We just talked about the different types of plant cells, but now we need to understand
how those cells organize themselves to form larger structures. First,
let’s review the levels of organization in living organisms. Cells are the building blocks of life,
but cells are usually organized into tissues, and these tissues will often be organized into organs,
and this is true of plants just as it is for animals.
Plants have three main types of tissues, and all of these tissues are comprised of the plant
cells we discussed in the previous tutorial, so let’s go through these types of tissue now.
First up, ground tissue makes up the majority of a plant’s body, so to speak, and it’s broken
up into three subgroups based on cell type. Those are the parenchyma, the collenchyma,
and the sclerenchyma, which we just learned about. Ground parenchyma tissue is the most common tissue
in a plant. It appears in a variety of locations and does many jobs.
Parenchyma tissue is responsible for the photosynthetic layer in leaves,
called the mesophyll, where the plant performs gas exchange and creates sugars, making its own
food. Parenchyma tissue is also how a plant stores excess energy in the form of starches,
which are complex polysaccharides. Starch-filled parenchyma tissue can be found in a plant’s roots,
and parenchyma tissue also makes up the majority of a seed so that the starches can feed the
embryonic plant until it’s able to photosynthesize on its own. Additionally, parenchyma tissue is
so prevalent throughout a plant that it also takes on the role of growing to cover wounds
and replace other tissues lost through physical trauma or disease. Wound closure is an important
function for plants just like it is for us, because if a plant has an open wound then all
sorts of pathogens like fungi and bacteria could invade the plant and quickly kill it.
The other two subtypes of ground tissue, ground collenchyma tissue and ground sclerenchyma tissue
are also composed of cells by the same names. As we now know, both collenchyma cells and
sclerenchyma cells have thick cell walls made of cellulose, and in some cases, lignin,
which provide structure for a plant. Therefore, ground collenchyma and ground sclerenchyma tissues
can be found throughout a plant, wherever structural support is most important.
But as we said, ground tissues are just one of the three kinds of plant tissue, and these ground
tissues are essentially sandwiched between the other two kinds of tissue in a plant.
On the external surface of a plant, we can find dermal tissues. This name makes sense because
“dermal” is a word that relates to the skin or exterior of a living organism,
so these tissues essentially form a sort of “skin” for the plant.
A plant’s skin is called the epidermis, and it’s a layer of cells only one cell
thick. Most of these cells don’t have chloroplasts or other specialized organelles, they’re primarily
there just there to serve as a protective layer to shield the more important tissues beneath.
As extra protection, most epidermal tissues secrete a waxy substance called cuticle
that prevents excess water from escaping the plant and also protects the plant from invasion
by pathogens like fungi and bacteria. This cuticle is one of the main evolutionary advantages that
land plants exhibit over their aquatic ancestors. Some epidermal cells can specialize to take on
hairlike shapes which help the plant with specific gas and nutrient transfer functions, but these
hairs can also be useful in deterring insect herbivores that might try to graze on the plant.
A plant also needs some openings in the epidermis in order to let water and gases travel in and out,
so as to maximize the surface area available for material exchange. These openings are called
stomata. However, if they were left open all the time then pathogens could infiltrate
the plant through these areas. Therefore, some specialized epidermal cells called guard cells
are utilized to cover the stomata. These curved cells appear in pairs on either side of a stoma
and work together to open or close the stoma as needed by the plant. The function of guard
cells is especially important for plants living in very dry areas that need to keep water from
evaporating away during the day, so the stomata will often remain closed until the sun goes down.
You can see this happen in warm-season grasses and other plants growing in arid environments.
In older sections of a plant that aren’t growing as fast,
the epidermis may transition into a thicker layer of dead cells called the periderm. The periderm
is able to provide greater protection to the inner layers of the plant than the epidermis, but it’s
a less active tissue which doesn’t really grow, though it still allows for limited gas exchange.
The final group of plant tissues is not actually present in all kinds of plants.
Vascular tissues are the main characteristic that separates vascular plants, like trees,
from nonvascular plants, like mosses, and it allows vascular plants to have a wider
variety of growth strategies. Vascular tissue is important for large plants like shrubs and trees
because it redistributes water and nutrients throughout a plant’s body,
allowing for trees to grow tall without losing the capacity for nutrient transport
between distantly-separated parts, like the branches and the roots. Vascular tissue is what
allowed the ancestors of modern plants to abandon their reliance on living in or near water sources,
meaning that we can now find plants in almost every environment on Earth, regardless of how
dry they seem. We will discuss these aspects of plant evolution a bit later in the series.
Vascular tissue can be further broken down into two types, xylem and phloem. Xylem is
a vascular tissue made of dead cells called tracheids and vessel elements.
These are both elongated cells whose walls are strengthened with lignin, the substance that
makes woody plants so stiff and strong. Xylem is the vascular tissue responsible for transporting
water and mineral nutrients upwards. The roots of a plant absorb water and minerals from the soil.
The xylem then allows these substances to move up and
throughout the plant due to the cohesive and adhesive properties of water, in this case
referred to as capillary action, which we discussed in the general chemistry series.
At the top of a plant, excess water is released through the stomatal openings in the leaves by a
process called transpiration, which occurs when water exiting a plant’s leaves evaporates into
the air. The mechanism of transpiration promotes further capillary action in the xylem, meaning
that water will continue flowing up through the plant even though the xylem cells are dead.
The other kind of vascular tissue we mentioned is phloem, and it’s composed of living cells
called companion cells and sieve cells. Companion cells regulate the function of the phloem,
while the sieve cells execute this function. Phloem tissue is responsible for transporting
the sugars produced through photosynthesis in the leaves to all of the other parts of the plant.
Sieve cells are connected by sieve plates, which are membranes with pores through which the sugar
solution can pass. Although phloem relies largely on gravity to move sugars down from the leaves,
it also needs some input of water from the xylem in order to thin the sugary sap and allow
it to flow through the sieve plate pores. In this way, xylem and phloem vessels act sort of like the
arteries and veins that comprise the circulatory system in our bodies, in that they shuttle
important substances around so that they can be made available to all the cells in the organism.
So that covers the three types of plant tissues, those being ground tissue,
with its three subtypes, parenchyma, collenchyma, and sclerenchyma,
dermal tissue, and vascular tissue, which can be divided into xylem and phloem. Now that we know
the different kinds of cells in a plant and how they group together to form different tissues,
it’s time to see how those tissues form the different organs, or parts of a plant.