At one time, biologists thought that fungi were merely forms of plants that had lost their chlorophyll and had returned to saprotrophy to gain food. why is this no longer considered a solid theory?

Answer & explanation:

Fungi have long been regarded as organisms that were part of the Plantae kingdom (plants), mainly because they have cell walls in their cells, reproduce by spores, and the fact that some of them are sessile (attached to the ground).

However, fungi are no longer considered as plants because they do not have chlorophyll or specialized cells, such as chloroplasts and vacuoles. In addition, fungi have heterotrophic nutrition (plants are autotrophic), needing to absorb organic substances to survive.

Nowadays, fungi are studied in isolation and are inserted in their own kingdom, the Fungi kingdom.