Back in the old forum days, I used to write on specific organisms frequently. Now that I’m doing Botany, I think this little spot would be missing a lot if I didn’t give some spotlight to the greater picture, especially in regard to groups of organisms most of us take for granted, such as plants.
This last week brought us undergrads face-to-face , for the first time, with real hardcore terrestrial plants, and the first such plants were a group of organisms called Ferns.
Even though I’m alt-tabbing the wiki article for fact verification (and digging up fun facts as well), I can, sans wiki, sum up what are the interesting differences between Ferns and all the other plant taxa we’ve learnt of so far.
Ferns are similar to mosses in some respects, and like mosses and all evolutionary descendants of mosses, they’re embryonic plants, with distinct sporophytic stages that develops from a protected embryo that is grown and shielded within the parent fern.
Ferns actually have independent sporophytic stages, which is a bit odd. Flowering plants don’t have that, and neither do mosses (which can be very roughly considered the evolutionary “befores and afters” of Ferns). In mosses, the sporophyte is, if not completely “parasitic” on top of the gametophyte, is still an attached (above-ground) outgrowth of it.
In flowering plants, the gametophyte is situated atop the sporophyte, which is the reverse for mosses. I won’t get any deeper into that, since I haven’t studied about them yet 🙂
Ferns are distinguished in the plant kingdom as the first truly Vascular Plants. It’s not that more primitive plants don’t have some means of relaying organic material and water around the body of the plant, but in Ferns, we witness the first instance of complex, all-body vascular organs, namely, the Xylem and the Phloem. The X and P are just fancy words for “tube for shifting organic compounds” and “tube for shifting water”, respectively. As the first hardcore terrestrial plants, vascular organs are a must-have adaptation. Growing taller is a logistic nightmare, but with the enormous selection pressure on short plants that compete on the same sunlight, it’s a must. It’s a good evolutionary explanation for why those Ferns went through all the trouble, and this is actually a distinguishing feature in Ferns: they’re specialists. Their penchant for being taller is just the tip of the iceberg (they’re also adapted to hostile habitats, habitats which constrain the flowering plants but not Ferns).
The most revealing innovation in Ferns is the organ that most of us seem to readily associate with plants: Leaves.
To begin with, I was simply delighted to finally understand what this organ actually is. Up until next week, leaves to me, as they are to most laymen, were simply “green bits on them flowers and whatnot”. There’s more to that, or merely, a more accurate description. Leaves are firstly defined as the photosynthetic organs. In short, what the mouth does for heterotrophs like us, the leaves do for autotrophs like plants. In short, it’s the plant’s way of getting chow. Up until now, photosynthesis wasn’t confined to specialized organs, and hence, leaves are truly a hallmark of evolutionary innovation.
As an aside, it’s interesting to note that evolutionary innovations are often a precursor to two things:
A.Enormous comparative fitness (evolutionarily-speaking, as opposed to simpler organisms)
B.An evolutionary dead-end. Jacks-of-all-trades have more “promotion possibilities” than “Masters-of-one-trade”. This is why bacteria outlived many metazoa (and will probably outlast us!)
Since I’m an evolution afficionado, I want to have the finishing part of this post to focus on some interesting evolutionary tale, but I think I can combine that with some cool info on Ferns in general. What I mean by that is that you can actually see for yourself the evolutionary “nodes” in Fern evolution by observing the various stages of leaf evolution.
Like Is said, leaves are the photosynthetic organs of plants, but leaves haven’t sprouted de novo out of ancient moss-like thalluses (even though even weeds have leaflike apparatuses).
The first instance of leaves comes in the shape of protophylls (ancient leaves). Protophylls are nothing but dandruff like scales without any actual vascular tubes for carrying the photosynthetic products to the body of the plant. Since the protophylls are usually small and aggregate, this is not a big problem, and obviously this is an ample condition for evolutionary advance: now that we have the specialization in order, all we have to do is grow some tubes. 🙂
Psilotum - a protophyllic fern
The second and third stages of leaf evolution are very similar: Microphylls and Macrophylls. The noted difference between the two is that microphylls have only one artery-like tube and macrophylls have a branching like web of vascular tubes. It’s quite easy to imagine how one evolved to the other, but not so easy to come up with how protophylls evolved into either, or should I say, to one and then the other. 🙂
Lycopodium - a microphyllic fern
So, yet again, we come across an oft-taken-for-granted plant group and find that it tells us fascinating evolutionary stories. Mainly, that those cheeky bastards are opportunistic little buggers that probably gave us the precursors for modern plants, meaning that Shakespeare and other like-minded cupid-heads should give them some credit. The true journey to dry land starts with Ferns, and so the true evolution for the plants we hold as familiar starts with them.