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A holdfast is a rootlike structure used by some plants and algae to firmly affix themselves to substrates such as rock. Many people associate holdfasts with seaweed, although they can also be found on land; ivy, for example, produces holdfasts which help it climb rock and brick walls. If you have ever attempted to pull a piece of seaweed from a rock, you can understand the name of this plant structure; you will typically tear the seaweed apart before forcing the holdfast loose.
Holdfasts work in different ways, depending on the plant and the function of the holdfast. Seaweeds, for example, use their holdfasts to firmly fix themselves in place in the ocean. The holdfast may look like a bundle of roots, but unlike roots, the holdfast does not pull up any nutrients for the parent algae. Ivy, on the other hand, does gather nutrients with the holdfast.
Seaweed holdfasts come in a number of different shapes. Discoid holdfasts look like discs, and they attach to the substrate with the form of a natural glue generated by the seaweed. Stolon and claw types, on the other hand, are made with a bundle of structures called haptera, which look like fingers gripping the substrate. These types of holdfasts are also attached with glue, ensuring that the plant cannot be dislodged.
This rootlike formation makes a pretty remarkable anchor, and holdfasts are often a topic of interest for people who study things like kelp forests. A kelp forest consists of a huge collection of individual strands of kelp, anchored to the ocean floor with their holdfasts. Kelp forests not only look like forests on dry land, they also act like tree forests, providing shelter to other animals and creating a unique microclimate. The flexible fronds of seaweed can move with the water, but they won't drift off, thanks to their firm grip on the substrate.
Many aerial plants also create holdfasts to take advantage of available substrates. People who cultivate such plants at home may offer them rocks to grow on so that their holdfasts have something to grab on to. Such plants must typically be kept misted with water and nutrients to ensure that they stay healthy, because the holdfast will not pull nutrients from the substrate, as the plant does not want to damage the rock it is anchored to.