Field of Science

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!


  1. Hi Jessica,
    These pictures are great! Its hard to find detailed close up photos of these cells for class. Thanks for sharing. When you say that "the walls of these cells are not strengthened by the compound lignin, so they are not termed xylem and phloem", one can assume that phloem cell walls have lignin. I believe that phloem cells in vascular plant don't have lignin either. Am I wrong?

  2. Mostly it is just more complicated than I let on with that statement. Technically Xylem and Phloem are complex tissues that consist of multiple cell types. You are right that the phloem cells that do the transporting (sieve cells or sieve-tube members) and the cells that assist (companion cells) are not lignified. However the phloem does include both fibers and sclerids that are lignified. Add in some additional parenchyma cells that function in storage and radial transport (non-lignified), and phloem is pretty complex.

    Thanks for pointing out the need for some clarification of this point. I am always challenged with relaying complex information in a simplified way in this blogging format without sacrificing accuracy.

    I consulted my favorite Plant Anatomy text for the above information. (Anatomy of Seed Plants by Katherine Esau)

    Thanks again!

  3. Jessica, you do a great job in showing complex information in a simplified way. This is why I indicate your blog to my students! I'm a botany teacher in São Paulo, Brazil and I know how difficult it is to deliver a complex subject... specially when most of the literature is in English.

    1. Thanks for the compliment on the blog. I am glad to hear that you are directing your students here and hope that they find it useful too. Feel free to use the images from any of my previous posts. Having them up and available for other educators is one of my goals with this whole blogging endeavor. Cheers - Jessica

  4. thanks for this blog too - teaching botany in Prague, Czech Republic; getting very usefull overall information from yout blog, keep track :-)



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