In biology, a microspore is a small spore as contrasted to the larger megaspore. This combination is found only in heterosperous organisms. Most plants that reproduce by spore without seed only produce one class of spore. In the ferns, the only megasporous plants are aquatic or semi-aquatic, including the genera Marsilea, Regnellidium, Pilularia, Salvinia, and Azolla. This condition is also known in the lycopod genus Selaginella.