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Mosses are 'an old model' of plants that have very limited 'differentiated tissue'. This means that they only have a few types of cells. Like angiosperms consist out of roots, leafs, stem, etc. which all hold different cell types that perform different functions, the moss plant itself doesn't have any of that. * They don't have specialized roots that transport moisture from the soil to the top of the plant which means that the entire plant has to stay moist in order for it to survive. That's why you find them on forest floors which are shielded from the sun etc. This limits distribution throughout and across eco-systems. * They don't have any seeds with shells and a oily reserve (like apple seeds) that can which keeps moisture in, and makes it possible for the seed to wait for years for a good time to sprout. * Because mosses don't have roots they can't grow tall and win a battle for light with plants that do have roots. These are just a few examples of the advantages that cell-differentiation (angiosperms) has over non-cell differentiation(mosses). But ask yourself this; if mosses have been around for longer than angiosperms and still thrive very well, are those new features really an advantage? If you ask me, these features and evolutionary improvement of them is rather an advantage over their fellow angiosperms because they grow in the same part of an ecosystem. Mosses and angiosperms can live together just fine because mosses can grow under trees because they don't really like a lot of sunlight and the angiosperms keep mosses nice and in the shade.
Thank you very much.