As far as I know, roots of plants grow to the direction of Earth's gravity - this is called gravitropism.
But what happens if plants are in space? Are they able to perceive gravity in state of weightlessness? Either way, where do they grow their roots in those circumstances?
Note: this question may fit better either to Space.SE or to Astronomy.SE, but I was not sure about it.
Astronomers on the International space station have done exactly that and grown Arabidopsis Thaliana in space. You can read the whole article here. Plant roots apparently grow away from the seed exactly like on earth seeking out nutrients.
Roots are negatively phototropic in addition to being positively gravitropic. Both growth patterns are mediated by the same hormone, auxin. Removing one factor or the other will not prevent roots from growing "down."
Plants have specialized organs that help them survive and reproduce in a great diversity of habitats. Major organs of most plants include roots, stems, and leaves.
Roots are important organs in all vascular plants. Most vascular plants have two types of roots: primary roots that grow downward and secondary roots that branch out to the side. Together, all the roots of a plant make up a root system.
There are two basic types of root systems in plants: taproot systems and fibrous rootsystems. Both are illustrated in Figure below.
- Taproot systems feature a single, thick primary root, called the taproot, with smaller secondary roots growing out from the sides. The taproot may penetrate as many as 60 meters (almost 200 feet) below the ground surface. It can plumb very deep water sources and store a lot of food to help the plant survive drought and other environmental extremes. The taproot also anchors the plant very securely in the ground.
- Fibrous root systems have many small branching roots, called fibrous roots, but no large primary root. The huge number of threadlike roots increases the surface area for absorption of water and minerals, but fibrous roots anchor the plant less securely.
Dandelions have taproot systems grasses have fibrous root systems.
Root Structures and Functions
As shown in Figure below, the tip of a root is called the root cap. It consists of specialized cells that help regulate primary growth of the root at the tip. Above the root cap is primary meristem, where growth in length occurs.
A root is a complex organ consisting of several types of tissue. What is the function of each tissue type?
Above the meristem, the rest of the root is covered with a single layer of epidermal cells. These cells may have root hairs that increase the surface area for the absorption of water and minerals from the soil. Beneath the epidermis is ground tissue, which may be filled with stored starch. Bundles of vascular tissues form the center of the root. Waxy layers waterproof the vascular tissues so they don&rsquot leak, making them more efficient at carrying fluids. Secondary meristem is located within and around the vascular tissues. This is where growth in thickness occurs.
The structure of roots helps them perform their primary functions. What do roots do? They have three major jobs: absorbing water and minerals, anchoring and supporting the plant, and storing food.
- Absorbing water and minerals: Thin-walled epidermal cells and root hairs are well suited to absorb water and dissolved minerals from the soil. The roots of many plants also have a mycorrhizal relationship with fungi for greater absorption.
- Anchoring and supporting the plant: Root systems help anchor plants to the ground, allowing plants to grow tall without toppling over. A tough covering may replace the epidermis in older roots, making them ropelike and even stronger. As shown in Figurebelow, some roots have unusual specializations for anchoring plants.
- Storing food: In many plants, ground tissues in roots store food produced by the leaves during photosynthesis. The bloodroot shown in Figurebelow stores food in its roots over the winter.
Mangrove roots are like stilts, allowing mangrove trees to rise high above the water. The trunk and leaves are above water even at high tide. A bloodroot plant uses food stored over the winter to grow flowers in the early spring.
Roots have primary and secondary meristems for growth in length and width. As roots grow longer, they always grow down into the ground. Even if you turn a plant upside down, its roots will try to grow downward. How do roots &ldquoknow&rdquo which way to grow? How can they tell down from up? Specialized cells in root caps are able to detect gravity. The cells direct meristem in the tips of roots to grow downward toward the center of Earth. This is generally adaptive for land plants. Can you explain why?
As roots grow thicker, they can&rsquot absorb water and minerals as well. However, they may be even better at transporting fluids, anchoring the plant, and storing food (see Figure below).
Secondary growth of sweet potato roots provides more space to store food. Roots store sugar from photosynthesis as starch. What other starchy roots do people eat?
Geotropism is the influence of gravity on plant growth or movement. Simply put, this means that roots grow down and stems grow up. Geotropism comes from two words, “geo” which means earth or ground and “tropism” which means a plant movement triggered by a stimulus. In this case, the stimulus is gravity. Upward growth of plant parts, against gravity, is called negative geotropism, and downward growth of roots is called positive geotropism.
What makes geotropism happen?
In plant roots, the very end of the root is called the root cap. It makes the roots turn downward as they grow. The root cap is vital for geotropism since it contains cells with sensors called statoliths. Statoliths are specialized parts of the root cell that settle to the lowest part of the root cap in response to the pull of gravity. This makes the cell expand faster in a downward direction.
A similar mechanism is known to occur in plant stems except that the stem cells are programed to elongate upward, the exact opposite of the cells in the roots.
This upward and downward growth will continue even if the plant is turned sideways or upside down. In other words, no matter what you do to a plant within Earth&aposs atmosphere, it will still grow roots down, stem up. The reason for this comes from the nature of a plant, and it&aposs general response to gravity.
Another example of geotropism is the movement of nutrients. minerals and water in a plant. This transport is accomplished by specialized parts of the plant, the xylem (pronounced zylem) and the phloem (pronounced flowem) are the straw like parts of a plant’s stem that move the stuff up and down.
The xylem moves the water and nutrients from the roots to the branches, stems and leaves of the plant. The phloem moves the sugary sap from the leaves to the roots.
An easy way to remember what moves things up or down is to remember what the old native American said - “River flow𠆞m downstream.” Phloem moves things 𠇍ownstream,” too.
Growth in plants occurs as the stems and roots lengthen. Some plants, especially those that are woody, also increase in thickness during their life span. The increase in length of the shoot and the root is referred to as primary growth, and is the result of cell division in the shoot apical meristem. Secondary growth is characterized by an increase in thickness or girth of the plant, and is caused by cell division in the lateral meristem. Figure 4 shows the areas of primary and secondary growth in a plant. Herbaceous plants mostly undergo primary growth, with hardly any secondary growth or increase in thickness. Secondary growth or “wood” is noticeable in woody plants it occurs in some dicots, but occurs very rarely in monocots.
Figure 4. In woody plants, primary growth is followed by secondary growth, which allows the plant stem to increase in thickness or girth. Secondary vascular tissue is added as the plant grows, as well as a cork layer. The bark of a tree extends from the vascular cambium to the epidermis.
Some plant parts, such as stems and roots, continue to grow throughout a plant’s life: a phenomenon called indeterminate growth. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases when a plant part reaches a particular size.
Most primary growth occurs at the apices, or tips, of stems and roots. Primary growth is a result of rapidly dividing cells in the apical meristems at the shoot tip and root tip. Subsequent cell elongation also contributes to primary growth. The growth of shoots and roots during primary growth enables plants to continuously seek water (roots) or sunlight (shoots).
The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.
The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems, which are lacking in herbaceous plants. Lateral meristems include the vascular cambium and, in woody plants, the cork cambium (see Figure 4).
Figure 5. Lenticels on the bark of this cherry tree enable the woody stem to exchange gases with the surrounding atmosphere. (credit: Roger Griffith)
The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem (tracheids and vessel elements) to the inside, and secondary phloem (sieve elements and companion cells) to the outside. The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength.
In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells (bark) containing a waxy substance known as suberin that can repel water. The bark protects the plant against physical damage and helps reduce water loss. The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm. The periderm substitutes for the epidermis in mature plants. In some plants, the periderm has many openings, known as lenticels, which allow the interior cells to exchange gases with the outside atmosphere (Figure 5). This supplies oxygen to the living and metabolically active cells of the cortex, xylem and phloem.
Figure 6. The rate of wood growth increases in summer and decreases in winter, producing a characteristic ring for each year of growth. Seasonal changes in weather patterns can also affect the growth rate—note how the rings vary in thickness. (credit: Adrian Pingstone)
The activity of the vascular cambium gives rise to annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter and their primary cell walls are not extensively thickened. This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids. It results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem (Figure 6). An examination of the number of annual rings and their nature (such as their size and cell wall thickness) can reveal the age of the tree and the prevailing climatic conditions during each season.
RESPONSES OF ROOTS TO SIMULATED WEIGHTLESSNESS ON THE FAST-ROTATING CLINOSTAT
Sedimentable cell particles are distributed randomly along the horizontal axis of the fast-rotating clinostat. They neither sediment in the direction of gravity, nor in the direction of the centrifugal force, nor in the direction of the resultant force of both. The effect of this“weightlessness”and that of very small centrifugal forces on the perception of mass acceleration was examined using young primary roots of Lepidium sativum L. (Cruciferae) during their early development on the fast-rotating clinostat. The results of the experiments show: 1) there is no response of the roots in the direction of gravity, 2) at small centrifugal forces (< 2.2 × 10 −2 g) a curvature response occurs in the direction of the stimulus, 3) the threshold value for the perception of mass acceleration lies at 4.3 × 10 −3 g, and 4) below the threshold value the existence of an autonomous root curvature has been proved for the first time, which is not caused by mass acceleration.
Deciduous plants such as Forsythia respond to the lack of light and warmth in winter by entering a resting period. In preparation, the plant produces chemicals that weaken the leaf stalks, so the leaves fall. Over winter, the plant does not need to make food. Its shoots and buds are inactive. When spring comes, the plant produces chemicals that make buds and shoots start to grow again.
Some plant parts respond to contact. Climbers, such as pea plants and this passion flower, put out long, reaching shoots called tendrils. When a tendril reaches something solid ? such as a garden cane or the stem of another plant ? it coils around it. By grasping at supports in this way, the plant is able to climb even higher.
Evolution of Plants
As shown in Figure below, plants are thought to have evolved from an aquatic green alga protist. Later, they evolved important adaptations for land, including vascular tissues, seeds, and flowers. Each of these major adaptations made plants better suited for life on dry land and much more successful.
From a simple, green alga ancestor that lived in the water, plants eventually evolved several major adaptations for life on land.
The Earliest Plants
The earliest plants were probably similar to the stonewort, an aquatic algae pictured inFigure below. Unlike most modern plants, stoneworts have stalks rather than stiff stems, and they have hair-like structures called rhizoids instead of roots. On the other hand, stoneworts have distinct male and female reproductive structures, which is a plant characteristic. For fertilization to occur, sperm need at least a thin film of moisture to swim to eggs. In all these ways, the first plants may have resembled stoneworts.
Modern stoneworts may be similar to the earliest plants. Shown is a field of modern stoneworts (right), and an example from the Charophyta, a division of green algae that includes the closest relatives of the earliest plants (left).
Life on Land
By the time the earliest plants evolved, animals were already the dominant organisms in the ocean. Plants were also constrained to the upper layer of water that received enough sunlight for photosynthesis. Therefore, plants never became dominant marine organisms. But when plants moved onto land, everything was wide open. Why was the land devoid of other life? Without plants growing on land, there was nothing for other organisms to feed on. Land could not be colonized by other organisms until land plants became established.
Plants may have colonized the land as early as 700 million years ago. The oldest fossils of land plants date back about 470 million years. The first land plants probably resembled modern plants called liverworts, like the one shown in Figure below.
The first land plants may have been similar to liverworts like this one.
Colonization of the land was a huge step in plant evolution. Until then, virtually all life had evolved in the ocean. Dry land was a very different kind of place. The biggest problem was the dryness. Simply absorbing enough water to stay alive was a huge challenge. It kept early plants small and low to the ground. Water was also needed for sexual reproduction, so sperm could swim to eggs. In addition, temperatures on land were extreme and always changing. Sunlight was also strong and dangerous. It put land organisms at high risk of mutations.
Vascular Plants Evolve
Plants evolved a number of adaptations that helped them cope with these problems on dry land. One of the earliest and most important was the evolution of vascular tissues. Vascular tissues form a plant&rsquos &ldquoplumbing system.&rdquo They carry water and minerals from soil to leaves for photosynthesis. They also carry food (sugar dissolved in water) from photosynthetic cellsto other cells in the plant for growth or storage. The evolution of vascular tissues revolutionized the plant kingdom. The tissues allowed plants to grow large and endure periods of drought in harsh land environments. Early vascular plants probably resembled the fern shown in Figure below.
Early vascular plants may have looked like this modern fern.
In addition to vascular tissues, these early plants evolved other adaptations to life on land, including lignin, leaves, roots, and a change in their life cycle.
- Lignin is a tough carbohydrate molecule that is hydrophobic (&ldquowater fearing&rdquo). It adds support to vascular tissues in stems. It also waterproofs the tissues so they don&rsquot leak, which makes them more efficient at transporting fluids. Because most other organisms cannot break down lignin, it helps protect plants from herbivores and parasites.
- Leaves are rich in chloroplasts that function as solar collectors and food factories. The first leaves were very small, but leaves became larger over time.
- Roots are vascular organs that can penetrate soil and even rock. They absorb water andminerals from soil and carry them to leaves. They also anchor a plant in the soil. Roots evolved from rhizoids, which nonvascular plants had used for absorption.
- Land plants evolved a dominant diploid sporophyte generation. This was adaptive because diploid individuals are less likely to suffer harmful effects of mutations. They have two copies of each gene, so if a mutation occurs in one gene, they have a backup copy. This is extremely important on land, where there&rsquos a lot of solar radiation.
With all these advantages, it&rsquos easy to see why vascular plants spread quickly and widely on land. Many nonvascular plants went extinct as vascular plants became more numerous. Vascular plants are now the dominant land plants on Earth.
Hydrotropism is directional growth in response to water concentrations. This tropism is important in plants for protection against drought conditions through positive hydrotropism and against water over-saturation through negative hydrotropism. It is especially important for plants in arid biomes to be able to respond to water concentrations. Moisture gradients are sensed in plant roots. The cells on the side of the root closest to the water source experience slower growth than those on the opposite side. The plant hormone abscisic acid (ABA) plays an important role in inducing differential growth in the root elongation zone. This differential growth causes roots to grow toward the direction of water.
Before plant roots can exhibit hydrotropism, they must overcome their gravitrophic tendencies. This means that the roots must become less sensitive to gravity. Studies conducted on the interaction between gravitropism and hydrotropism in plants indicate that exposure to a water gradient or lack of water can induce roots to exhibit hydrotropism over gravitropism. Under these conditions, amyloplasts in root statocytes decrease in number. Fewer amyloplasts means that the roots are not as influenced by amyloplast sedimentation. Amyloplast reduction in root caps helps to enable roots to overcome the pull of gravity and move in response to moisture. Roots in well-hydrated soil have more amyloplasts in their root caps and have a much greater response to gravity than to water.
Section through a maize root tip as seen through a confocal microscope.
© Jim HaseloffWellcome ImagesB0005172 CC-BY-NC-ND 4.0
Students create a simple plant experiment chamber, and use corn or bean seeds to test the effects of gravity (gravitropism) on root growth.
This activity is from the Plants in Space Teacher's Guide, and is appropriate for all grade levels.
Developed and conducted in collaboration with BioServe Space Technologies of the University of Colorado, and the United States National Aeronautics and Space Administration.
Plants respond directly to Earth&rsquos gravitational attraction, and also to light. Stems grow upward, or away from the center of Earth, and towards light. Roots grow downward, or towards the center of Earth, and away from light. These responses to external stimuli are called tropisms. Plants&rsquo growth response to gravity is known as gravitropism the growth response to light is phototropism. Both tropisms are controlled by plant growth hormones.
Indoleacetic acid, or auxin, is a plant hormone that, in high concentrations, stimulates growth and elongation of cells in stems, while retarding the growth of root cells. When auxin is distributed uniformly throughout a stem, all sides of the stem grow at the same rate, thereby enabling the plant to grow toward light and away from gravity (see illustration on page 5). If the plant is tipped over on its side, auxin concentrates on the lower side of the stem, causing the cells on the lower side of the stem to elongate. This process turns the stem so that it once again grows upward, presumably toward the light.
Roots also will change direction when a plant is tipped on its side. Auxin concentrates on the lower sides of the roots and inhibits the elongation of root cells. As a result, root cells on the upper side of the root grow longer, turning the roots downward into soil and away from the light. Roots also will change direction when they encounter a dense object, such as a rock. In these cases, auxin concentrates on the lower side of the roots, enabling the roots to change direction and find a way around the rock so that normal growth can resume.
investigate the effects of gravity
To learn the effects gravity has on growing plants, students create a simple germination chamber from a Zip-loc ® -type plastic bag and a moistened paper towel.
Note: For in-depth information regarding the role auxins play in plant growth and development, and about Brassica rapa, please download the Plants in Space Teacher's Guide.
Objectives and Standards
Ask a question about objects, organisms and events in the environment.
Plan and conduct a simple investigation.
Use appropriate tools and techniques to gather data and extend the senses, and analyze and interpret data.
Use data to construct a reasonable explanation.
Think critically and logically to make the relationships between evidence and explanations.
Use mathematics in all aspects of scientific inquiry.
Communicate investigations and explanations.
Reproduction is a characteristic of all living systems because no individual organism lives forever, reproduction is essential to the continuation of every species.
All organisms must be able to obtain and use resources, grow, reproduce, and maintain stable internal conditions while living in a constantly changing external environment.
Behavior is one kind of response an organism can make to an internal or external stimulus.
An organism&rsquos behavior evolves through adaptation to its environment. How a species moves, obtains food, reproduces, and responds to danger are based in the species&rsquo evolutionary history.
Earth and Space Science
Gravity is the force that keeps planets in orbit around the sun and governs motion in the solar system. Gravity alone holds us to Earth&rsquos surface and explains the phenomenon of the tides.
Materials and Setup
For complete list of materials, material options, safety issues and setup information, please download the PDF.
Materials per Student Group or Student
1&ndash2 large seeds, such as corn or bean
Cardboard square, cut slightly larger than the sandwich bag
One sheet of white paper toweling
Procedure and Extensions
Fold a piece of paper towel to fit inside the sandwich bag.
Moisten the paper towel until it is uniformly damp. Empty any excess water from the towel and place the towel in the bag.
Position one or two seeds on top of and in the center of, the moistened towel. The seeds should be visible through the bag. Seal the bag.
Position the bag in the center of the cardboard, and secure the corners with cellophane tape. Stretch the bag tightly to prevent sagging, and to help hold the seeds in place. Stand the cardboard upright on its side and lean it against a wall.
Observe the seed and record its appearance over the next few days.
When the first root has formed and grown one to two centimeters long, turn the cardboard 90 degrees, as shown below.
Roots and Worms Science Lesson
All About Earthworms
Earthworms live in the soil of every continent in the world except for Antarctica! There are about 2700 different kinds of them.
They aren’t much to look at (they may even seem a little gross), but earthworms are really good at what they do. You might be surprised to learn that their job is a very important one. So, what do they do? They dig tunnels through soil in the ground. As they go, they eat, digest their food, and then excrete it. That doesn’t sound very important. Well, it turns out, the “waste” that worms excrete is actually very valuable for soil. It is full of nutrients that help plants grow. The tunnels they form also help keep the soil healthy by supplying it with oxygen and making it easier for water to soak into the ground. Worms periodically come up to the surface of the ground to find food, then go back down and continue tunneling. This process helps mix up the richer soil from farther down in the earth with the soil at the top. This is important because lots of the nutrients in topsoil have already been used up by plants and the soil down below has more nutrients. All of these things make the soil better for plants to grow in. This is important for us since most of our food comes from plants or from animals that eat plants.
Earthworms are excellent recyclers! They eat things like fallen leaves and decaying animals. They can also eat food scraps, fruit and vegetable peels, eggshells, and some garbage (like coffee grounds and tea bags). Organic matter – something that came from a living thing, such as a plant or animal – will break down on its own eventually, but an earthworm can eat and digest an amount of food and dirt equal to its own weight in a single day, so the process goes much faster with their help! This keeps the soil full of helpful nutrients.
Worms need food, oxygen, and moisture to live. They breathe through their skin instead of with lungs. Oxygen from water in the ground can pass through a worm’s skin to keep it alive. They like the soil to be damp so that their skin can stay moist and slimy, but not too wet. If you go outside after a rainstorm, you might be able to spot some earthworms on the sidewalk. Sometimes after heavy rain, earthworms come up to the surface because they’ve gotten too much water while in the ground. UV rays from sunlight can kill worms very quickly, though, so if the rain storm happens during the day and the sun starts shining again, earthworms that have come up to the surface often get burned by the sun’s rays and die. If you happen to see any earthworms on the sidewalk, it’s a good idea to use a stick to move them back to an area with dirt.
Anatomy of an Earthworm
Earthworms are very simple creatures. They don’t have arms, legs, or ears. Instead of eyes, they have special cells on the outsides of their body that are very sensitive to light. Those cells help them see light, but nothing else. They have small simple brains that are used to help them move their bodies. They can also have up to five hearts to help pump blood through their long bodies.
An earthworm’s body is divided into lots of segments and they have a head end and a hind end. The very first of the tiny segments is the earthworm’s mouth and the last segment is its anus, where waste, called castings, exits its body. Both ends look similar, but you can tell the head end by the thick ring-like segment that is located near it.
An earthworm’s mouth is very small, but it is strong enough that it can hold onto a leaf and drag it around as the worm moves! When an earthworm eats, it uses a muscle in its throat to move the food down into a little space called a crop. The food stays in the crop for a little while, sort of how food stays in your stomach for awhile. Then it is pushed into another space called a gizzard. The gizzard has large grains of sand and small stones in it from the sand and dirt the worm has eaten. To digest the food, the gizzard squeezes in and out and the sand and stones rub together and grind up the food! From there it passes through the worms intestines where the worm gets all the nutrition it needs from the food. Then it exits the worm’s body as castings.
All About Roots
Most plants start their life as some sort of seed. A seed has all of the information it needs to grow into a plant, but before it can grow, it needs certain conditions to be right. When it has everything it needs (warmth, oxygen, and water), it will sprout. The sprouted seed will soon grow a stem above the ground. Below the ground, it will grow roots. The roots grow downwards into the soil. Roots are very important for plants. They help hold the plant in place in the soil while it grows. They also provide water and nutrients that the plant can’t live without. The roots soak up nutrients and water from soil, then the nutrients move up the roots into the stem of the plant to reach the leaves, flowers, and fruit. Roots have tiny hairs on them to help absorb water and nutrients from the soil. Sometimes plants use their roots to store extra food, especially during the winter.
Do you remember what plants need in order to grow? They need sunlight, air, and water. They also need nutrients. The best way for plants to get nutrients is from soil. As you’ve already learned, earthworms help provide soil with lots of great nutrients. A plant’s roots are the parts that allow a plant to use the nutrients that the worms provide. Roots help plants grow, and then earthworms eat the leftover parts of plants and the cycle starts all over again!
There are several different kinds of roots. Some plants have many roots and some just have a few. Trees have large systems of roots – some really big ones to help hold the tree up safely in the ground, and lots of smaller ones to help the tree get water and nutrients. Some vegetables, like carrots, radishes, and turnips, are actually roots! They are called taproots, because they just have one long main root. Sugar comes from a type of root, called a sugar beet, which is similar to a root vegetable.
Besides keeping a plant in place, roots can help keep soil in place. For example, the roots from trees growing along the edge of a river or near an ocean can help hold the soil in place when water washes over it.
This worksheet can be used as either a matching game (by cutting out all eight root and plant pictures) or as a cut-and-paste review of different roots that plants can have. Take a moment to discuss how different kinds of plants use their different types of roots and why they might be shaped the way they are.