We just learned about nonvascular plants, so it’s time to step into the world of vascular plants.
The first type of these we will examine are called lycophytes, or club mosses. To be clear,
even though they have “moss” in their name, lycophytes are vascular plants,
unlike the nonvascular mosses and other bryophytic plants we just discussed. In fact, lycophytes are
the oldest living type of vascular plant. Fossils of ancient lycophytes have been found dating
back to the Silurian period, around 425 million years ago. At certain points in Earth’s history,
lycophytes were the dominant land plants, with some growing as large as modern trees.
But modern lycophytes are usually small and grow close to the ground in the forest understory,
or as epiphytes, which means small plants that grow on other, larger plants. In either case,
lycophytes hold themselves to their substrate with rhizomes. The above-ground portion of the plant
usually has dichotomous branching, meaning the branches fork in two at each intersection. But
instead of the more complex leaves of more recently-evolved vascular plants,
lycophyte branches bear the small predecessors of leaves, called microphylls.
One major difference between lycophytes and the bryophytes they evolved from
is that with lycophytes, the diploid sporophyte is the dominant generation,
or the generation that we’re most aware of seeing, rather than the haploid gametophyte. The top of
the sporophyte develops into a strobilus, which is a structure that looks kind of like a pinecone.
The strobilus is where meiosis happens and the spores develop. The haploid spores can develop as
part of the sporophyte body, or they can develop separately, either above or below the soil.
There’s quite a bit of variation within lycophytes in terms of how their spores
develop into gametophytes, and there isn’t a lot known about how lycophyte gametophytes develop.
What we do know, however, is quite similar to what we learned for bryophytes,
in that the gametophytes have two types of reproductive heads. Antheridial heads
have multiple antheridia, or places where sperm cells are produced through mitosis,
because the gametophyte is already haploid. Archegonial heads have multiple venters where egg
cells are produced, also through mitosis. When there is sufficient water in the environment,
the sperm cells can swim to the archegonial heads and fertilize the egg cells. Once the
egg cells are fertilized, they germinate into new diploid sporophytes to start the cycle over again.
Interestingly, club moss spores are good for more than just producing more club mosses.
Humans have discovered a wide range of uses for the spores because of their high fat content,
which causes them to be both hydrophobic and flammable. In the early days of photography,
club moss spores or “lycopodium powder” were used to create the flash needed to take pictures.
Lycopodium powder, sometimes referred to as “vegetable sulfur”
has also been used in fireworks and magic tricks because of its explosive properties,
and it has been used as a coating on pills and latex gloves, again due to
its hydrophobic properties. Lycophyte spores are quite fascinating, but there is another group of
vascular plants that reproduces via spores, so let’s move forward and talk about those next.