Land plants (Class Embryophyceae) include liverworts, mosses, ferns and their allies, gymnosperms, and the flowering plants. The earliest embryophytes made the transition from water to land. This transition required three important morphological adaptations: the cuticle, guard cells, and spores. The cuticle is a waxy layer on the exterior of the epidermis that helps to retain the proper osmotic balance of water within the plant when exposed to the air. While the cuticle kept water in, it also did not allow carbon dioxide, in which is necessary for photosynthesis to occur. Specialized cells known as guard cells created openings, stomata, in the leaves to allow for the diffusion of gases. Spores are single-celled gametes that are able to disperse overland and have very thick walls to protect the embryo from drying out. It is currently hypothesized that land plants developed from stoneworts. While both classes within Streptophyta are considered multicellular, land plants vary from stoneworts by having complex, specialized, and very protective reproductive structures (i.e. spores in sporangia or seeds in fruits), whereas stoneworts have gametes within a simple structure, the oogonium, prone to drying out when exposed to air