This is a gametophyte of the fern Asplenium scolopendrium. It is only about 2mm across and is composed of a heart shaped sheet of cells that is only one cell thick. Fern gametophytes are easy to grow and you can do it at home.
Sowing the spores
|Scalding soil before sowing fern spores.
|Enclosing the pot in a bag to give a humid atmosphere and to avoid needing to water the pot.