And God said, Let there be light: and there was light. And God called the light Day, and the darkness he called Night.
Genesis 1: 3, 5, KJV
Flowering plants move through their life cycle smoothly, from seed to plant to flower to seed, but there are other plants (as well as mushrooms) who have a more complicated life cycle; scientists call this the alternation of generations. In this type of life, the creature has not one but two forms; this would be like an oak tree that looked like a sunflower when it was young or a kitten looking like a fish when it grows up. For a moss, fern, or mushroom, this dramatic difference in appearance at various points their live is quite natural, like their life has a morning and a night – different but still part of the same day.
Consider, for example, the mushroom. As you will recall, the main part of this fungi is not the admirable stalk and cap of the mushroom itself but rather the mycellium (and the microscopic hyphae strings that make it up) hidden in the dead log or under the ground where it grows. You could say that the hyphae and the mushroom are the morning and night of this fungus. The mushroom is what we call the sporophyte – the form of the creature that makes spores. Spores, as we have said, are kind of like seeds in that they can spread the mushroom, but they are microscopic and lack the protection and food that seeds are given. (If you want to see spores, you can make a spore print: cut off the stalk of a mushroom and set it under a bowl on a piece of white paper with the gills down. In the morning you will see black markings on the paper where the spores have landed.)
These spores spread in the wind, and those that are lucky enough will land on the right kind of fallen log and will start to grow. The hypha that forms is the part of the life cycle called the gametophyte (and recall that this is the more common appearance of the fungus). The fungus now goes through life, eating dead things, growing, excreting, and looking for a compatible hypha with which it can reproduce. When they meet, they create these rather bizarre cells with two nuclei that grow into a knot of hyphae underground. When the conditions are just right, that knot will finally produce a mushroom and the cycle can begin again.
For moss, morning and night are largely the same as mushrooms, with the soft green gametophyte bed that you see on the sides of trees or along the edges of sidewalks being the more common form of the plant. This greenery makes microscopic gametes – the male sperm and female eggs; when the moss gets wet, the sperm swims to the egg and fertilizes it (that is why moss always grows where it is damp). Then special sporophyte stalks shoot out, like dew drops balanced on the end of a slender blade of grass towering over the moss below. When the spores mature, the sporophyte opens and the moss can spread to new areas.
When it comes to algae, liverworts, and ferns, however, morning and night are seemingly switched. The familiar spindly fern fronds that grace many a forest floor are actually the sporophyte: on the underside of the leaves you might find little tan circles known as spore sacs that open when mature to spread the fern spores. Often, however, the spores fall straight down and you will find the delicate heart-shaped gametophytes that grow from them in the shade of the fern. Like with moss, the sperm that the gametophyte produces swims to the egg when it gets wet; once they meet, they grow into the familiar feathery fronds to continue the life cycle.
What about pine and fir trees – those prickly giants that deck many a hall at Christmas time? These plants do not have flowers, so does that mean that they produce spores? Conifers (those trees that have cones, like pine and fir trees) are actually like most other trees in that they have pollen which is carried by wind from tree to tree to fertilize seeds. These trees have two kinds of cones – male cones that produce the pollen (like the stamen on a flower), and female cones that receive the pollen and grow the seeds (like the flower’s stigma). Plants can have some surprising life cycles that help to give our world wonderful variety.