More from The Signature of All Things

***Spoiler Alert***

This post may contain plot details and quotes from The Signature of All

In the midst of the drama and intrigue that plays out in this book are some great mossy points for discussion. 

At one point Alma mentions that mosses have no internal skeleton to support themselves growing tall, thus they are relatively short. Additionally they cannot transport water within their bodies. Bryophytes are typically called non-vascular because that lack the conducting tissues of xylem and phloem. These tissues transport water and sugars to and from the roots and leaves. 

This is a distinction that I would often point out between bryophytes and other plants. More recently I have come to question and debate this point. Some mosses do have cells that move water and sugars internally from one part or their body to another, called hydroids and leptoids. They are similar to the cells of xylem and phloem of vascular plants. Some of them are dead at maturity and/or have modified end walls with perforations, allowing for faster transport. What these cells lack is the compound lignin in their cell walls. Lignin both strengthens the cell walls and makes them impermeable to water. Creating stronger and less leaky transport tubes. Lignin is what gives wood its strength and enables trees to grow tall. Mosses have some of the chemical precursors to lignin (Ligrone et al. 2008), but they did not evolve this compound. So I get that lignin is important, but some bryophytes do have conducting cells that move water and sugars around in their bodies. I wouldn't call them vascular, but they are not lacking internal water transport either. 

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Alma describes mosses as being defined by what they lack. No flowers, no seed, no fruits, no roots, and no internal skeleton. Mosses also do not engage in sex. All these points are true except for the last one. Mosses do in fact produce offspring by sexual reproduction. They have eggs and swimming sperm that fuse to form the sporophyte offspring. My guess is that this inaccuracy was intentional. 

The alternation of generation in plants was elucidated in 1851 by Wilhelm Hofmeister (Kaplan and Cooke, 1996). Though Alma is described as having corresponded with researchers around the world, she may not have read Hofmeister's work. He was based in Germany and his 1851 work was printed by his family's publishing company. I am not sure how widely the work would have circulated at that time. Thus at this time the reproduction of mosses was a "mystery to the naked human eye". This aspect lead to their being known by the evocative name Cryptogamae, which means hidden marriage. 

So Alma's statement that mosses do not engage in sex was an accurate statement for that time in our scientific knowledge of plants. Kudos to the author and her bryological guru for their attention to detail. I think it is good when we acknowledge that science is not a static bank of knowledge. We are constantly discovering and expanding our understanding of the world around us. Looking back at the history of where science has been helps us to appreciate how far we have come

This is part of a series of posts about the bryology in The Signature of All. 
Click here for all the posts in the series. 

References

Kaplan, D. and T. Cooke. 1996. The genius of Wilhelm Hofmeister: The origin of causal-analytical research in plant development. American Journal of Botany 83: 1647-1660.

Ligrone, R., A. Carafa, J. G. Duckett, K. S. Renzaglia, K. Ruel. 2008. Immunocytochemical detection of lignin-related epitopes in cell walls in bryophytes and the charalean alga Nitella. Plant Systematics and Evolution 270: 257-272.

Hofmeister, W. 1851. Vergleichende Untersuchungen der Keimung, Entfaltung und Fruchtbildung hoherer Kryptogamen (Moose, Farne, Equisetaceen, Rhizokarpeen and Lykopodiaceen). und der Samenbildung der Coniferen. {Translated Title: Comparative studies of the germination, development  and fruiting of higher cryptogams (mosses, ferns, Equisetaceae, Rhizocarp and Lycopodiaceae). and seed formation of conifers.} F. Hofmeister, Leipzig.

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!