Wednesday, May 28, 2008

Scrub Oak Part 2: How Angiosperms Work

So this post is going to be complicated. But I’ve done my darnedest to make it as clear and straightforward as possible, with simple pictures, no (well few) big words, and almost no terms we haven’t seen before. Anyone with a 6th-grade or better education can make it through this post, and should.

Preamble – Why This is Really Worth Understanding

So I think one of the things that semi-smart people struggle with on an ongoing basis is whether or not to invest the effort in understanding how something really works. In other words, there are lots of fairly complicated things in the world around us: automatic transmissions, refrigerators, hedge funds, REITs and oil refineries. If our lives and/or careers are closely tied to one of these things – like we’re a hedge fund manager or a refrigerator repair-person- then we probably need to know exactly how that specific complicated thing works, but if not, then we can just get along fine without understanding it. Still, we may feel kind of lame if we don’t understand why our car doesn’t work or how our beer stays cold or what’s going on with our investments. So we constantly debate ourselves as to whether it’s worth making the effort to really understand something that isn’t directly tied to our trade, and predictably, most of us understand relatively little of how the world works.

Tangent: The huge glaring exception to this observation in my life is my father. The guy understands all of the above examples I listed, how they work and how to go about doing or designing them. Plus a whole bunch of other things, like how to design bridges, invest in real estate, and find and smelt iron ore. And if that weren’t enough, the guy is an amazing artist.(below)

Like most people, and- sadly- unlike my father, I all too often don’t take the time to really understand how stuff works. It’s not that I’m stupid; I’m just busy, distracted, and like most people, a bit lazy.

But as I started learning about, and becoming more interested in plants, I came to realize that How Angiosperms Work is really worth understanding, and the best analogy I can give you as to why it’s worth understanding is the transistor.

One of the most dramatic changes in the world over the past couple of generations that has affected all of us has been the explosion of high technology. From ultra-reliable cheap watches to computers to the Web to cell phones to iPods to televisions to fuel-injection to coffee-makers, technology touches all of us. Now it’s fairly unreasonable to expect an average joe to understand how all of these things actually work, but there’s one fundamental building block, one common root, one thread, upon which all of these technologies and so many others are based: the transistor. If you understand how a transistor works, you understand the basic functional mechanism of all of these technologies.

Tangent and Disclaimer: My college degree is in Electrical Engineering. At one time, more than 20 years ago, I actually understood exactly how a transistor worked down to the sub-atomic level. Though I’ve long since forgotten, never use my degree, and am today- ironically- almost embarrassingly inept when it comes to anything electrical or electronic, to this day I derive a deep satisfaction from having understood how a transistor worked at least for a while, and knowing that there really is a science and reason and a logic behind how all of these things work, and that it isn’t all just voodoo black magic or some weird, wild dream from which I can’t awake.

If a transistor is worth understanding, then Angiosperms are at least 10 times more worth understanding. Because Angiosperms affect us, and have affected every human who ever existed, so much more than transistors. Almost everything we eat- whether directly or via animal feed- is an Angiosperm. With over 300,000 species, Angiosperms comprise the vast majority of the world’s plant biomass, and account for nearly all of the oxygen we breathe. By any objective measure, the Earth is ruled by Angiosperms, and we’re just along for the ride.

Given their significance in our lives and the world, it’s fascinating how complicated and weird Angiosperm reproduction is. In fact it’s so complex, elegant and strange that it’s generally thought to have evolved but once, most likely around 140 – 150 million years ago. All Angiosperms, including so many of the plants we’ve looked at- Willows, Cottonwoods, Maples, Glacier Lilies, Milkvetch, Dandelions, Palm trees, Crabgrass, Bearclaw Poppies, Dyers Woad, Musk Thistle and Joshua Trees- use the same mechanism. And here in Northern Utah and the end of the May, there’s no better plant to illustrate it than Gambel Oak.

How Angiosperms Work

We talked a while ago about the basic parts and structure of males and female flowers. Oaks are monoecious, with both male and female flowers and they’re wind pollinated, which means that Oak catkins disperse gazillions of grains of pollen in hopes that some small number will wind up on the stigma of a female Oak flower.

A female Oak flower has 3 separate pistils, each topped by a sticky stigma. Each pistil leads down to 2 chambers, each of which contains 2 ovules. Now strangely, only one ovule in one chamber ever gets fertilized. The other 5 ovules and 2 chambers are aborted, usually during the process of pollination.

Prior to actual pollination, each ovule contains a megaspore, which is a fancy word for a “starter” cell that divides a bunch of times. The megaspore undergoes a complex development of its own via meiosis. Each megaspore divides via meiosis into 2 cells, each of which in turn divides again- via mitosis*- resulting in 4 cells, 3 of which are spontaneously aborted. The 4th cell becomes the Embryo Sac Nucleus, and undertakes a mitotic process of its own, dividing twice, resulting in 8 haploid cells.

Side Note: My earlier posts on Dandelion genetics and polyploidy in Sagebrush explained the terms: haploid, diploid and triploid. In the case of Oak, the diploid number is 24, the haploid number is 12.

Of the 8 haploid cells, 5 are spontaneously aborted, 1 becomes the egg, and 2 become polar nuclei, whose function we’ll see in a moment.

Tangent: The obvious question in all this is: why all the waste? Why the extra chambers and ovules and potential embryo sacs and nuclei and eggs? And how is it determined which cells are aborted and which maintained? The answer is we just don’t know; this is just what evolution came up with. It may not be tidy or designer-perfect, but it works, and works great.

When the pollen grain lands on a stigma of the same species (or same-enough species), the sticky pad of the stigma holds it in place.

A pollen grain contains two (diploid) cells, each of which performs a specific function. The first cell starts growing a tube downward through, and along the length of, the pistil, toward the ovule chamber, and the three haploid cells (1 egg and 2 polar nuclei) within.

The second cell divides meiotically into 2 sperm cells, which travel down the tube- or more accurately, are carried downward by the burrowing tip of the pollen tube, since the sperm can’t move/swim on their own- toward the ovule chamber.

The downward growth and penetration of the pollen tube can take days, weeks or months, depending on the specific Angiosperm in question. When it reaches the ovule chamber, it delivers one sperm to the egg, which unite to form the diploid embryo. The other sperm is delivered to the 2 polar nuclei, which combine to create a separate triploid (36 chromosomes) embryo. The triploid embryo develops into what is called the endosperm, which surrounds and feeds the diploid seed embryo. It is the food for the seed, and in many of the plants that we eat – wheat, corn, coconuts- it’s primarily endosperm that we’re consuming.

And that’s Double Fertilization. There’s still a lot that has to happen before another tree or shrub or weed is successfully produced, but we now have a fully fertilized seed with its own self-contained food source, ready to be carried, blown, flown or dropped to a location where it might just possibly sprout and take root. It’s what happens in every Angiosperm, and as weird and complicated and wasteful as it may seem, every oak tree, every acorn, every wheat stalk, every clump of grass, and every fluffy Cottonwood seed was produced exactly the same way. From the standpoint of a living thing, it is the mechanism that makes the world work. This, right here, is the Beauty of the World.

There are a zillion things I’ll never understand or figure out. But I’m glad I lived long enough to understand this.


SLW said...

Just awesome! My admiration is unbounded... you should be teaching this stuff! (well, you are...) I appreciate (oops, I mean, well, sorry, I mean appreciate) the refresher, having forgotten all this before you were... nevermind!

Watcher said...

Thanks Sally. I always felt that this one was my best, and most worthwhile post. I'm so glad someone finally read it!

WheelDancer said...

I also came to understand the lowest level of transistor theory in what now seems like a past life and this is way cooler.

Enel said...

" Each megaspore divides twice via meiosis, resulting in four cells"

I think the first division is meiosis, then subsequent divisions are mitotic otherwise the resulting cells would be quarter or eighth ploid.

My biology is rusty in this arena, so feel free to correct me.

Otherwise, great post.

Watcher said...

Enel- You are correct and thanks for the catch! Finally got around to cleaning this one up (text & graphic).