The evolution of land plants or embryophytes from a haplontic algal ancestor some 470 million years ago is characterized by the innovation of multicellularity in both the haploid and diploid generation, commonly known as the alternation of generations. While early diverging land plants or bryophytes feature a sporophyte generation that is terminal and completely dependent on the gametophyte, all later diverging land plants or tracheophytes are characterised by a stepwise reduction of the haploid gametophyte generation and elaboration of the diploid sporophyte generation. The haploid-to-diploid transition during fertilization is directed by gamete specific TALE-class homeodomain (HD) proteins KNOX and BELL in phylogenetically diverse eukaryotes, including fungi, brown algae, red algae, the unicellular chlorophyte alga Chlamydomonas reinhardtii and the liverwort Marchantia polymorpha. However, land plant genomes encode two classes of KNOX genes. In seed plants, ferns, and mosses, KNOX1 activity is associated with continued sporophyte cell proliferation, including sporophyte apical meristem activity. KNOX2 genes in flowering plants are involved in secondary cell wall deposition in the vasculature and seed mucilage coating. While the genome of Arabidopsis thaliana encodes 4 class I and 4 class II KNOX genes, the liverwort Marchantia polymorpha encodes only one full length gene for each class. Recent research suggests that the Marchantia polymorpha KNOX2 share a similar role in secondary cell wall and mucilage deposition, although during sporogone wall and spore development.