by Don Culwell, SFNC docent
This space in the December E-Newsletter described the tiny forest of moss “stem and leaves;” they are really no stem and leaves at all, for they lack any vascular tissue to conduct food and water up through the tiny plants (but they do not need this vascular tissue, for they are only tiny plants and can absorb moisture from their surfaces…they do it quite well).
Well, plants of this “moss forest” are involved in plant sex, and it is not the mating of tiny springtails and water bears that live among their leaves, although they may be there and doing sex, too. (You see, sexual reproduction is vitally important to all organisms that can manage it. This process produces new combinations of genes that form offspring that are a bit different from their parents, offspring on which natural selection by the environment can act …a subject for another discussion in this space).
Located in the tops of green moss plants sit tiny, thin walled containers where eggs and sperm are made, sperm that have flagella (whip-like hairs) that propel them to the eggs. These flagellated sperm can swim through the film of water on the surfaces of moss leaves, water not only from splashing of rain drops bouncing sperm all over, but water that accumulates on moss leaves from a heavy dew-fall or mist on a drippy day. Many sperm cells are made inside each tiny, thin walled, ovoid sac we call an antheridium. You see, many sperm must be made which insures the fertilization of just one egg, since these sperm are splashed about all over the place, not just toward an egg.
So, these sperm cells swim to the eggs, eggs that are found in archegonia, one in each archegonium. Archegonia are tall, thin, vase-like structures with an enlarged base big enough to hold just one egg…each archegonium has a tall, thin neck with an opening all the way through it…it is down through this long neck that a sperm swims. Ultimately it reaches the egg and “ZAP,” it’s been fertilized…the sperm and egg fuse (one set of genes from the male plant and one from the female plant)…a zygote is made with 2 full sets of genes in the cell, one from each plant. Now, isn’t that just the neatest thing, the best way to make a new moss plant?!
This new zygote cell grows into a taller, whisker-like hair still attached to the top of the tiny, green moss plant…many of these are spread over the top of a pad of moss plants at some time of the year…many of them, sometimes much taller than the green, leafy moss plants they are on top of. And this gives a very “hairy” look to the “moss forest”.
Way at the top of each of these tall “hairs,” one from each plant bearing it, is a rounded or elongated capsule inside of which many, very, very tiny spores will be made. Each spore cell contains only one set of genes (not the 2 sets that were put together by fertilization making the zygote cell). This one set of genes is put in each spore through a process we call meiosis, or a cell division that reduces the number of chromosomes in each cell from that division process. At just the right moment when these spores are mature, each capsule bursts open by teeth under the cap at the top and spores are released into the wind. These spores are blown everywhere and germinate to grow into the first green cells (protonema) that will form green mosses that you so easily can see.
So, now you know how mosses come to be…and they usually are seen growing in large masses on soil, on tree trunks, or on rocks in shady places in the environment where it is more moist. Here their cells can easily get water when they need it…this is just SPLENDID!