Bryophytes are considered "nonvascular plants" which means they do not contain a true vascular system such as xylem and phloem. In them a simpler conducting tissue is found that too only in mosses. These conducting cells that transport nutrients and water throughout the moss plants are called hydroids and leptoids. They are similar to xylem and phloem except they lack specialized, lignin-containing wall thickenings. Their cell walls are much thinner and yet very permeable to water and solutes.