Word of the Week: Holdfast

holdfast [ hohld-fast ] noun: a rootlike or disk-like structure that serves to attach certain organisms to a support

 

In botany, a holdfast does exactly what it says–it holds organisms (like certain plants, algae, or lichens) firmly to substrates or supporting structures.

Few plants outside of the tropics have holdfasts, but two such plants are native woody vine Parthenocissus quinquefolia (Virginia creeper) and non-native invasive Euonymus fortunei (wintercreeper). The tendrils of Virginia creeper are negatively phototropic, that is they grow toward the darkness, where they might find a support, rather than toward open light. When its tendril touches a vertical structure, the tip enlarges into a disk or holdfast that secretes a resinous adhesive, which firmly secures it to the solid surface. “Tendrils ten and fifteen years old have been tested and found to be elastic and the holdfasts resisted detachment until the pull exerted was over two pounds.” (Lofgren 1973) Because these holdfasts are difficult to remove, vines should not be allowed to grow on wood shingles, painted surfaces, gutters, and wires. As a ground cover, wintercreeper spreads quickly overtaking native plants in its wake. Holdfasts allow it to climb 40 to 70 feet on vertical structures like shrubs and trees, smothering them and their seedlings.

Kelp forests grow mostly along the Pacific coast from Alaska to Baja, California. Giant kelps, more abundant in the southern range, can grow up to 175 feet and anchor their oscillating bodies to underwater rock with rootlike structures called holdfasts that can withstand strong ocean currents. The density of the kelp creates a drag on the currents and a more tranquil habitat for a diversity of marine species. Kelp meadows are found in the subtidal rocky shallows of the North Atlantic, including along the New England coast. At 3–13 feet tall, Laminaria digitata (horsetail kelp, oarweed) can be the dominant alga in these meadows and their holdfasts provide refuge to a wide variety of crevice dwelling species.

Some lichens, the symbiotic unions of a fungus and an alga, anchor themselves to a substrate like a rock or tree with mostly non-penetrating fungal filaments called rhizines or a central peg-like extension from its body (thallus) called a holdfast. Like the holdfasts of kelp, it functions only as an attachment mechanism and does not absorb nutrients as do the roots of plants.

References

Jasper C. Hill JM. 2015. Laminaria digitata on moderately exposed sublittoral fringe bedrock. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. (accessed October 7, 2021).

Kelp Forest Habitat on the West Coast. NOAA Fisheries. Last updated by West Coast Regional Office on June 25, 2020.

Laminaria digitata. Marine Algae of Cape Cod, Massachusetts and Adjacent Islands. iNaturalist. (accessed October 13, 2021).

Lichens. U.S. Department of Agriculture, Forest Service Rangeland Management & Vegetation Ecology—Botany Program. (accessed October 7, 2021).

Lofgren DO. 1973. Climbing Plants. American Horticulturalist Spring, 52(1): 27–31.

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