the protonema is the first stage of the gametophyte in mosses, developing directly from a haploid spore; thus, its cells are haploid . In contrast, the Microspore Mother Cell is diploid and undergoes meiosis to form microspores . The Primary Endosperm Nucleus (PEN) in angiosperms is formed by triple fusion (fusion of a male gamete with two polar nuclei), making it triploid .