Seeds have been an important development in the reproduction and spread of conifers and flowering plants. Plants such as mosses, liverworts and ferns do not have seeds, and use unprotected spores and other methods to propagate themselves. Before the upper Devonian period, land plants, like modern ferns, reproduced by sending spores into the air. The spores would land and become new plants only in favourable conditions. Spores have little food stored, and may be just single cells rather than embryos.