the longer side chain of poison oak has some indications of this plant's urushiol oil being slightly more potent.
Phew - does that help? Here are some references on the net to more information:
Do a search on Google and you'll find many more references. |
