While volunteering a few weeks ago in the CNPS co-sponsored San Luis Creek restoration project with the City of San Luis Obispo, several of us were removing weeds and planting natives along both sides of the creek in front of the Old Mission Church. This project is now entering its third year with nearly 200 native plants placed in this scenic landscape.
While working on the creek bank a few weeks ago, I noticed a number of shoots of giant horsetail, Equisetum telmateia, emerging on the east facing slope (see photo). When I brushed these odd-looking shoots, a yellow-white powder flew out of the cone-shaped apex, as if pollen grains were being released in search of a mate. I thought to myself, “Where is the female target on this plant – where is the pollen going? Perhaps, like a cattail, the two sexual parts are located one above the other, on the same shoot?” These structures appeared to me as very strange, nothing like a rush or sedge that often grows near water, so I decided to investigate the subject later that day.
It turns out that, similar to the ferns, horsetails have two distinct plant morphologies: a diploid (2n) sporophyte plant and a haploid (1n) gametophyte plant (see diagram). And the nonphotosynthetic, cone shaped, spore-bearing structure atop the long stem of the fertile shoot (called a strobilus) is a different structure than the familiar green, reed-like structure of the vegetative shoot, which we see often in wet areas. These two stems make up the two components of the sporophyte, the fertile shoot emerging in the spring and the vegetative shoot showing up later in the year.
The gametophyte plant on the other hand – just like with the ferns – is rarely seen due to its small size. It hides under leaf litter and does its work by releasing two different gametes, female egg and male sperm cells. They eventually find each other, create a union called a zygote, and then proceed to grow into plants like the one I saw along the creek. Thus, it was the sporophyte half of the life cycle I saw that day, and the so-called pollen being released were the haploid spores looking for a new home in soil to grow as a gametophyte. Case solved!!
It is interesting that Equisetum is called a “living fossil” and has been noted in the prehistoric record as far back as the late Paleozoic era, up to 500 million years ago. Their decomposed bodies are an abundant component in coal deposits, now used to partly power the grid. Back in the day, a now extinct species of this plant family often grew to over 100 feet tall. We think of these ancient plants that still exist on earth as primitive and/or simple. Well, I’d say the way this species maneuvers through life with its different plant forms may be primitive, but it is anything but simple.