Haustoria are specialized structures formed by certain parasitic plants, such as mistletoe and dodder, to extract nutrients and water from their host plants. These structures penetrate the host's tissues, allowing the parasite to access vital resources necessary for its growth and survival. In addition to plants, haustoria can also be found in some fungi, where they help the organism absorb nutrients from their surroundings. This adaptation is crucial for the survival of these organisms, as they often rely on other living organisms for sustenance, making them dependent on their hosts for energy and nutrients.