Species commonly identified as Euphorbias

The focus of this post is to explain the differences between Euphorbia and the species the non-euphorbs they are most commonly misidentified as. It is important to start with a little understanding of the basic flower structure and terminology.

All Euphorbia species have a cyathium. A cyathium is technically an inflorescence, but the important question here is how do we tell it apart from a normal flower? The main thing to look for is a gland on the rim of the floral cup (technically the involucre). This will be a round or oval structure that often has a depression in it. The color can range from green to yellow to red to nearly black. This gland may have something on it that looks like a white or pink petal known as the petaloid appendage or glandular appendage. To better understand this, I recommend looking at observations in this project and finding the structures. Also, Euphorbia PBI has some good information about the structure.

But, what if the plant isn't flowering? There is no reliable way to tell if the plant is a Euphorbia if it isn't flowering. However, there are a number of plants that are commonly mistaken for Euphorbia that I have listed below.

Asclepias (milkweeds):
This confusion comes primarily from the similarity of milky sap that is released when plants are damaged. Other than floral differences, members of Asclepias often will have thicker leaves and stems. You should look up the species of Asclepias in your area before making the call if the plant isn't flowering or fruiting.

If the plants are flowering or fruiting, the differences are quite obvious with the flowers of Asclepias being larger than almost any cyathium and typically more colorful (and often all one color). The fruits of Asclepias are narrower and not lobed, while those of Euphorbia are 3-lobed.

Cactaceae (Cacti):
This confusion doesn't really apply to US natives with the possible exception of Candillia (E. antisyphilitica). It is easy to tell the difference once you learn what an areole is. This is basically an area of stem that has a lot of potential to bud and generally takes the form of a fuzzy or spiny round spot that, spines, new stems, and flowers all come out of. In Euphorbia species that have spines, the flowers, and stems will come out above the spine or spine cluster, not through it. To learn more, I highly recommend familiarizing yourself with areoles. Wikipedia has a decent explaination of this, but there are several good cacti books that go into greater detail.
If the plant is flowering or fruiting, identification is extremely easy. In flower, cacti will have more than 7 (and typically many, many more) large, showy petals. In Euphorbia, only a handful of species have more than 5 glands and/or corresponding appendages. None are native to or established in the US. Occasionally, there will be an abnormal cyathium that has 6 or even 7 glands but look at the entire plant to find the average. Cacti fruits are unlobed and fleshy, Euphorbia fruits are 3-lobed and dry at maturity.

Bupleurum spp.:
When in flower, Bupleurum spp. look very similar to the larger members of subg. Esula. In addition to the different flower shape, the bracts are whorled below each flower cluster. In some species, each leaf blade completely surrounds the stem (perfoliate). The bracts of members of subg. Esula are typically whorled only at the base of the inflorescence, but become opposite above that. No species of Euphorbia in the US have perfoliate leaves.

Polygonum spp.:
These plants only really look like the members of sect. Anisophyllum, so I will focus on the differentiation from them. Polygonum species have alternate leaves instead of opposite leaves. Also, the stipules clasp around the stem, which is very unlike Euphorbia in general.

Portulaca oleracea (purslane):
This is most often confused with members of sect. Anisophyllum. None of the Euphorbia species in the US have succulent leaves like those of Portulaca. Portulaca also lacks the white milky sap of Euphorbia. The flower and fruiting structures are also quite different.

This is sometimes confused with members of sect. Anisophyllum, but have alternate leaves. The flowers are actually flowers and, as such, lack the glands that Euphorbia has. These are held on the underside of stems in many species.

A few other plants that have been misidentified as Euphorbia:
Pilea microphylla: confused primarily with sect. Anisophyllum.
Tidestromia: confused primarily with sect. Anisophyllum.
Mitracarpus hirtus (and other similar members of Rubiaceae): confused primarily with the upright, large leaved members sect. Anisophyllum.
Guilleminea densa: confused primarily with sect. Anisophyllum.
Achyronychia cooperi: confused primarily with sect. Anisophyllum.
Other prostrate members of Amaranthaceae: confused primarily with sect. Anisophyllum.
Kallstroemia: confused primarily with sect. Anisophyllum.
Polycarpon: confused primarily with sect. Anisophyllum.
Acmispon: confused primarily with sect. Anisophyllum.

Please let me know if there are any other groups that you would like to see listed here.

Photo showing Euphorbia maculata, Polygonum, and Portulaca oleracea

由使用者 nathantaylor nathantaylor2017年07月18日 15:32 所貼文




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