How do marine plants stay anchored?

Many marine plants are free floating and, therefore, are not anchored to the bottom of the ocean.  Most of those that do anchor themselves, such as kelp and other seaweeds are anchored by a root-like holdfast that does not transfer nutrients to the plant.  These holdfasts come in a variety of shapes and sizes.  Some are disc-shaped and others are more like claws.  There are a few species of flowering marine plants that have true roots to hold them in place and to provide their nutrients.