All seed plants, some ferns, and some lycophytes, are heterosporous. They produce small dispersing male spores and large, usually static, female spores. The remaining plants are homosporous, producing only one type of spore. Over 50 years ago, researchers noted that homosporous plants have many more chromosomes that heterosporous ones. Chromosome number usually increases through the process of genome doubling (polyploidy). After decades of theory and studies we still do not have a satisfactory explanation for how the condition of spore types can be related to chromosome number. This synthesis brings together ideas and data that have accumulated over 30+ years from my lab, and from others. Using available databases, we are exploring traits that correlate with spore type. We are also looking at gene presence and expression patterns in the genomes of representative homosporous and heterosporous plants. Others involved in this project include Sylvia Kinosian, Pam Soltis, Doug Soltis, Blaine Marchant, and Jim Leebens-Mack.