Topics in Depth
The Theme of Primary Growth and Secondary Growth in Plant Biology
Primary Growth of Shoots
Do plants grow from the top or the bottom? If you carve your name in a tree trunk, will it be at the same place in 10 years or will it move up the trunk? To know the answers to these questions, you need to understand primary growth and secondary growth.
First, let’s look at primary growth. Primary growth extends the length of a plant both aboveground and belowground. Since humans generally live aboveground, we usually only see the aboveground parts of a plant: the shoot system. The entire shoot system, no matter how large or small, owes its beginnings to a small region of the plant called the shoot apical meristem.
An apical meristem is a region of high cell division (lots and lots of mitosis) that contributes to the extension of the plant. The shoot apical meristem is an apical meristem that is in the shoot system, as opposed to the root apical meristem that is, you guessed it, in the roots. It is only through the activity of the shoot apical meristem that the plant grows taller.
The shoot apical meristem is found at the tip of the plant stem, so growth extends upward from the top of the stem, not the bottom. Those bottom leaves aren’t going anywhere until they fall off the plant. That means if you carve your name into the trunk of a tree, it will still be there many years later (but don’t do that, it hurts the tree like a tattoo hurts human skin; watch this clip from Fern Gully if you don’t believe us).
One more meristem is the intercalary meristem. This is a region of rapid cell division at the base of nodes. This type of meristem is only found in monocots, so don't go looking for it on eudicots. You’ll be looking a long time. These are particularly important to monocots because they allow stems to elongate quickly and also for leaves to regrow quickly if they have been damaged.
Just like a human body has all its different parts (arms, legs, torso, head), a plant body has parts that are the same on every plant, though they may look different in different species.
The parts of a shoot system are the:
- Stem (nodes + internodes)
- nodes are where leaves attach to the stem
- internodes are the spaces on the stem in between the leaves
- Leaf (petiole + blade)
- Branches, which grow out of axillary buds
- Reproductive parts (the flowers and fruit)
A leaf is made up of a blade and a petiole. The blade is the flat green part that you usually think of as the leaf, and the petiole is just the little stem that attaches the blade to the main stem. In between the leaf primordia, where new leaves form, and the stem below, are the axillary buds. These will form branches, which will have their own apical meristems on the ends. Axillary buds are often protected by bud scales. A bud scale is a modified leaf that covers the delicate bud until it starts to grow into a shoot.
Most of the parts named above are visible as they originate on the shoot apical meristem. The shoot apical meristem is comprised of leaf primordia, which turn into leaves, and the apical dome, where the stem elongates. Under a microscope, the tip of a plant shoot looks like this:
Sometimes stems are modified, and specialized stems may look and function differently than "regular" stems. For example, a rhizome is a stem that grows horizontally underground. Just because a plant part is growing underground doesn’t mean it’s a root! A rhizome can have axillary buds that shoots grow out of. Irises have rhizomes, as do ginger and potato plants. Many people know that potatoes grow underground and are called tubers, but they actually are not roots. Potatoes are enlarged ends of rhizomes, storing sugars and acting as storage organs for the plant. A rhizome looks like this:
In the picture above, see how the tubers are at the ends of the rhizomes, and the true roots are below?
Primary Growth of Roots
The root system also has an apical meristem, known as the root apical meristem. This acts in much the same way as the shoot apical meristem, causing extension growth. The main difference is this growth goes down into the ground, and roots, not leaves and branches, come from the root apical meristem.
Roots have really important jobs, and they don’t get a lot of credit for their hard work because they are underground all the time. Roots are responsible for:
- Anchoring the plant into the ground
- Absorbing water and nutrients
- Storing nutrients
- Associating with soil microbes in symbiotic relationships
As roots grow, they travel downward through the soil, dodging rocks and other obstacles that might be in their way. Just as you should wear a helmet when riding a motorcycle or playing hockey, roots have their own type of helmet: a root cap. The root cap protects the root apical meristem as the root pushes its way through the soil. It also secretes slimy ooze that lubricates the soil around the tip of the root, aiding the root on its journey through the harsh soil.
Roots can take on many different forms, and root form depends on whether the plant is a eudicot or monocot. In eudicots, the first root to form is the primary root. It grows straight down and is the dominant root, also known as a taproot. The taproot can produce lateral roots that grow out to the sides. Common eudicots include tomato plants, roses, maple trees, oak trees, and raspberry bushes.
In eudicots, branch roots soon join the taproot in its hunt for nutrients. These branch roots form from an area called the pericycle. Branch roots don’t grow as long as taproots, but they expand the plant’s ability to take up water and nutrients from the ground.
In monocots, the primary root usually dies soon after the plant germinates and is replaced by roots that form on the stem, called adventitious roots. Adventitious roots are lateral roots that anchor the plant. Monocots don’t have taproots but instead have shallow, fibrous root systems that trap lots of soil. Some examples of monocots are corn, orchids, lilies, and magnolias.
When seeds first start to germinate, the most important thing for the young plant is to get a good hold in the ground. The plant produces more roots than shoots when it is young, but as it gets older the amount of root structure is roughly the same as the amount of shoot structure. In fact, the underground root system often mirrors the aboveground shoot system.
Secondary Growth of Shoots
Now we know how a plant gets taller and its roots get longer. But what about wider? Even a big tree with an enormous trunk starts out as a puny seedling. Popeye eats a lot of spinach to grow big and strong, but what do spinach plants eat?
The width of a plant, or its girth, is called secondary growth and it arises from the lateral meristems in stems and roots. As with apical meristems, lateral meristems are regions of high cell division activity. However, the cells they make grow outward rather than upward or downward. Eudicots use lateral meristems to add to their width; monocots, however, do not experience secondary growth. We’ll come back to them later.
The lateral meristems that produce secondary growth are called cambiums, which just means a tissue layer that adds to plant growth. The two important ones for secondary growth are the vascular cambium and the cork cambium. The vascular cambium produces more vascular tissue (xylem and phloem), which provide support for the shoot system in addition to transporting water and nutrients. Because the xylem and phloem that come from the vascular cambium replace the original (primary) xylem and phloem, and add to the width of the plant, they are called secondary xylem and secondary phloem. Here is what that looks like:
The vascular cambium is only one cell thick and forms a ring around the stem of a plant. On its interior, it adds secondary xylem and on its exterior, it adds secondary phloem. In trees, the layers of secondary xylem form wood. The layers of the secondary phloem form bark. Over time, the tree sheds older layers of bark and replaces them with newer layers. If you look at a cross section of a stem, the width of the wood gets bigger over time but the bark always remains a narrow band.
Over time, the older wood in the inner part of the trunk goes becomes transformed. It doesn’t turn into an alien and fight Decepticons, but it does increase its defenses. The inner wood goes goes through a genetic process that makes it harder and more resistant to decay. The wood’s cells are dead, and it is now called heartwood. Heartwood is sometimes, but not always, darker than the surrounding wood. You can think of it as the "heart" of the tree, keeping the tree strong and sturdy because it is in the middle of the tree. However, it does not actually contribute to keeping the tree alive—trees can live with their heartwood completely decayed!
The wood that surrounds the heartwood is called sapwood. Sapwood is the living wood where transport of water occurs. Sapwood, unlike heartwood, is vital to the tree’s health because it is carrying the water and nutrients the tree needs to survive. Sapwood is softer than heartwood, so if you have to decide which to build your house out of, choose the heartwood.
In this cross section of a stem, the stuff in the middle, labeled Pi, is called the pith. The pith is made up of primary cells (originating from an apical meristem). The area labeled with an X is the xylem, and the P is the phloem. The area labeled BF is a region of bast fibers, which are strong supporting fibers in the phloem. These are not present in all plants. The outer dark region labeled C is the cortex, which surrounds the vascular tissue. And last but not least is the epidermis, which is the outermost layer of cells.
In temperate areas with a distinct summer and winter, the vascular cambium takes a nice long rest during the winter, kicking its feet up and watching marathons of Friday Night Lights for a few months. When it starts up its cell division again in the spring, the new cells are much bigger than the last cells made during the fall because water and nutrients are more available in spring. The parts of the wood with wider cells made in the spring are called springwood. Wood made later in the season is called summerwood and is often composed of thinner cells. This cycle of growth in the spring and summer and Friday Night Lights in the winter, repeats every year and forms annual tree rings.
The cork cambium makes cork, which is a tough, insulating layer of cells. These cells have wax in them, which helps them protect the stem from water loss. The cork layer also protects the plant from insects and pathogens such as fungi and bacteria, and can insulate the tree from fire. This cork is indeed the same cork found in wine bottles, which usually comes from the cork oak tree (Quercus suber). Harvesting cork from these trees maintains the ecosystem: in areas of Europe where cork harvesting has been abandoned, the cork oak habitats have become overgrown by flammable shrubs, causing an increase in wildfires. Cork is also part of the bark, and it falls off over time.
Monocots don’t have secondary growth. Usually monocots do not get very wide. However, some monocots, such as palms, can get pretty thick in the middle. How? When palms shed their leaves, they don’t lose the entire leaf. The base of the leaf stays attached to the stem, and layers of old leaf bases accumulate over time. This makes the palm stem wider even without having secondary growth.
Not all roots are underground. Mangroves are tropical plants that grow in flooded soil, and they have huge aboveground root systems to help them get air while the rest of their roots are underwater. Many orchids and other plants are epiphytes, which grow completely on top of other plants. Their roots dangle in the air or are plastered onto tree trunks, and are usually pretty small since the orchid has found someone else to lean on and don’t need the roots to keep them upright.
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