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.
