Moss. To those of us in Colorado, moss conjures images of soft green blankets cushioning the forest floor, or carpeting the north side of a tree in a verdant forest. To my daughter in western Washington, moss is that soggy green mat choking out the turf in her shady backyard, or keeping the shake roof too wet, allowing rot to set in. But love it or disparage it, moss is one fascinating plant.

Before we get to just why I find moss so interesting, we need to take a detour back to your high school biology class. (No quizzes, I promise.) You might recall that most organisms have two sets of corresponding chromosomes. For example, humans have 46, two sets of 23. These matching chromosomes provide the instructions for creating and maintaining us.

When gametes—eggs and sperm—are formed, those paired chromosomes have to split. One chromosomes of each pair goes into one gamete; the other goes into another gamete. Thus, each human gamete has 23 chromosomes. When the egg and sperm unite, the resulting fertilized egg once again has a full complement of 46 chromosomes, half from the mother and half from the father. This is why you might have your father’s brown eyes, but your mother’s pointy nose.

With each generation, the chromosomes are reshuffled. This is called sexual reproduction, and it allows for more genetic variability. In turn, that gives a species a greater chance to survive changes in the environment. This is a good thing.

Okay, why go into all this biology? Weren’t we talking about mosses? Hang on, almost there.

Like animals, plants also take advantage of sexual reproduction, but the way they do it is more complicated than in animals. Plants have what we call an “alternation of generations.” That means that one generation reproduces asexually (without recombining their chromosomes). Then the new generation reproduces sexually. Then that generation reproduces asexually, and so on. (Some animals, such as the sea anemone-like hydras, do this as well.)

There’s more. The generation produced asexually will have only half the full number of chromosomes. (We say that it’s “haploid,” “hap” meaning “half.”) These haploid plants reproduce by creating gametes—eggs and sperm. That’s why haploid plants are called gametophytes—they produce gametes. An egg and a sperm then combine sexually into a zygote having both sets of chromosomes.

Each zygote grows into a plant. Because it has both sets of chromosomes, we say that plant is “diploid, ” “di-” meaning “two.” This diploid plant will mature and produce haploid spores, so it’s called a sporophyte.

So, sporophytes are diploid and gametophytes are haploid.  Confused? Maybe this illustration will help.

Most of us are familiar with ferns. You may have noticed the ordered series of black spots beneath their fronds. These are sacs filled with spores. When the spores are released, they grow into tiny gametophytes, only about half an inch tall. These, in turn, produce gametes which combine (in water) to form a zygote which grows into a new fern plant. This is why you find ferns growing primarily in wet places. (When working the Master Gardener help desk, I would occasionally get callers concerned about the insect or disease afflicting their ferns. I was happy to reassure them that their plants were merely trying to reproduce.)

Most of the plants we see are diploid. They have both sets of chromosomes. Therefore, they’re also sporophytes—they produce spores. Daisies, redwood trees, and Kentucky bluegrass are all diploid sporophytes. You never see the spores, or the haploid, gametophyte generation, because it is very small—only a few cells—and grows inside the plant’s flowers or cones.

So—what makes moss so special? In this instance, the moss we see growing on trees or carpeting our roof is the haploid, gametophyte generation. It’s the sporophytes that are tiny and inconspicuous.

(Mosses aren’t the only plants where the gametophytes live longer and are not dependent on their sporophyte hosts. Liverworts and hornworts also have a dominant haploid generation. Both of those are types of small, non-vascular plants.)

Moss plants are either male or female. Male plants produce sperm, and female plants produce eggs. (Remember, parent plants already have only a single set of chromosomes, so no special cell division (meiosis) is needed to produce haploid eggs and sperm.)

Since moss lacks flowers and pollinators, the sperm have to fall into water so they can swim to the female plants. This is why we only find mosses in damp places such shaded streams or pretty much all of western Washington. It’s also why we never see a moss tree. How could the sperm swim up to the top of the tree where the female gametes are waiting?

Once the sperm reaches the egg, they fuse and grow into a diploid sporophyte.

This sporophyte has no chlorophyll, so it can’t manufacture food from sunlight. It is completely dependent on the parent plant for protection and nourishment. Eventually, the sporophyte creates haploid spores—and these grow up to become the familiar green carpet we see in the forest.
_____
Chart attribution: Alternation of generations.svg: Peter coxheadderivative work: Peter coxhead (talk) – This file was derived from  Alternation of generations.svg:, Public Domain, https://commons.wikimedia.org/w/index.php?curid=18439050