Hello, Internet! It’s been an awfully long time, hasn’t it? It sure feels like it, to me. I’ve gotten my first physical chemistry and my last biochemistry ([sobs]) under my belt since we met last, and I’m back with a vengeance to take down the last of my undergraduate coursework. Or, you know, to be taken down by my coursework, kicking and screaming.
(I’m not kidding. With the way Physical Chem II is going, it feels an awful lot like it’s going to be mostly screaming. And crying. Lots of crying.)
I’m taking a break from feeling hopelessly, hopelessly lost to study something makes me feel hopeful—plant biology! (Yes, you read that right. I’m taking plant biology, much to my roommate’s exasperation, because I’m not planning on being a doctor, and plants are freakin’ cool, okay?)
Alright! Now that all the formalities are out of the way, let’s get down and dirty (hah) with a lil’ post on the growth of plants.
Doesn’t that sound incredible?
Plants are, in all honesty, really, really weird. I often jokingly tell Sapphire, when she accuses me of treason for taking this class, that plants are like aliens among us, and I have every intention of being on the right side of the next alien uprising. I’ll stand by that statement here. Plants are awesome, and I welcome our tree overlords with open arms.
One of the things that seems to captivate the human imagination when it comes to plants is the way that they just… grow in the way that they do. You stick this tiny, shrivelled, potentially old, maybe dethawed seed in the dirt, and you’ll soon have a little green plant poking out of it. The stem will always grow up, the roots will always grow down, and, provided that your selected spot has enough water, light and nitrogen, the plant as a whole will be perfectly happy there for the rest of its life.
“So,” you ask, “how does a plant do its thing?”
“I mean,” you continue, “I kind of already know, but I’m assuming you have more to say on the topic, and I’m already here, so I might as well listen.”
(I apologize if I do a lot of writing in the second person—I’ve been binge-listening to Welcome to Night Vale.)
Well, if you’re going to talk about growth in an organism, the first thing you have to do is talk about growth on the smallest scale—the cellular scale. In order for anything to grow, the fundamental units from which it is composed (cells, in case you missed the memo) must replicate themselves. You’ll already be familiar with the term for cell division—yup, we’re talking about mitosis!
As you probably already know, the general gist of mitosis is that a cell duplicates its genome, distributes that information into two new nuclei, and then splits itself apart (a technically separate process called “cytokinesis”). Plant cells undergo mitosis this way, too, but plant cells are also… special.
You see, plant cells have some organelles that animal cells don’t, the most prominent of which (other than, you know, chloroplasts) is the central vacuole. This is an organelle that modulates the internal pressure of plant cells, and it’s also the dumping site for enzymes and anything the cell deems dangerous. (You like onions, right? Because you’re weird? The compound that makes you cry when you cut onions is syn-Propanethial-S-oxide, and it’s made from enzymes that are released when you bust the central vacuoles of onion cells. Serves you right. Hah.)
The central vacuole is great, but not when you’re a humble nucleus tending to cell division. That’s where the phragmosome comes in. Strands of cytoplasm (aptly called “cytoplasmic strands”) slice the vacuole into smaller pieces, and actin filaments drag the nucleus to the center of the cell. This turns into a sheet of cytoplasm that marks the plane of cell division. During cytokinesis, the phragmosome turns into a phragmoplast, which “serves as a scaffold for cell plate assembly.” The cell plate, our final plant-specific structure, is where the plant cell’s rigid cell wall forms.
It’s also worth noting, while we’re at it, that plant cells are capable of forming secondary cell walls, which essentially involves adding thickness to the inside of their primary cell walls in three layers. These become so thick that, usually, the cell traps itself, and it dies. (Don’t worry—this is rather the point.)
Okay, so, now that we’ve talked about cell growth, we can talk about mature plant growth, right? Wrong, dearest reader. There’s a very important link between cell division and a mature plant. What do we call that?
Yup. We gon’ talk about plant embryology.
(“Embryology?” Yep. What’d you think was in that seed?)
Shh, calm down, it’s not as hard as it sounds. I know everyone loathes plant reproduction, but it’s okay! We’re going to start with a zygote, so we don’t have to deal with double fertilization. Are you chilled out now…?
Okay, good. So, if you want to make an adult plant, you have to start from a baby plant. If you want a baby plant, you have to start with a zygote. This is not so unfamiliar—animals, including humans, do this mess too. What’s different is the stage at which polarity (a difference in ends of the embryo). In humans, you can split cells off of an embryo until the blastula stage, and you’re good to go. (That’s how you get identical twins.) In plants, we’re not so lucky—the different ends of the embryo are different within minutes of fertilization.
The first time the cell divides, it produces two inequivalent daughter cells: a terminal cell and a basal cell. The basal cell develops into the suspensor, which anchors the embryo and feeds it nutrients from the endosperm. (This is not an embryonic root. However, the top part of it, called the “hypophysis,” does become part of the root cap.) The terminal cell develops into the embryo.
As the embryo develops, it passes through several “adorably-named” stages. A globular embryo already has layers of cells with different fates. A heart-stage embryo (and yes, it’s heart-shaped) has bilateral symmetry. A torpedo-stage embryo has cotelydons (two, if it’s a dictot) and all the right meristems in all the right places.
Okay! Now that we’ve taken a detour to talk about embryogenesis, we can talk about plant growth. Don’t get too hype, though—it’s not as weird as it might seem.
Basically, there are two kinds of growth in plants—primary and secondary. Primary growth is growth “up”—length in the stems and roots, branching, leaf-, flower-, and fruit-production. Secondary growth is growth “out”—think of trees getting wider by the year. (You may have heard this in the form of a joke from a biology teacher, as in, “I no longer undergo primary growth, but I sure do undergo secondary growth!”)
The hotspots for plant growth are the meristems, and they come in two varieties (unless you’re grass). Apical meristems contribute to primary growth, forming protoderm (pre-dermal tissue), ground meristems (pre-ground tissue) and procambium (pre-primary-vascular tissue). Lateral meristems, on the other hand, contribute to secondary growth, in the form of vascular meristems (pre-secondary-vascular tissue) and cork cambium (pre-periderm).
(If you’re a grass, you have intercalary meristems, which are found at the internodes of grasses.)
Each plant has two apical meristems: a shoot apical meristem, and a root apical meristem. The shoot apical meristem is a tiny, sensitive part of the plant, and at its center is a central zone, which is essentially a pool of indefinitely undifferentiated cells undergoing division. Some of the cells produced here get pushed out into the peripheral zone to become lateral organs, or into the rib meristem, to form stem. Further down, vascular tissue also develops.
The same is true for roots (division at the tip, elongation further down, and, finally, cell differentiation), but the root meristem also gets a bit of extra technology. You see, while the shoot apical meristem is up at the top of the plant, shielded from the world by leaves and living the cushy life, the root meristem (the most sensitive part, mind you) is literally driving itself head-first into the dirt. To keep the root meristem from smashing itself to smithereens, the plant equips it with a root cap, which protects it and helps it push through the dirt.
That’s all fine and dandy, but what about secondary growth? What about the lateral meristems? (“I NEED ANSWERS,” you might be growling.)
Well, the most important example of secondary growth is in the formation of secondary xylem and phloem. You might think you’re unfamiliar with these things (because I haven’t written the tissues post yet!), but you’re, like, so totally not. What happens when you add thickness to, say, one of our tree overlords? Yup. You get wood.
Secondary xylem accumulates on the interior of a plant, forming in a ring every year that the plant grows. (Xylem is deader than bread, and is basically a network of tubes that pipes water up from the roots.) This is what forms wood, with its rings that can be used to determine a hecka ton about the life of a tree. Phloem, in the meantime, accumulates on the outside, and only the newest of it is useful. (It’s still alive, and it shuttles sugar and water up and down. Where do you think we get sap?)
Whale, that’s it for plant growth! It’s not that hard, right? (“No, June! It’s really easy, and also I’m glad you’re alive!” Aww, thanks, reader.) Now we can move on to more specific things—things like, you know. Leaves.
Wanna fangirl with me, since my roommate clearly doesn’t? Hit me up, fam.