Hill's pondweed (Potamogeton hillii) COSEWIC assessment and status report: chapter 3

Species Information

Name and classification

Scientific name:

Potamogeton hillii Morong

Synonym:

Potamogeton porteri Fern.

Common names:

Hill’s pondweed; potamot de Hill

Family:

Potamogetonaceae (Pondweed family)

Major plant group

Monocot flowering plant

The type specimen of Potamogeton hillii was collected by Rev. Ellsworth J. Hill in Manistee County, Michigan, August 5, 1880.

Potamogeton porteri was originally determined to be a separate species, distinguished from P. hillii by its broader leaves. Closer examination of both herbarium specimens and field collections by Haynes (1974) resulted in the conclusion that there was a large variation in leaf morphology of the species and that P. porteri is a morphological extreme of P. hillii. This view has been largely accepted in subsequent treatments.

Description

Hill’s pondweed is a submerged aquatic with green to olive stems, 30-60 cm long, and 0.5-1.0 mm in diameter bearing linear, green to olive leaves. Leaves are 3 nerved, 2-6 cm long, 1-2.5 mm wide (sometimes up to 4 mm) and are bristle-tipped; 1-2 rows of air cells (lacunae) are present along the midrib. The leaf-like stipules are free, delicate, rarely shredding at the tip, 7-16 mm long. The clusters of flowers (spikes) are 4-7 mm long and held above the water surface on recurved stalks (peduncles) 0.6-1.4 cm long. Fruits are brown, 3-keeled, 2.3-4 mm long. A good line drawing is available in Crow and Hellquist (2000). Figure 1 is provided courtesy of Agriculture and Agri-Food Canada.

Sub-section Pusilli of the genus Potamogeton is comprised of the submerged linear-leaved species of pondweeds, and they are notoriously difficult to distinguish from one another due to their vegetative similarity and morphological plasticity (Haynes, 1974). Haynes concludes that flowers or fruit are necessary to distinguishing the species of the sub-section Pusilli, much as it is for species of Aster or Solidago.

Potamogeton hillii could be mistaken for P. pusillus, P. foliosus, P. friesii, P. strictifolius, or P. ogdenii, especially when not in fruit. Potamogeton hillii’s bristle-tipped leaves help distinguish it from P. obtusifolius, P. pusillus, and P. foliosus. P. hillii can also be distinguished from P. foliosus by its 3-keeled fruit and longer leaves. The similar, narrow-leaved pondweeds are best distinguished through the use of a key such as that in Crow and Hellquist (2000). No genetic markers for Potamogeton hillii have been documented.

Figure 1. Overall habit of Potamogeton hillii and details of selected plant parts (illustration courtesy of Agriculture and Agri-food Canada):

  1. habit
  2. terminal portion of leaf
  3. flowers seen from top
  4. fruits
Figure 1. Overall habit of Potamogeton hillii and details of selected plant parts: A, habit; B, terminal portion of leaf; C, flowers seen from top; D, fruits (illustration courtesy of Agriculture and Agri-foodCanada).

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