May 6, 2012 | 5
A fern may seem a simple thing. It’s a leaf; it sprouts from the forest floor. But it’s much more than that. Ferns were one of the first plant forms to evolve, and they retain features that show it.
For instance, unlike mosses and their wacky buddies the liverworts, ferns possess true stems (which usually run along or under the ground as stolons or rhizomes), roots, and leaves (you may think mosses have leaves, but they don’t – not “true” ones that share a common origin with most other leaves). But unlike conifers and flowering plants, ferns do not make seeds or fruit.
Similarly, when green plants evolved in the ocean, their eggs were fertilized by swimming sperm. The first land plants likely did likewise, and so do mosses and ferns. Even today, for a fern to complete its life cycle, a hapless sperm must swim to a waiting egg.
But where on the fern does this happen?
It doesn’t. At least, it doesn’t happen on the plant *you* think of as a fern. It happens on a completely separate plant that is also a fern, but looks nothing like a fern you would recognize. A secret ninja fern. We bio-nerds call it a gametophyte.
Gametophytes make gametes: eggs and sperm. Like the gametes they produce, the plants are haploid, that is, they contain one copy of all their chromosomes instead of two. For this to happen, reductive cell division – meiosis, as you may recall from high school biology – must take place. Our eggs and sperm are likewise haploid, while our bodies are diploid. If we did not make haploid gametes, fertilized eggs would contain double the number of chromosomes we do, with dire genetic consequences*.
In ferns, the gametophyte is a milquetoast little guy – a heart- or kidney shaped green film of cells (not unlike the filmy fern we saw last time) that lacks true leaves, roots, or stems.
It sends some long cells into the soil to serve as root-like objects called “rhizoids” and lives long enough to do its job. The zygote formed when egg and sperm meet then grows into the object we think of as a fern. It usually grows right out of the gametophyte, perhaps ultimately obliterating it in the process.
There is no equivalent to the gametophyte in the animal world; for there to be so, the cells that would otherwise form your eggs and sperm would need to divide into a multicellular organism that, to be analagous to a fern gametophyte, lived independently of you. Also, these alter-humans, whatever they were, would need to be the ones to produce actual eggs and sperm and get the sperm from point A to point B. Then the new human would have to be incubated in and grow out of this other thing. It’s a strange concept, to say the least.
But for plants, this is a fact of life. Believe it or not, all plants have gametophytes. Every plant on Earth undergoes this “Alternation of Generations”, gametophyte alternating with the diploid sporophyte (in the case of ferns, this is the thing you think of as the fern), and so on, but only in the most early-evolving forms do they exist as free-living plants separate from their alter egos.
Where is the gametophyte in pine trees and daffodils? They are hidden – tiny clumps of cells tucked inside pine cones and flowers. In flowering plants, the gametophyte usually doesn’t exceed a handful of cells. But in more “primitive” plants like mosses, the gametophyte is the plant you know and love. It is the sporophyte that is small and easily overlooked. We’ll take a look at a few of these before this series is over.
Yes, gametophytes are weird. But their existence also means that the sporophytes need a way of making little gametophytes. If it’s not eggs and sperm, what is it? The answer is contained in their name: spores.
Spores are cells used for propagation that are formed asexually, by regular cell division, AKA mitosis. The many ways that ferns make, package, and distribute their spores is a thing of great beauty, and a chief reason I find them fascinating.
When you look at the back of a fern, you might think you are looking at a pile of spores
but you would be wrong. Look more carefully. What you are looking at is a pile of sporangia collected in a patch called a sorus (pl. sori). Sporangia are little spore making factories, and they can be stunningly complex and beautiful in ferns. One of the most classic shapes looks a bit like a Roman helmet complete with an “annulus” or crest that functions both to rupture the sporangium and disperse the spores.
When the sporangium is ripe, it breaks, and the crest pulls back on the rupture, releasing the spores. The pattern in which the sporangia and sori are formed or the shapes or configurations of sporangia in sori can be surprisingly beautiful in ferns. We will see that in upcoming posts on the New Zealand ferns.
This is why I was so excited to see them. Though I’d seen pictures and flattened dried specimens when I took botany and taxonomy of vascular plants, there’s nothing like going out into the field, turning over a leaf, and waiting to be delighted and surprised by what’s under there. And on that note, we’ll take a look at a few just such delightful surprises next time.
*Down Syndrome in humans is the result of three copies of Chromosome 21 instead of the usual two whwn one of the pair of chromosomes fails to separate fully during routine meiosis. There must be some sort of “sticky bit” on 21 that has a bad habit of failing to separate after replication the older mothers get A blanket quadrupling of our genomes would end in catastrophe – a zygote or embryo that spontaneously aborts. Interestingly, in plants this doesn’t seem to be nearly the disaster it is in animals. Spontaneous doubling, quadrulpling, etc. of plant genomes has been a major mechanism for generating new species in some plant groups, and for generating new cultivars of flowers and varieties of wheat among plant breeders.