Some science behind the scenes
It is a remarkable fact that well into the space age, most modern school text books divided the world of the living into just two categories – plant and animal. Micro-organisms hardly featured. Amoebas and similar single celled organisms were treated as proto-animals and algae as proto-plants. Bacteria were usually lumped in with plants too, even though everyone knew they didn't belong there.
Many organisms in the visible world were also poorly served by the traditional division. Fungi, the group that includes mushrooms, moulds, mildews, yeasts and puffballs were nearly always treated as botanical objects, though in fact almost nothing about them – how they reproduce and respire, how they build themselves – matches anything in the plant world.