Video Transcript
In this video, we will learn how to
describe the basic structure of a dicot plant stem. We will see which simple and
compound tissues the stem is composed of and the specific structures that provide
the stem with its useful functions.
Plant stems are vital to their
survival. Stems are usually long stalk-like
structures that form the main body of a plant. They tend to rise above ground,
though some stems can also be found underground, like in this potato plant. The stem can help steady and
support a plant under harsh environmental conditions, allow a plant to move to
access light, and even to transport essential substances to the different plant’s
organs.
Stems need to be really well
adapted for these functions, as some plants, like this North American Hyperion tree,
which is sometimes called a redwood, can have stems reaching as high as 115
meters. To understand just how large this
plant is, this is approximately how a tree of this size would compare to the typical
size of a giraffe, the tallest animal on Earth.
In this video, we’ll be learning
about how the structure of plant stems can facilitate survival, including in these
huge organisms. A dicot, which is short for
dicotyledon, is a type of plant that produces seeds that have two seed leaves. These embryonic leaves are called
cotyledon, and they’ll form eventually the first leaves that we’ll see above
ground. In this video, we’ll be looking at
the structure of dicot stems. But it’s worth noting that
monocots, which is short from monocotyledon and produce seeds with only one seed
leaf, will have a slightly different stem structure. An easy way to remember how many
cotyledons each of these flowering plant types have is that mono- means one while
di- means two.
Let’s look at a cross section
showing the typical structure of a dicot stem. The epidermis is a single layer of
cells that covers a plant stem as well as its leaves, flowers, and roots and forms a
boundary to the external environment. It effectively acts as a tough skin
for a plant, and it can sometimes produce a waxy cuticle to protect it. The waxy cuticle helps to protect
the stem, especially the cells sitting just below the epidermis, from mechanical
damage or excess water loss. It can also help prevent the
entrance of dangerous microorganisms, like pathogens, that might cause
infection.
Moving inwards from the epidermis,
there are several layers of spongy tissues that make up a region called the
cortex. The cortex is an outer layer of
tissue that’s found immediately below the epidermis of a stem or of a root. And it’s composed of several
different types of simple tissues. A simple tissue is made up of cells
that are structurally and functionally very similar to each other. The majority of the cortex is made
up of parenchyma tissue, which is one example of a simple tissue. Parenchyma cells make up the soft
fleshy tissues inside various parts of the plant, such as the leaves, stem, and
roots. Parenchyma tissue has plenty of
intracellular spaces between its cells that provides aeration to promote gas
exchange. Parenchyma cells also contain
chloroplasts to carry out photosynthesis.
Directly below the epidermis within
the cortex of growing stems, there is another region of simple tissues called
collenchyma tissue. Collenchyma cells are typically
longer than parenchyma cells, and they have thickened cellulose and pectin cell
walls. Like parenchyma cells, collenchyma
cells usually contain chloroplasts to carry out photosynthesis. Collenchyma cells are usually found
below the epidermis of leaf veins and stems, particularly young stems, and are
essential in growing regions of the plant for providing structure and some
flexibility. So this region of the stem
highlighted in orange would have a high proportion of collenchyma cells just below
the epidermis.
Sclerenchyma tissues are the
toughest of the three simple tissue types found in plants. Sclerenchyma cells have thick cell
walls with a chemical called lignin deposited in them. This provides structure and
mechanical support, particularly to mature stems and leaves. Sclerenchyma cells are usually
dead. So they’re often found in the
cortex of more mature regions of stems and leaves, like this region in pink. The region in orange is less likely
to contain sclerenchyma as it’s still developing.
As our plant is still fairly young,
let’s focus on these regions that will contain more collenchyma and parenchyma in
the cortex. The innermost layer of the cortex
is called the endodermis, which can be identified in our diagram as this orange
structure. The endodermis is sometimes called
the starch sheath, as it’s responsible for storing starch, in addition to regulating
the movement of water, ions, and plant hormones in the plant’s transport system.
This transport system consists of
several structures called vascular bundles, one of which has been circled here. The vascular bundles are
responsible for moving essential materials around the plant to the different organs
that require them. Though vascular bundles will be
present in the roots and leaves as well as here in the stem, their arrangement
differs depending on their location. Each vascular bundle consists of
phloem tissue, identified here in pink; xylem tissue, identified here in blue; and a
layer called the cambium that sits between them. Each vascular bundle is supported
by a tough section of sclerenchyma tissue called the pericycle, shown here in
red.
Let’s take a closer look at the
distribution of these simple tissues in the plant stem. We already know that the cortex of
growing stems is made up primarily of collenchyma and parenchyma tissues, and in
more mature regions sclerenchyma tissues, and that the pericycle contains
sclerenchyma cells to provide structural support to the vascular bundles. Between each vascular bundle are
regions of parenchyma tissue called the medullary rays. And the bulk of the middle of the
stem is called pith. Pith is a spongy tissue in the
middle of plant stems and is made up of parenchyma cells. Both the pith and the medullary
rays mainly function as storage tissues, though the medullary rays also transport
materials from the vascular bundles to the pith for storage.
Let’s look at the composition of a
vascular bundle in the stem in more detail. Vascular systems are essential in
most multicellular organisms. Single-celled organisms, like this
amoeba, can usually obtain the substances they need, like sugars, water, or oxygen,
directly across their surface by diffusion and can usually release waste products in
the same way, as they are small organisms with a large surface area relative to
their volume. As soon as organisms are made up of
more than one cell, diffusion of these substances becomes much slower, as all of
their many cells will require these substances. When organisms become as large as
multicellular plants that can have millions of cells, if not more, this process
becomes nearly impossible.
Most multicellular organisms, like
plants and animals, are unable to transport the substances they need across their
surface, as it would take far too long to transport them to their innermost
cells. Therefore, most multicellular
organisms have vascular systems that are specifically adapted to transport all of
these substances that they need to every single cell in the body. This plant, for example, is
multicellular, and it has vascular bundles to transport substances like sugars and
water around its body. The vascular bundles not only
transport water and mineral ions from the roots to the leaves, stem, and other parts
of the plant that require them, but it also requires vascular bundles to move
solutes like sugars and amino acids from the leaves, where the majority of them are
made, both up and down the stem to the other plant organs that require them.
This diagram shows a close-up view
of one vascular bundle in the stem. And you can see that it consists of
two main regions, the xylem and the phloem. While the phloem is responsible for
the transport of these sugars and amino acids, the xylem is responsible for
transporting water and minerals. Let’s take a closer look at xylem
tissues first. Xylem tissue consists of two main
types of cell: xylem vessels and xylem fibers. Some parts of the xylem might
include cells called tracheids, especially in nonflowering plants. These are structurally similar to
xylem vessels, but they are tapered and closed and often longer and thinner. As they’re made up of cells that
differ in their structure and function, xylem is often described as a compound
tissue, or sometimes as a complex tissue.
Compound tissues differ from simple
tissues as they’re composed of various types of cell that vary considerably in both
their structure and their function. Xylem vessels are made of
sclerenchyma cells. And when they’re mature, they’re
usually dead. Like other sclerenchyma cells,
xylem vessels have lignin in their walls, so they’re referred to as lignified. The cells in xylem vessels are
stacked end to end, with their end walls broken down to form a hollow tube. These tubes are waterproof because
of the lignin in their walls. This allows water molecules and
dissolved mineral ions to move up through the xylem vessels without leaking out, as
if being moved through a straw.
The lignin in the xylem vessel
walls not only waterproofs them but also makes them stronger. The xylem fibers are also
lignified, which helps to provide structural support to the xylem vessels and
prevents them from collapsing. The main function of the xylem is
to transport water and mineral ions from the roots, where they’re absorbed from the
soil, to the other parts of the plant that require them.
Water is a key reactant in
photosynthesis. So it’s needed in the
photosynthesizing parts of the plant, like the leaves. Water is also a key medium for
transport among its other functions in plants, such as filling vacuoles and
maintaining cell shape. The mineral ions that are
transported in the xylem also have many functions in the plant, for example, for
building up amino acids that can then form proteins for growth and support or for
forming the photosynthetic pigment chlorophyll.
Let’s take a closer look at the
phloem next. Phloem tissue is another example of
a compound tissue, as it’s made up of four main different types of cell: sieve tube
members, which are otherwise known as sieve tube elements; companion cells, which
can be identified here in pink; and fibers and sclereids, which have not been shown
in this diagram as their main function is to provide mechanical support to the
phloem. The role of the phloem is to
transport sugars and amino acids primarily from the leaves and other photosynthetic
parts of the plant to the other plant organs that require them.
Sugars like glucose are mainly
synthesized in the parts of the plant that are exposed to light, as these parts will
be carrying out photosynthesis. But these sugars are needed in all
parts of the plant, as they will all be carrying out cellular respiration to release
energy for the plant’s various processes. Therefore, these sugars are
transported via the phloem to all the parts of the plant that might require them for
respiration. The phloem is also responsible for
transporting amino acids to the various plant organs.
A special feature of the phloem is
that it can transport these solutes both up and down the stem depending on which
direction they’re required in. This is different from the xylem,
which can only transport water and minerals up the plant from the roots where
they’re absorbed. To facilitate the movement of these
substances, there are sieve plates between each sieve tube member. These sieve plates have large holes
in them, much like a sieve, that allow certain substances to pass through them.
The companion cells, which are
linked to the sieve tube members, contain many mitochondria to release the energy
that’s needed for transport in the phloem. The companion cells and sieve tube
members are linked to each other and pass substances through their walls through
gaps called plasmodesmata. This forms a continuous flow of
cytoplasm between the two types of cell.
Let’s see how much we can recall
about the structure of the stem by having a go at a practice question.
The diagram provided shows a
simplified structure of a dicotyledonous plant stem. What structure is indicated by the
question mark? (A) Epidermis, (B) cortex, (C)
pith, or (D) vascular bundle.
This diagram shows us a cross
section of a dicot stem. And we need to identify one of the
structures within it. To do this, let’s look at the dicot
stem structures that are mentioned in these answer options.
The epidermis is a single layer of
cells around the plant stem that forms a boundary to the external environment,
acting as a tough skin for the plant. The epidermis often produces a waxy
cuticle to coat it that can protect the stem from mechanical damage or water loss
and may help prevent the entrance of dangerous microorganisms that may cause
infection.
Just below the epidermis are
several layers of cells that make up a region called the cortex. The cortex is a spongy region made
up of many simple tissues, such as parenchyma tissues, and in growing stems
collenchyma tissues, right below the epidermis. While parenchyma cells make up a
lot of the soft fleshy tissues within various parts of the plant, collenchyma cells
provide some structure and flexibility to growing stems.
Pith is a spongy tissue found in
the center of dicot stems. Pith is also mainly made up of
parenchyma cells, as its primary function is storage of substances like water and
sugars.
The question mark is pointing to a
structure called a vascular bundle. The vascular bundles are the
plant’s transport systems, which move essential nutrients, like water and sugars,
around the plant to the different organs that require them. Around the exterior of each
vascular bundle is a tough region of sclerenchyma tissues, which helps to support
the vascular bundle. The vascular bundle also includes
phloem tissues, which transport sugars and amino acids around the plant, and has
been shown in this diagram in blue.
Moving closer to the pith, the
vascular bundle also contains xylem tissues. Xylem is responsible for
transporting water and dissolved mineral ions from the roots of the plant to the
other parts that require them. It’s shown in this diagram in
red. Therefore, the structure that was
marked with a question mark in this diagram was the vascular bundle.
Now that we’ve had a go at applying
our knowledge, let’s review the key points that we’ve covered in this video. The typical structures in a dicot
stem include the epidermis; the endodermis, which is sometimes known as a starch
sheath; and the cortex, which is positioned between the two. There are also several vascular
bundles, and between each vascular bundle are the medullary rays. The medullary rays transport
materials from the vascular bundles to the pith in the center of the stem for
storage. The vascular bundles are the
transport system of the plant, and they include phloem tissues and xylem
tissues. While the phloem transports sugars
and amino acids from the photosynthetic parts of the plant to the other organs that
require them, the xylem transports water and dissolved mineral ions from the roots
to the rest of the plant.
