TRANSPORT- GET MOVIN’

Transport is the movement of things from one place to other. It happens all the time. For example, you might transport the stinky bag of trash in your kitchen to the curb for garbage pickup. Or you might be transported from the bus stop to school or work. Transport happens inside our bodies, too. Our heart is connected to a superhighway network of veins and blood vessels that make up our circulatory system, which is responsible for transporting nutrients from the burger you ate throughout your body from your nose to your toes.

TRANSPORT IN PLANTS?

What about transport in plants, how does a Redwood, one of the tallest trees in the world, move water from the soil to the needles on its tallest branches over 300 ft in the air? (That’s over 30 stories high!) Or how does a carrot transport the sugars made in its green, leafy tops below the surface of the soil to grow a sweet, orange taproot? Well, certain types of plants (vascular plants) have a system for transporting water, minerals, and nutrients (food!) throughout their bodies; it’s called the vascular system. Think of it as the plant’s plumbing, which is made up of cells that are stacked on top of one another to form long tubes from the tip of the root to the top of the plant. To learn more about it, let’s study the stem.

STEM OVERVIEW

Ah, the stem, the part of the plant that connects the leaves to the roots! But, not all stems are similar! For example, cactus stems are swollen and store water. Some stems twist and have grasping tendrils like the pea plants growing up a garden trellis or lianas in the tropics.

Other stems are covered in thorns, providing lyrical inspiration for 80s power ballads and making the stem less palatable to herbivores. Stems give a plant structural support so they can grow upright and position their built in solar panels (leaves) towards the sun, but stems are also flexible allowing them to bend in the wind and not snap. Despite the shape or modification, inside every stem of a vascular plant is a bundle of tubes, and this my friends is where transport happens in the plant.

STEM VISUALIZATION

To understand transport in plants, let’s start with a little stem anatomy. Imagine that you’re holding a handful of drinking straws and chopsticks with a rubber band around them.

This bundle is your imaginary plant stem.

The rubber band, the drinking straws, and the chopsticks represent the three types of tissues found in vascular plant stems. The rubber band symbolizes the dermal tissue that covers the outside of the plant stem, and like our skin it acts as a protective layer. Ideally the rubber band would completely cover your makeshift stem bundle, so you’ll just have to use your imagination. The chopsticks fill in the space between the rubber band and the drinking straws and represent what is called ground tissue. Ground tissue is made up of cells that provide structural support to the stem. The drinking straws represent the third tissue type, the vascular tissue. Depending on the type of plant, the drinking straws might be arranged in the stem in a very organized way or scattered throughout haphazardly. Regardless of their arrangement each straw has a job to do; either transport water and minerals or transport sugars.

XYLEM: DRINK UP!

In our example, the straws that transport water and minerals up from the roots to the leaves are called xylem (zy-lem). Now imagine that each straw is actually a certain type of cell stacked one on top of the other creating a tube. Depending on the type of plant, xylem tissue can be made up of one or two different types of cells. Plants like ferns and conifers have xylem “straws” that are made of slender cells called tracheids. At maturity these cells die, leaving behind a rigid cell wall scaffolding tube to conduct water and minerals. Flowering plants have an additional type of xylem tissue called a vessel element. Like tracheids, vessel elements are dead at maturity, but unlike tracheids, vessel elements are much wider – more like a smoothie straw! This means that they can transport more water at a faster rate. Just think of how much faster you can slurp a soda with a wider straw! Just because vessel elements are wider, doesn’t necessarily mean that they’re better. Vessel elements are prone to getting little air bubbles caught in them, and once an air pocket occurs, the party is over and it is very difficult to move water up the stem.

PHLOEM: IT’S ALIVE!

Back to our imaginary plant stem, the remaining straws transport food made in the leaves to the rest of the plant and are called phloem (flo-um). Phloem tissue is also made up of two types of cells that are less rigid and much more lively than their water carrying compatriots (no really, they don’t die at maturity like xylem cells do). One cell type does the heavy phlo-ing, while the other is the wingman. Here’s how it works: sieve tube elements are masters of flow. They stack one on top of the other separated by perforated plates creating the tube-like structure we’re familiar with. Sieve tube elements clear almost everything out of their cells that could slow the flow including organelles and even their nucleus! Anything that’s leftover gets squeezed up against the cell wall like pushing all the chairs to the side of a room so you can break dance in the middle. The sieve tube elements are busy, but they couldn’t do it alone. Directly connected to the sieve tube elements through holes in their cell walls are their faithful buddies the companion cells. These cells have all the necessary cellular machinery to keep themselves and their adjacent sieve tube element alive and kickin’. And while companion cells don’t conduct food along the stem of the plant, they do play an integral role in loading food into and out of the sieve tube elements.

PRIMARY AND SECONDARY GROWTH: IT TAKES TWO TO TANGO

But don’t forget, plant stems can grow in two directions. Our imaginary plant stem helps us to visualize what the inner workings of a soft, green herbaceous stem – similar to what a dandelion stem might look like.

The dandelion stem will grow in length until it’s taller than the grass around it in your lawn – making it an easy target for the lawnmower. We call the increase in stem length primary growth. How does a stem actually get longer? Do the individual cells along the stem just keep getting bigger and bigger? Nope! (But individual cells and their cell walls will elongate to a certain size.) Primary growth originates in the apical meristems or places of rapid cell division, which are located at the top of the growing plant and at the tips of the roots. New cells are made in the apical meristems, so plant length increases by adding these new cells to the end of the stem, just like if you were using wooden blocks to build a tower. Each block you add to the top increases the height of the structure.

But what about stem growth in a tree? How does the trunk of a tree grow to be so much thicker than a dandelion stem? A tree seedling stem will start off green and flexible but over time, the tree will grow larger, become woody, more massive, and will need structural support to keep itself from falling over. The tree does this by increasing the width of the stem, which is called secondary growth.

Stems get wider at two places: the vascular cambium and the cork cambium. The vascular and cork cambium are also places in the stem where cells are dividing rapidly – the difference is where they are located. Cork cambium is a circular band of dividing cells found just beneath the outer covering of the stem. Its job is to make cork, or the outer most layer of bark that you see on trees. The vascular cambium is also a circular band of dividing cells, but it is located deeper into the stem between the two types of vascular tissue we talked about earlier: xylem and the phloem. The vascular cambium is a jack-of-all-trades. Cells in the vascular cambium divide and if the new cells are located toward the outside of the stem they become phloem, and if they are located toward the inside of the stem the cells become xylem. The vascular cambium will continue to divide creating new layers of cells in two different directions on either side of itself, and over time the stem will become thicker.

WATER AND NUTRIENT TRANSPORT: WHAT MOVES YOU?

So now we know what parts of the stem are responsible for transporting water (xylem) and nutrients (phloem), but we don’t know yet how they move or what drives their movement. Keep in mind that one requires energy and one does not.

Let’s start with water. The movement of water in a plant is like a one-way street, it is unidirectional and it travels along this route: soil -> roots -> stem -> leaves -> air. The movement of water throughout a plant is driven by the loss of water through it’s leaves, or transpiration. The water molecules that move through the xylem are connected in a continuous “stream”. They are able to do this because 1) water molecules really like each other (a property called cohesion) and 2) they also like to stick to other substances (a property called adhesion), and these two properties allow water to move up the xylem “straw” we visualized earlier. As water evaporates into the atmosphere from the surface of the leaf, it “tugs” the adjacent water molecules inside the leaf, which “tugs” on the water molecules in the stem, which “tugs” the water molecules from the roots, which “tugs” water molecules into the roots from the soil. So water evaporating from the leaf initiates the “tug” or pull of water through the stem. But, the important thing to remember is that this movement of water is passive, meaning that it doesn’t require any energy to transport water through the plant.

Now let’s move onto the sweet stuff, phloem. The movement of sugars in a plant is much different than the movement of water. First of all, phloem can move both up and down a plant, which comes in handy when a plant needs energy down below to grow new roots, or when a tasty apple is developing on a high branch. The sugars are made in the leaves as a product of photosynthesis. To get the food made in the leaves to other parts of the growing plant requires energy. So, with the help of some water from the xylem, sugars are actively loaded into the phloem where the sugars were made (which is called the source) and actively offload where they are needed (which is called the sink). Ever seen a dumb waiter in an older home? Phloem loading and unloading works sort of the same way. Someone in the kitchen can open the door and put a plate of food inside the mini elevator, then with the help of some energy and a pulley system, the tray of food is taken up the elevator shaft to another floor where someone opens the door and retrieves it. In plants the movement of nutrients through the phloem is driven by where the sugar is most needed for the growth of the plant.

Land plants are amazingly diverse and adaptable, but they owe their leap out of the pond as it were to some hereditary assistance 400 mya from their closest living relatives, green algae. Think for a moment about the transition that land plants underwent, from an ancestral state of growing either partially or completely submerged under water to growing on land, surrounded not by water, but… air! It was a gamble, because it can be a tough, dry, desiccating world out there, but it paid off and plants thrive in incredibly diverse environments all over Earth.

Before we dive into plant diversity, let’s first take a little trip down memory lane of Earth’s evolutionary past. To understand the evolutionary relationships of a group of organisms, biologists construct a phylogeny. It’s kind of like a family tree, except instead of focusing only on a few generations; it zooms way out and looks at relationships over evolutionary time. It’s helpful to know which organisms arrived on the scene early (ancestral types) and which ones arrived later (descendants). Once biologists figure out ancestor/descendent relationships, they build a phylogenetic tree using different types of information such as the structure and shape of the plant (morphologic data) and DNA sequences of certain genes (molecular data). Plants are organized onto the phylogenetic tree based on similarities and differences in these data.

Land plants had to undergo exciting structural and reproductive changes to adapt from an aquatic to terrestrial lifestyle. Considering their vast diversity, all land plants have at least one very important characteristic in common – they undergo a unique life cycle that is referred to as an Alternation of Generations. This means that the life cycle of any land plant has two distinct phases. The amount of time spent in each phase varies depending on the group to which the plant belongs.

The first phase is called the gametophyte generation (phyte=plant, gameto=gamete). In this phase, we are referring to a plant (multicellular, not just a single cell) that has a single set of chromosomes (haploid) that produces gametes (egg and sperm cells) that are also haploid. Here’s the really interesting part, the egg and sperm cells are produced via mitosis, cell division resulting in two cells that are identical to the original cell.

For those of us more familiar with our own lifecycle than that of plants, the “egg and sperm cells produced via mitosis” part of the gametophyte generation description should sound some bells and whistles in your mind as a major difference to the way human produce their own egg and sperm cells. Our gametes are produced through meiosis or the division of a single cell into four cells that have half the genetic information of the original cell.

The second phase is called the sporophyte generation (again phyte=plant, sporo=spore). In this phase, we are referring to a plant (multicellular, not just a single cell) that has two sets of chromosomes (diploid), which is the result of the fusion of two gametes (sperm and egg). In the most basic sense, we’re talking about a plant that produces spores. More specifically, the diploid sporophyte produces haploid spores. How do they do it? Well, there’s only one type of cell division where the overall chromosome number is reduced during division and that’s meiosis. So the sporophyte undergoes meiosis to produce haploid spores.

Now that we understand the nuts and bolts of Alternation of Generations, let’s begin our tour through land plant diversity and see how this lifecycle plays out in each group.

BRYOPHYTES

The first group that emerged after green algae was the nonvascular plants or bryophytes. Remember that the vascular system is the “plumbing” that plants use to transport water, minerals, and nutrients. Vascular tissue is broken up into two types: xylem transports water and minerals, while phloem transports nutrients. Therefore nonvascular plants like mosses, liverworts, and hornworts do not have xylem or phloem, which also means they don’t have true roots, stems, or leaves. Some have rhizoids which might look like little roots, but they serve to anchor the plant. Rhizoids are not absorptive.

Without a well-defined system to transport water throughout their bodies, where would you guess that these types of plants are found? And if you had never seen a bryophyte before, but you knew it had no vascular system, would you suspect the plant to be tall or have a low stature? Bryophytes and mosses are found in moist environments, and they are typically low to the ground, because they lack stems or a vascular system to transport water.

There are several key innovations that our buddies the bryophytes had to undergo to make the evolutionary leap from an aquatic to terrestrial environment. Two structural changes allowed these plants to survive in a dry, terrestrial atmosphere. First, bryophytes developed cuticles, different from the layer of skin at the base of our fingernails and toenails. A plant cuticle is a waxy layer that covers the plant that keeps water in and keeps the plant from drying out. Second, bryophytes developed stomata, which are pores in the cuticle that allow gas exchange. Sure, plants release oxygen, but they also need to take in carbon dioxide for respiration, so pores are critical for gas exchange to take place.

The last key innovation for bryophytes is a reproductive adaptation. Remember, that in bryophytes the gametophyte generation is dominant. Being that this plant is a gamete producing plant, they do it with style. Bryophytes developed gametangia or specialized gamete-forming structures. There are two types: the male, sperm producing structure (antheridium) and the female, egg producing structure (archegonium). Bryophytes, for all the strides they made to adapt to living on land, have a carryover from their aquatic ancestry – they still need water for reproduction. Sperm released from the antheridium will swim to the archegonium to fertilize the egg. The developing zygote actually grows up out of the gametophyte into the spore-producing generation (sporophyte). It’s kinda like lipstick coming out of it’s tube . The diploid sporophyte is totally dependent upon the gametophyte for survival. Lipstick can’t survive without its case.

SEEDLESS VASCULAR

The key innovation for this next group, and for all plants from this point forward in our survey of plant diversity, is vascular tissue: xylem to transport water and minerals, and phloem to transport sugar. Vascular plants have true leaves, stems, and roots. Vascular plants also have a special substance called lignin which is a compound in the cell walls of plants that gives them additional strength and stability. Considering the plumbing and structural advantages vascular plants have, it’s no wonder that they are, in general, much larger plants than bryophytes.

The seedless vascular plants include club mosses, whisk ferns, horsetails, and ferns. Some of the plants in this group still need water for fertilization. Unlike the bryophytes, the sporophyte generation is dominant. The gamete-producing plant, or gametophyte generation is free-living but very small.

SEED PLANTS

The key innovation for the remaining plants is the development of seeds. This may sound simple, but the development of seeds was a major adaptation in the evolution of plants. Seeds are hearty, and most importantly, they can endure dry conditions. The adaptation of the seed meant that plants were free from their dependency on water for reproduction, and consequently they could colonize drier environments.

Here’s a little insight into “the seed”. It’s a pretty remarkable adaptation that wouldn’t be possible without the evolution of a characteristic in seed plants known as heterospory. Basically this means the production of two distinct types of spore producing structures, and therefore two distinct types of spores: microspores and megaspores. The microspores develop into pollen, or the male gametophyte. The megaspore develops into the egg, or the female gametophyte. The egg and sperm fuse to form a zygote that develops into an embryo, which is protected inside several layers and wrapped in a protective coat. The whole package is the seed!

We group the seed plants into two major groups: gymnosperms and angiosperms. The exciting aspect of these two groups is that the sporophyte is dominant and the gametophyte is so reduced that it is dependent upon the sporophyte for survival. The sperm and egg develop within the sporophyte, and the female gametophyte is retained within the tissues of the sporophyte. There are three main types of gymnosperms: cycads, ginko, and conifers. Gymnosperms all rely on wind for pollination. Wind can be a finicky thing, and not always the most reliable, especially when something as important as ensuring viable offspring (fertilization and the production of seeds) is at stake! Our next group really upped their pollination strategy with a couple fabulous innovations.

ANGIOSPERMS

Angiosperms are an incredibly successful group, radiating all over the globe. There are two secrets to angiosperm success: flowers and fruit. Aside from attracting people (floral industry rakes in billions of dollars a year!) flowers also serve the role of attracting pollinators. This is an advantage over gymnosperms that rely upon a gust of wind to transport their pollen!

Another key innovation of angiosperms is fruit. Fruits are sweet, delicious, and hidden inside of them is all the genetic material for the next generation of plant – the seed! It was very clever marketing to package such precious cargo in an outer covering that is soft, fleshy, sweet, and nearly irresistible to animals.

So as you can see, plants had to undergo a bunch of structural and reproductive changes to adapt from an aquatic to terrestrial lifestyle. But keep in mind the overarching trend in plant diversity – a shift from gametophyte dominant plants in the byrophytes, to an evolutionary intermediary step where the sporophyte dominant generation is dominant and the gameophyte generation is independent, to seeds plants where the gametophyte generation is so reduced that it is dependent upon the sporophyte for survival.