Why were fungi once classified as belonging to the plant kingdom?
For a long time taxonomists considered fungi to be members of the Plant Kingdom. This early classification was based mainly on similarities in lifestyle: Both fungi and plant are mainly sessile (meaning they are not able to move around), have similarities in general morphology and growth habitat (like plants, fungi often grow in soil, in the case of mushrooms forming conspicuous fruiting bodies, which sometimes bear resemblance to plants such as mosses). Moreover, both groups possess a cell wall, which is absent in the Animal Kingdom. However, the fungi are now considered a separate kingdom, distinct from both plants and animals, from which they appear to have diverged approximately one billion years ago. Many studies have identified several distinct morphological, biochemical, and genetic features in the Fungi, clearly delineating this group from the other kingdoms. For these reasons, the fungi are placed in their own kingdom.
