Long-form article by CVN member Véronique McIntyre (text and photos).
Let’s go cycle!
But I am warning you: it will be a cycle of life. The life of mosses to be precise, using the information gleaned during the Botany Walk in February 2024 led by Jocie Brooks.
Mosses exist in two forms. The one we see year-round and call “moss” is a plant (phyto in Greek) that produces gametes. To sound educated, you can call that form a gametophyte. The gametophyte of each species looks different, which enables us to name them.
Here, for example, is a gametophyte of the step moss (Hylocomium splendens) obligingly showing the step that earned it its common name.
The second form is only visible a few weeks every year, and many people don’t notice it. That form produces spores and is called, you guessed it, a sporophyte. Each species has a different sporophyte which also enables us to name them, but it is less useful than the gametophyte for that purpose since it is not visible year-round.
The sporophyte does not look at all like the gametophyte.
Fig 2. A sporophyte of Menzies’ neckera (Neckera menziesii).
Luckily, sporophytes are attached to the gametophyte of their corresponding species, which makes it easier to identify each moss.
Fig. 3a. A sporophyte of Menzies’ neckera attached to the gametophyte.
Fig. 3b. Four sporophytes of lanky moss on the gametophyte.
In fact, sporophytes are only found attached to gametophytes. They don’t lead independent lives. Many sporophytes are not green—the gametophytes feed those.
Fig. 4a. Close-up of the attachment of palm tree moss sporophytes.
Fig. 4b. Close-up of the attachment of badge moss (Plagiomnium insigne) sporophytes. Notice the “foot” on the badge moss that both anchors the sporophyte on the female gametophyte and transfers water, nutrients, hormones and various other signals to the growing sporophyte.
Are sporophytes parasites then?
No, not more so than a fetus is in its mother’s womb. Sporophytes are a generation of moss that look different from the gametophyte generation. Why call them generations you ask? Because by definition, a generation is a reproductive cell (a spore or a gamete) that develops into an organism that produces reproductive cells. It is easy in humans—gametes give rise to an individual who produces gametes for the next generation. Mosses, like all plants, present an alternation of generations: a gametophyte carries a sporophyte that gives a gametophyte and so on.
Gametophytes produce gametes. Just like in humans, they produce female and male gametes (also called ovules and spermatozoids). A difference with humans is that in some species of mosses the male and the female gametophyte-forming parts are on separate plants and in other species they are on the same plant. Here for example are the male gamete-forming parts (called antheridia) of two different mosses:
Fig. 6a. Antheridium of juniper haircap (Polytrichum juniperum).
Fig. 6b. Two antheridia of palm moss (Leucolepsis acanthoneuron).
The corresponding female gamete-forming parts (called archegonia) are hidden inside the female gametophyte. Gametes are very precious to an organism (they form the next generation!) so it makes sense that in smaller mosses they are formed at the top of the moss, well protected (or at least as well as possible in such a small organism) from voracious herbivores.
Fig. 7. Archegonium at the top of juniper haircap, hidden by the summit leaves.
Moss sperm cells come equipped with two flagella, which allow them to move as long as there is water around. Sperm cells swim from one plant to the next until they reach an archegonium, go inside, and fertilize an ovule. Of course, the chances of a specific sperm cell reaching an archegonium are very low but given that mosses have been on Earth for the last 410 million years according to the fossils we find, that system obviously works for them. Fertilization is the fusion of the nucleus of the male sperm cell with the ovule. It can only be observed through a microscope.
Some species increase their chances of success at this stage by growing a carpet of female gametophytes next to a carpet of male gametophytes that are slightly higher. Palm moss displays this strategy beautifully, and so do species of badge moss (Plagiomnium spp.) and juniper haircap.
Fig. 8. Left: female gametophytes; right: male gametophytes. They are on a log that slants slightly downward toward the left side following the gentle slope of the trail.
The egg resulting from fertilization on the female gametophyte divides and the sporophyte becomes visible:
Fig. 9. The green oval mass at the top of the gametophyte of electrified cat’s tail moss (Rhytidiadelphus triquetus) is a very young sporophyte.
The sporophyte then grows a supporting stalk called a seta that raises it higher than the gametophyte, making it more difficult to reach by herbivores. At the same time a capsule appears on top of the seta.
Fig. 10. Young sporophyte developing on juniper haircap female gametophyte. The red seta (stalk) shows it can’t photosynthesize and points to its dependency on the gametophyte for its food. The white capsule is where spores will form. The white veil is a protective part from the gametophyte.
Very small mosses tend to develop very long setae compared to the moss height, which underlines the protection effect of the seta. This is the case in the wall screw-moss (Tortula muralis).
Fig. 11. Very long setae raise the capsule containing the spores of Tortula muralis a bit more out of reach of herbivores.
As the sporophyte matures, cells inside it divide using a special type of division called meiosis. This ensures that each resulting cell, called a spore, contains only one half of the number of chromosomes of the fertilized egg (called the haploid number of chromosomes, n for short). We humans use the same division process, but only in our gametes.
The germination of a spore results in a haploid gametophyte (contain ½ of the organism’s full set of chromosomes) and the gametes they form (by another type of division, called mitosis, which conserves numbers of chromosomes) are also haploid. Fertilization restores the full set of chromosomes (called diploid, or 2n), and the resulting sporophytes are diploid.
Cells in the diploid sporophyte carry both male (Y) and female (X) chromosomes, but thanks to meiosis each spore carries either an X female chromosome or a Y male chromosome (half female spores and half male spores). So, each spore germinates into either a female or a male gametophyte, although in some species the male and female parts are borne on the same gametophyte. The carpets of single-sex gametophytes observed in some species come from the germination and subsequent asexual reproduction of one spore, or from environmental factors that result in differential germination of spores.[1]
Fig.12. Mature sporophytes of broom moss (Dicranum scoparium) ready to release the spores they contain. The white veil that partially covers two of them is a remnant of the gametophyte that protected the young sporophyte (see Figure 9).
This works great for plants—each egg leads to the production of up to a millions spores. Thanks to this multiplying effect of having spores, plants can be eaten by herbivores in great numbers and still survive.
Spores spread a few metres around the sporophyte. The capsules can have teeth that guide a bit the direction the spores go. When it is wet the capsule stays closed—spores travel farther in dry weather.
Fig. 13. Empty spore capsule of bristle moss (Orthotrichum sp.). Note the teeth around the capsule.
To summarize, ovules form on the female gametophyte, which is a fixed plant. Sperm travel onto that plant to fertilize the ovule, thus recombining the genes from both parents and increasing genetic diversity of the species. The resulting sporophyte grows on that fixed gametophyte. The many spores that then form are released “far” from the fixed gametophyte they came from—spores allow the extension of the range of the moss. This is why we see carpets of mosses. Both generations, gametophyte and sporophyte, collaborate in mosses to extend their range and at the same time increase their genetic diversity which might allow them to adapt to new environments.
So, now that we have seen many steps of the life cycle of mosses, we can draw the whole cycle.
Just for the beauty of it, see the emerging new step in the step moss below… Who can’t agree that mosses are both fascinating and beautiful? Who would be so cruel as to use moss killer?
References
- Glime, J. M. (2017). Ecophysiology of Development: Gametogenesis. Ch. 5-8. in: Glime, J. M. Bryophyte Ecology. Volume 1. 5-8-1 Physiological Ecology. [PDF]
