What is a moss?

For those of you unfamiliar with mosses:

Polytrichum commune

Mosses are small, green, photosynthetic plants. Instead of producing flowers and seeds, they produce spores. Most of them have a stem with leaves on it similar to ‘higher’ plants, but their leaves are usually only one cell thick. They have things that look like little roots, called rhizoids, which are used for anchoring the plant to its substrate. They have no true vascular tissue and all water and nutrient uptake is done on the outside of the plant. They have evolved many interesting structures that help in the quick hydration of the plant. Mosses have the amazing ability to dry out completely, suspending all growth and then, when water becomes available again, resume photosynthetic metabolism. Due to their small size and seeming insignificance, many people assume they are a ‘lower’ plant form. However, they are very specialized and an extremely successful group of plants which occupy every possible niche available to them.