the lesson: the diploid phase of a plant life cycle begins with a fertilized egg called a zygote. a zygote divides by meiosis and grows into a mature sporophyte. a sporophyte is a spore-producing plant. when it is mature, it will produce haploid spores through meiosis. a spore will divide by mitosis and grow into a mature gametophyte. a gametophyte is a gamete-producing plant. when a sperm meets an egg, fertilization takes place and a new sporophyte is produced. this sporophyte starts the cycle all over again.