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.