Plants are able to grow throughout their lifetime due to undifferentiated tissues called meristems. Apical meristems add primary growth and cause stems to increase in length, while lateral meristems add secondary growth and cause plants to increase in girth. Flowers are the reproductive structures of angiosperms that contain both male and female organs in some species. A seed consists of a dormant embryo surrounded by a food supply and protective seed coat. It undergoes germination through imbibition and metabolic changes that allow the embryo to resume growth.