Conducting Cells in Mosses

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 present in the peristomate mosses (those with teeth around the mouth of the capsule) which includes the Bryopsida, the crown group of mosses, and the Polytrichopsida, the hairy-capped mosses. Hydroids are lacking in some of the earliest diverging lineages: including Sphagnaceae, Andreaeaceae, and Andreaobryaceae.
(Paragraph Updated 16 April 2014:  An earlier version of this post incorrectly stated that hydroids are only present in the Bryopsida. However, in Ligrone et al 2000 they state that hydroids are present in the Bryidae. This older name for the group includes both the Bryopsida and the Polytrichopsida and thus has been updated in the paragraph above.)

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.

R Ligrone, J G Duckett, and K S Renzaglia. 2000. Conducting tissues and phyletic relationships of bryophytes. Philos Trans R Soc Lond B Biol Sci. 355: 795–813. 

To give you a visual of these conducting cells, I remembered that I came across some during the course of my dissertation research. Below is a transverse section through the midrib of a gametophyte leaf of Funaria hygrometrica.

The cells with the thickest walls in the middle-center are stereids, which help to support the leaves. Directly above them are two cells that have a 'blown out' appearance. These cells do not have any cytoplasmic contents and have a very thin wall between them. I would interpret these cells as hydroids. Above and slightly to the right of the hydroids is a large cell with intact cellular contents and a large number of pores in one of the cell walls. Thus I think that this is a food conducting cell or conducting parenchyma cell. 

I have blown up the image and added some letters to help orient you to the larger image above. (Key: stereid = s, hydroid = h, food conducting cell = fcc)

It has been a while since I have looked at and interpreted electron micrographs. Well maybe not so long ago. I did a lot of that for my dissertation but the interpretation stage seems like, and was a couple of years ago. Interpreting this micrograph was super fun and tells me that I need to get back to the electron microscopy lab and generate some more images and data to interpret and think about!