Lycophytes: Early Vascular Plants

 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.  Archegonal 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, then  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  knowns 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.