Sclerid reinforced with lignin. Some living land plants, namely mosses, do not contain xylem and phloem. Instead, the gametophytes of many mosses contain water conducting cells known as hydroids . Like tracheids, hydroids are elongated cells with oblique end walls, however they lack secondary ornamentation characteristic of tracheids. Also like xylem, hydroids lack cell contents at maturity and so appear empty.
Some mosses also have solute-conducting leptoids surrounding a central bundle of hydroids.
Leptoids are elongate cells that have nuclei and living protoplasts and thus closely resemble the most generalized phloem cells of some vascular plants. Hydroids may also be found in moss sporophytes , but leptoids have been found only in the sporophytes of a few genera. P-type cells characteristics of Psilophyton and many common living plants. The fossil record of early land plants preserves a variety of other conducting-tube forms. Some tubes are smooth and lack ornamentation.