Marsilea crenata

This small fern comes from Asia, where it often grows in rice fields. The leaves are tiny and rarely wider than 0.5cm. 
It is a hardy plant that tolerates poor light conditions and high temperatures. However, improved conditions are beneficial to the plant. Planted in small bundles that spread with offshoots. Creates a 1-3cm high and close carpet, well-anchored in the bottom layer.