I got an email a while back asking about the leptom and hadrom in mosses (sometimes both of these terms are spelled with an added -e at the end). Admittedly I had not heard these two terms before, but I was pretty sure that they referred to the hydroids (water conducting cells) and leptoids (photoshythate/sugar conducting cells) in mosses. Yes, some bryophytes do have specialized cells for conducting either water or sugars through their plant body, however, the walls of these cells are not strengthened by the compound lignin, so they are not termed xylem and phloem.
I headed to my handy reference shelf to look up the definitions of these two terms and here is what I found about the water conducting cells of mosses.
The hadrom is a term for all the of hydroids together in a structure. In mosses hydroid cells are unique to the Bryopsida, the crown group of mosses, and are lacking in some of the earliest diverging lineages: including Sphagnaceae, Andreaeaceae, and Andreaobryaceae.
The leptom (consisting of leptoids) is unique in mosses to the Dawsoniidae and Polytrichidae (the group of mosses that includes Polytrichum, the hairy capped mosses). Other groups of mosses have cells that could be termed 'conducting parenchyma cells', but they are not as specialized as leptoids.
These terms (leptom and hadrom) were introduced by the German botanist Haberlandt in 1879, which is probably why I hadn't heard of them before.
If you are interested in reading more, this paper has a very thorough and readable discussion of water conducting cells in bryophytes, which I consulted for the above information on hydroids and leptoids.