Usually these are little oases in the middle of a busy city and a perfect spot to sit down and enjoy the silence.
Bologna has one of the oldest botanical gardens in Europe, it was founded in 1568. There was already a Chair of Herbs, (a botanical professor) at the Bologna university, and this was mostly for the medical students who needed to know which herbs could be used as cures.
When the garden was founded, it was not in the same location as it is now, it has moved a couple of times.
Nowadays, it is still part of the university and admission is free.
There are a couple of greenhouses with tropical plants, but there are also different kinds of gardens, like a rock-garden.
Recently they have tried to re-create the old herb-garden with medicinal plants (garden of simples), using the same divisions with plants for the digestive system, the plants for the nervous system etc.