Toothworts in Vermont

In New England, genus Cardamine breaks into two distinct groups, the bittercresses and the toothworts. There are three species of toothworts in Vermont:

  1. Cut-leaved Toothwort (Cardamine concatenata)
  2. Two-leaved Toothwort (Cardamine diphylla)
  3. Large Toothwort (Cardamine maxima)

This article is about the latter two species, which are difficult to distinguish. Consider the following key characters from the indicated flora:

New Flora of Vermont [2015]:

  • Cardamine diphylla: Leaves normally 2, opposite or subopposite; margins of leaflets with appressed cilia
  • Cardamine maxima: Leaves normally 3, alternate; margins of leaflets with spreading cilia

Flora Novae Angliae [2011]:

  • Cardamine diphylla: Cilia on leaf margin mostly 0.1 mm long, ascending or appressed; rhizome continuous (i.e., of uniform diameter)
  • Cardamine maxima: Cilia on leaf margin mostly 0.2–0.3 mm long, spreading; rhizome alternately enlarged and constricted

Flora of North America (FNA) [2010]:

  • Cardamine diphylla: Rhizomes somewhat uniform in diameter; cauline leaves (sub) opposite
  • Cardamine maxima: Rhizomes not uniform in diameter (distinctly constricted at intervals); cauline leaves usually alternate, rarely subopposite

Since Go Botany is essentially the online version of Flora Novae Angliae, its online key is nearly identical to the printed key.

As far as I can tell, all of the above keys refer to stem leaves (not basal leaves, or rhizomal leaves, as FNA calls them). This implies that all keys require a flowering stem, that is, it may not be possible to distinguish the two species based on basal leaves alone (unless you're willing to dig up the plant in question).

DISCUSSION

The keys in the three flora overlap but there is no agreement: two of the flora include the arrangement of leaves on the flowering stem, two of them mention cilia along the stem leaf margins, and two them refer to the shape of the rhizome. The latter character is not very useful for identifying iNaturalist observations since observers rarely dig up their specimens (for obvious reasons).

Go Botany contrasts Cardamine diphylla with Cardamine maxima as follows:

  • Cardamine diphylla: with rhizome of nearly uniform diameter, leaves usually opposite, and cilia of leaf margin appressed to ascending
  • Cardamine maxima: rhizome alternately enlarged and constricted, leaves usually alternate, and cilia of leaf margin spreading

Although the stem leaves of the two species are usually opposite or alternate (resp.), the arrangement of leaves along the flowering stem is not sufficient. For a positive ID, the other two characters (rhizome morphology and leaf cilia) should also be considered. [Arthur Haines, personal communication]

For the sake of discussion, let's ignore Flora of North America for a moment. Both New Flora of Vermont and Flora Novae Angliae mention cilia on the margins of stem leaves, so that might be a good place to start. However, the cilia are very short, and so most of the photographs taken by iNaturalist observers will not show them.

Conclusion: The best above-ground character used to distinguish Cardamine diphylla from Cardamine maxima is the length and orientation of cilia on the leaf margins. The orientation of the leaves along the flowering stem, and the shape of the leaves themselves, are suggestive but not diagnostic.

Comments and suggestions are welcome.

RESOURCES

Publicado el 06 de junio de 2023 a las 03:09 PM por trscavo trscavo

Comentarios

Thank you for doing this research and compiling this summary of the available guidance, contradictory though it may be. This doesn't help with identification, but it is worth bearing in mind that C. diphylla is by far the most common and widespread of these three species in Vermont. C. concatenata is uncommon (S3), while C. maxima is rare to uncommon (S2S3).

Anotado por tsn hace 11 meses

Thanks @tsn at this point, I wouldn't know Cardamine maxima if I saw it. I need to start photographing the flowering stem and the leaf margins. Maybe I'll get lucky :-)

Anotado por trscavo hace 11 meses

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