Plant Organs

Leaves

Leaves play an important role in many processes vital to the plant's functioning. In this section, we will look at photosynthesis, transpiration, and the different layers of the leaf. The plant loses water through the leaves. This allows the plant to regulate its temperature and to draw up more water from the bottom of the root. The leaf is part of the shoot system but only works if the root system carries out its role.

Layers of a leaf

There are many different layers to the leaf. We can split the leaf up into 5 or so different layers:

• Waxy cuticle - waterproof layer found on top of the leaf.

• Upper epidermis - the thin upper layer of the plant cell.

• Palisade mesophyll - the compact layer of cells which carry out photosynthesis.

• Spongy mesophyll - the layer of cells involved in gas exchange.

• Lower epidermis - the lower part of the leaf. The stomata and guard cells are found here.

Photosynthesis

Photosynthesis is an important process for plants and occurs in the leaves. Photosynthesis is an endothermic process, meaning that the process requires energy and has to absorb light energy to take place. In photosynthesis, oxygen is created as a byproduct, whilst carbon dioxide is consumed from the environment.

Fig. 1 - Chloroplasts allow photosynthesis to take place in a plant cell

Transpiration

When discussing the leaf, it is important to mention transpiration. Transpiration is how plants lose water.

Transpiration is a critical process in the cell for a few different reasons. Firstly, the process of transpiration allows the plant to continue drawing water up. The loss of water from the plant creates a transpiration pull. This transpiration pull draws water up the plant in the xylem. Without this pull, water would be unable to move from the soil into the root. The pull creates a negative water potential in different areas of the plant, allowing water to move up the plant by osmosis. This movement of water up the plant provides the water essential for photosynthesis.

As well as this, transpiration helps to regulate the plant's temperature. Water evaporating from the surface of the leaf after diffusing from the spongy mesophyll cells allows the plant to cool itself down. Without this, the plant's temperature would rise too high. If the temperature was too high, the plant could not carry out metabolic processes like translocation and photosynthesis. At high temperatures, these vital processes slow down or even stop altogether.

Stem

The stem is another important organ in plants. The main role of the stem is to transport substances around the plant via the xylem and phloem. Have a look below for information on the transport of molecules.

The xylem

Water and ions dissolved in water are transported by the xylem vessel. The xylem vessel can only transport water upwards due to the movement of water in the plant being a passive process. Water can only move from the root of the plant, up to the leaf. There is no energy required for the movement of water in the plant. We describe this movement as being a passive process. The movement of water is aided by the loss of water from the top of the plant. As water leaves the top of the plant, it pulls water up the rest of the plant - this is known as the transpiration pull.

The xylem is adapted for the transportation of water in a few different ways. Firstly, the xylem vessel is made up of cells that have no end walls. The lack of end walls allows for easier water movement up the plant, with fewer obstacles in the way. As well as this, the xylem vessel also has no cell contents. Again, this allows for easier transportation of water, and these contents are not required as the cells in the xylem vessel are not metabolically active. Finally, the xylem vessel has lignin. Lignin is a waterproof material which wraps around the xylem vessel in different patterns depending on the age of the plant. Lignin ensures the xylem vessel remains rigid and strong but prevents water loss. In younger plants, there is less lignin present to allow for growth.

The phloem

The phloem transports solutes around the plant, both upward and downward. These solutes include sucrose as well as other sugars. Transport of sucrose in the phloem requires energy.

The phloem and xylem are found very close to each other in the plant. Together, they make up the vascular bundle of the plant. The phloem is found outside of the xylem in the vascular bundle. Have a look at Figure 2 to see the difference in location between the xylem and phloem vessel.

Fig. 2 - The Phloem and Xylem make up the vascular bundle

Sieve tube elements and companion cells

The movement of solutes through the plant, also known as translocation, requires energy. This requires the phloem vessel to have some adaptations to ensure there is enough energy present for the movement. When solutes move around the plant, we say they move from the source (where they are produced) to the sink (where they are used).

One adaptation of the phloem is that each sieve tube element is linked to a companion cell. The companion cell linked to the sieve tube element contains many mitochondria, and it carries out the metabolic activity for the sieve tube element so that energy is readily available for the transport of solutes. The sieve tube element and companion cell are linked via plasmodesmata. Two sieve tube elements are linked via gaps in the sieve endplate separating them called sieve pores.

Translocation

Translocation is the movement of solutes around the plant both upwards and downwards. As we have discussed above, translocation requires energy. This energy comes by way of the mitochondria in the companion cells.

Translocation is a two-way process. This means that solutes can move both up and down the plant. We can see that translocation is a two-way process when we add radioactive carbon dioxide to the plant. This radioactive carbon dioxide can be detected both above and below the site where it was added to the plant.

Root

Another important organ of the plant is the root. The root is an aspect of the root system rather than the shoot system. The root is the area of the plant below the ground, found in the soil. The root has a crucial role in the absorption of minerals, ions and water from the soil. These crucial substances then can make their way up the plant to other organs in the shoot system. The movement of water and other substances from the soil into the root requires osmosis and active transport to work collaboratively.

What are root hair cells?

Root hair cells are found in the roots of plants. They bridge the gap between the root of the plant and the soil. Have a look at Figure 3 to see how root hair cells look and consider why they look how they do.

Fig. 3 - Root hair cells are extremely important in the process of absorption for plants

Root hair cells play an important role in the absorption of nutrients and water from the soil into the plant. Due to their role in the absorption of nutrients, root hair cells are adapted to have a large surface area. This is to maximise the rate of absorption of nutrients. Did you notice their large surface area in Figure 3?

Absorption

As we mentioned above, root hair cells play an important role in the absorption of nutrients. Specifically, they play a role in the absorption of water, as well as ions like magnesium and nitrates. The movement of water into the plant from the soil is initially via osmosis.

However, this process only works passively whilst there is a steep concentration gradient between the soil and the root. When the concentration of water in the soil equals the concentration inside the root hair cell, water will stop moving into the plant. It is at this point that the plant needs to use active transport.

Active transport

Once the concentration gradient between the level of water in the soil and root hair cells levels off, osmosis will no longer move water into the plant. At this point, the plant needs to start 'pumping' ions from the soil into the root via active transport.

As ions get pumped into the root from the soil against their concentration gradient, the levels of ions in the plants build up. This starts to increase the water potential of the root hair cells, allowing water to enter the root from the soil again passively, down the concentration gradient. This shows us how active transport can be used alongside osmosis in the movement of water and ions into the root of the plant.

List of plant organs

Let's summarise the 3 different plant organs that we have discussed. So far, we have covered;

1. Leaf
2. Root
3. Stem

These 3 organs can be grouped together and described as the vegetative organs of the plant. These vegetative organs are the organs of the plant that support plant growth. As we mentioned above, these different systems work with each other to ensure key metabolic processes are able to function at a high level throughout different parts of the plant. Remember, we can break these organs into two different systems, the root system and the shoot system. The stem and leaf form part of the root system, while the root is the sole part of the root system.

Now that we understand the role that vegetative organs play in the plant, let's move on and focus on the fourth plant organ: Reproductive organs! Like humans, plants must reproduce. However, the process of reproduction in plants is quite different from that in humans. The process itself is very different, as is the anatomy of the reproductive organs.

Reproductive Organ of Plants

Aside from the vegetative organs, plants also contain reproductive organs. The role of vegetative plants is to support growth, whereas the role of the reproductive organs in plants is to carry out reproduction.

The male reproductive organ in plants is the stamen, and the female reproductive organ in plants is the pistil. These plant reproductive organs are both present in the flowers of plants. The majority of plants contain both male and female sexual organs in the same flowers, whilst some plants have some flowers that are completely female and other flowers that are completely male.

The stamen (male reproductive organ) is comprised of an anther attached to a filament. The anther produces the male sex cells (pollen). The ovary produces the female sex cells (contained in the ovules). The stigma is the top of the female part of the flower, which then collects the pollen grains. Now that we understand some basic anatomy behind plants' reproductive organs let's look at different types of sexual reproduction in plants.

Insect-pollinated flowers

Insect-pollinated flowers have bright petals and a sweet-smelling scent of nectar to attract insects. This can be described as an anatomical adaptation, deliberately used to attract insects to the reproductive organs of the plants.

The insect, which has been attracted to the plant's flowers by the bright petals and scent, lands on the flower to collect its nectar. As it lands on the flower, pollen grains will move onto the insect from the anther of the flower. The insect will then move on to another flower, again attracted by the scent and colour of the flowers. As it moves to the other flower, the pollen grains are transferred to the stigma, which can catch the pollen grains.

Wind-pollinated flowers

Rather than using insects to help in reproduction, some plants use wind instead. These plants (like wheat) do not need the anatomical adaptations that we discussed for insect-pollinated flowers. So, they do not tend to have bright colours or sweet smells.

For wind-pollinated flowers, their adaptations come in the length of the stamen attached to each anther. Each anther is attached to a long stamen, which moves in the wind. A gust of wind can easily carry the dust-like pollen grains away from the flower towards another plant. The pollen needs to land on the stigma of another flower, which has its own adaptations. The stigma of wind-pollinated flowers is long and feathery, which increases their surface area and the likelihood of the pollen landing on them!

Plant Organs Diagrams

It's important that we know each of these different organs, but also important that we know how to label pre-drawn pictures of the plant organs, as well as draw our own diagrams of plant organs. Please go over the article once again and make sure you can label each organ in an empty plant diagram. As an example, see the diagram of a plant leaf below and try to fill it out in as much detail as you learnt!

Fig. 4 - Empty leaf diagram. Fill out the names of the structures you see in the diagram