Eleocharis elliptica Kunth - a study on its variability
Eleocharis elliptica can be quite variable and specimens seem to intergrade between Eleocharis elliptica and Eleocharis compressa. Key morphological features that are used to separate these species are the cross section of the culm, the bifid nature of the scales and the rhizome internode spacing. All require careful collection and examination. Refer to Figures 1 and 2 for the following discussion.
Almost all Renfrew County (Eleocharis elliptica) specimens I have seen, show flattened culms, at least twice as wide as thick and up to five times wider than thick - only specimens from one location showed generally terete culms. The more flattened the culm, the more it exhibits a noticeable helical twist. Flattened culms with a helical twist are reported to be more characteristic of Eleocharis compressa but, Eleocharis elliptica and Eleocharis compressa may be better separated by the number of vascular bundles [Drapalik and Mohlenbrock, 1960] and not simply the aspect ratio of the culm cross section. However, specimens from Renfrew County show 8-11 ridges on the culm, indicative of 8-11 vascular bundles (see Figure 1) which, is intermediate betweeen Eleocharis elliptica with 6-8 vascular bundles and Eleocharis compressa with 9-14 vascular bundles [Drapalik and Mohlenbrock, 1960].
Bifid scales are seen frequently for Renfrew County specimens but these are shallow (nearly emarginate) and not deeply cut (cut depth shorter than scale width) as compared to Eleocharis compressa that should have a cut depth longer than the scale width.
A third characteristic for consideration is the rhizome internode spacing. Eleocharis elliptica is reported to have a rhizome internode spacing of 5 to 10 mm while Eleocharis compressa is only 2 mm [Smith, 2001, and S.G. Smith discussion in FNA]. Figure 2 shows internode spacing ranging from 3 to 7 mm but mostly 4 or 5 mm as is typical for all specimens from Renfrew County.
Given the above discussions, an assignment to Eleocharis compressa is poorly supported and at the very least, Renfrew County specimens encountered so far lean heavily toward Eleocharis elliptica.
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Figure 1. Microscopic images, random selections from four Renfrew County populations. Cross sections of culms were cut (mid culm) and imaged fresh, air pockets black, vascular bundles poorly captured. Leaf sheaths, scales and achenes imaged after drying specimens.
Figure 2. Rhizomes from two Renfrew County populations. Internode spacing marked by black lines.
References
Donald J. Drapalik and Robert H. Mohlenbrock, The Taxonomic Status of Eleocharis elliptica, The American Midland Naturalist, Vol. 63, No. 1 (Jan., 1960), pp. 143-148
S. Galen Smith, Taxonomic Innovations in North American Eleocharis (Cyperaceae), Novon , 2001, Vol. 11, No. 2, pp. 241-257