Fungi are in between animals and plants. Their cell walls are made of chitin—a substance never found in plants—which makes them more like insects. In addition, they cannot photosynthesize and depend on organic connections with other living beings they can feed on.
The survival of the fittest? Trees would just shake their heads—or rather their crowns. Their well-being depends on their community, and when the supposedly feeble trees disappear, the others lose as well.
Whenever the seedlings’ roots were exposed to a crackling at 220 hertz, they oriented their tips in that direction. That means the grasses were registering this frequency, so it makes sense to say they “heard” it.