How do plants grow? Photosynthesis and the parts of plants

This post answers the critical question: how do plants grow? We also cover the different parts of plants. We explain how those parts contribute to plant growth and why all of this matters to the gardener.

For most people gardening is about plants.

Admittedly, features like paths, walls, fences and decks play part in creating the overall garden picture. But without plants, it is very hard to consider any space to be a garden

Indeed, for most of us, it is plants that draw us to gardening in the first place – for me there was a kind of sudden awakening to the mind-boggling variety and beauty of the plants all around us. So much so, that I wanted to grow every plant I could get my hands on.

Anyway, I think it is pretty self-evident that if you want to learn to garden, and garden well, plants have to be the starting point of your studies.

When you garden, you need to know how to choose plants, how to keep them thriving, how to maintain them within their allotted space and, if possible, how you can get more plants from the plants you already have.

To help you fully understand those things, we first need to answer the question: ‘how do plants grow?

Because if we can understand how plants grow, we can understand what we need to do to help them grow well and then reap all the benefits that growing plants can bring us.

How plants grow – the building blocks

Like us, plants are living things.

We are able to grow and thrive because our bodies, remarkably, are able to take the energy from the food we eat and convert it into material can be used to create and renew the cells in our body.

Plants operate in a similar and equally remarkable way.  Plants fuel their growth by combining sunlight, air, water and soil nutrients.  Just think for a moment about that.

How can those ‘ingredients’, seemingly so lacking in weight or substance on their own, produce something as enormous, solid and long-lived as a mature oak or a California redwood?

How can those very same ingredients produce something so delicate and transient as an apple blossom, or something so sweet and succulent as a melon?

The answer is that plants achieve this magic by a process called photosynthesis.  Photosynthesis is unique to plants and is so specialised that science has never even been able to replicate it.

Photosynthesis

Put simply, photosynthesis involves the conversion of energy from the sun into the building blocks of plant life.

The actual process itself is quite complex but once we take into account a few simple scientific facts, we can easily gain an understanding of how it works.  This is what we need to know:

• First, we need to appreciate that the air around us contains carbon dioxide gas (CO₂).
• Second, we need to know that water (H₂O) is comprised of molecules of hydrogen and oxygen.
• Third, we need to know that the individual cells in the green parts of plants, such as the leaves, contain structures called chloroplasts.
• Finally, we need to know that each chloroplast is filled with a substance called chlorophyll. Chlorophyll is what makes leaves green and it is chlorophyll that takes the central role in the photosynthesis process.

Photosynthesis in five steps

So, with that knowledge, let’s look at how photosynthesis takes place. For the sake of a simple explanation, the process can be broken down into 5 steps.

1. The plant draws in water (H₂O) through its roots and carbon dioxide (CO₂) from the atmosphere and combines these in its chloroplasts.
2. The plant absorbs light from the sun, which leads to a reaction in the chlorophyll causing it to split the water molecules into their constituent parts of hydrogen and oxygen.
3. As a result of this reaction, the chlorophyll converts some of the energy absorbed from the sun into a simple form of sugar (C₆H₁₂O₆ in the illustration above). Meanwhile, the oxygen that has been liberated from the water finds its way out of the plant’s leaves back into the atmosphere.
4. The simple sugar molecules then combine with the hydrogen molecules that came from the water and together these create more complex forms of sugar, which are capable of being used to build the cells that form the plant’s structure.
5. These complex sugars then combine with water and nutrients that the plant takes up from the soil to form the various parts of the plant such as its roots, stems or flowers.

And that is pretty much it.  Combine sunlight, air, water and soil with the magic of chlorophyll and you have something that can provide you with shelter, beauty, warmth and food.

If we now turn to look at the main individual parts of the plant, we can gain an even better understanding of how plants grow and what we need to do as gardeners to help plants grow in the way that we want them to.

The Parts of Plants

Roots, leaves, stems and flowers are the principal parts of most plants (conifers and ferns are slightly different in that they don’t technically have flowers). These are what the process of photosynthesis creates.

Now that we understand how these come into being, we need to understand how they function, so that we can get the full picture of how plants work.

Roots

Roots are the underground parts of a plant. They have three main functions.

Firstly, roots anchor the plant in the soil.

They do this either by sending down long, sparse tap-roots deep into the soil or by spreading a complex network of fibrous roots in the soil near the surface around the plant.

Most plants use one or other of these tactics, but some use both.

Secondly, roots take in nutrients and water from the soil around them. These, as we have seen, are essential for photosynthesis and for the continuing growth of the plant.

At the tip of each root is an area of hardened cells called the root cap. This allows the root to burrow through the soil without damage.

Just behind the root cap, is a band of microscopic hairs. The root takes in the soil solution of water and nutrients through these hairs.

Thirdly, roots store food and energy.

Root vegetables, such as carrots are good examples of the storage capacity of roots. Bite into a fresh carrot, which is basically a swollen tap-root, and you can taste the sweetness of the stored sugars.

This stored energy is also what allows perennial plants that die back in winter to regenerate each spring.

Roots and the gardener

Where roots are concerned, the gardener’s job is to take care of the soil they are planted in, both at the time of planting and as the plant is in growth.

Roots need open (not compacted) soil, so that they can grow out and down and so that they do not become trapped in a waterlogged environment.

They need soil with an adequate supply of nutrients and they need enough, but not too much, water.

Soil cultivation is in fact one of the most important gardening tasks. If your plants’ roots are anchored in good, healthy soil everything else you do in the garden is likely to be a whole lot easier.

Stems

A plant’s stems form the framework from which it grows other parts that are essential for its continued life, both as an individual specimen (its leaves) and as a species (its flowers).

The framework on most plants usually consists of main stems and branches.

At the tip of each stem is the growing point – the apical meristem. This is the part that produces new leaves, buds and stem sections (or nodes) as the plant grows.

At the junction of each node you will usually find leaves and, between the base of the leaf stalk and the main stem, buds.

These buds are called axillary buds and it is from these that branches grow. Not all axillary buds will grow into branches. Most remain dormant, available for growth if the apical meristem is damaged or removed by pruning.

Apart from their structural function, stems also contain the tubes (called phloem and xylem).

Xylem transport water, sugars and nutrients up the plant from the roots up to the leaves and stems. Phloem transport the substances created by photosynthesis to the points where they are needed – e.g. the roots and the growing points of the plant.

Stems and the gardener

A gardener needs to understand these facts about plant stems in order to be able to bring some control to the plant by pruning it.

Pruning involves removing parts of the plant’s framework to achieve a desired outcome.

So, we prune to encourage leafy growth, which is achieved by removing the growing points from each stem, so as to induce the dormant axillary buds to grow.  This, for example, is what we do with hedges.

We also prune to maximise flowers or fruit. In this case we remove side shoots, so that all the plant’s energy can go into the remaining flowering (and fruiting) stems. This is what we do with tomatoes.

In all cases, bearing in mind the role of stems in carrying essential fluids around the plant, we should always cut back to just above a bud, so that there is less risk of the plant being damaged by infection entering the open wound.

Leaves

Leaves were the evolutionary development that enabled plants to come to dominate the earth.

They represent a scientific and aesthetic wonder.

Just consider how incredible it is, both as a matter of botany and beauty, to see a huge tree completely transformed in spring by its new green foliage.

Leaves are where photosynthesis takes place and this process is driven by light.

Leaves, therefore, have evolved to carry out the function of light collection.

This is why individually and/or collectively they present a large surface area to the sun. It is also why they are thin enough to let light penetrate into the their cell’s interiors.

Leaves have veins and midribs that carry water from the plant’s stem to the leaf cells and also carry sugars away from the cells for storage.

They also have microscopic openings, usually on their undersides, called stomata. These allow for the entry of carbon dioxide.

These stomata also allow water, in the form of water vapour, to escape from the leaves in a process called transpiration.

As water exits the plant through the leaves, the pressure in the plant’s xylem causes more water to be drawn through the system from the roots to replace the water that has been lost.

This is how the plant transports the nutrients that are drawn in at the roots to those parts of the plant where they are needed for growth.

If, on a hot day, you’ve ever felt how much cooler it is to stand in the shade of a tree rather than under man-made structure, then you have felt the power of transpiration and the cooling effect of the water vapour surrounding the tree’s leaves.

Leaves and the gardener

Leaves are very often what we cultivate the plant for, whether it be the edible properties of leafy vegetables, the aromatic properties of certain shrubs or the screening properties of hedging plants. 

Leaves are like plant barometers. The can tell a gardener a lot about a plant.

If they drop or become discoloured, we know that there may be something wrong with the plant’s growing conditions. So, for example, a yellowing of leaves, will often tell us that the plant is in need of nitrogen (which we must add to the soil).

Leaves can also tell us something about a plant’s character and preferences.

Plants with silvery, shiny or succulent leaves, or leaves covered in small hairs, are often well suited to hot, dry conditions. Those leaf characteristics are the way that the plant has adapted to be able to thrive in those conditions.

Softer (and therefore thinner) leaves or leaves of dark green (which attain that colour because of a greater concentration of chlorophyll) are often found on plants that prefer shade. 

Those adaptions enable the plant to photosynthesise, even in low light conditions.

Flowers

Flowers are what most plants and many gardeners live for.

Flowers are the means by which a plant engages in sexual reproduction. This is what plants have evolved to do. We, as gardeners, take advantage of this evolutionary urge when we grow plants for their flowers.

Plants contain male parts – the anthers and filaments (together, the stamen) and female parts – the stigma, style and ovary (together, the pistil).

The anther (orange in the picture) produces pollen, which is transported from one flower to another by means of insects, birds or wind.

When the pollen lands on the ‘sticky landing pad’ part of the pistil, called the stigma (light green in the picture), it is transported down to the base of the pistil to the ovary. There it fertilises the undeveloped seed (or ovule).

Fertilisation causes the flower to lose its petals as their job in attracting pollinators is done.

Seeds can develop in several ways, but one we are all familiar with involves the expansion of the ovary to produce the fruit that houses the fertilised seed. When the fruit drops it may be eaten by an animal but the seed will usually stay intact.

Once the animal digests the fruit, the seed is emitted within the animal’s droppings. In this way, the plant’s seed is spread to new locations and deposited with a helpful starter pack of nutrients in the form of the animal’s manure.

Seeds are also dispersed by the seed pods popping open and/or by wind blowing seeds off the plant.

Flowers and the gardener

Oddly enough, flowers can be precisely what the gardener tries to induce a plant to do or the very thing that the gardener wants to prevent.

We welcome the rose or the lily bud but look to prevent our leafy vegetables from flowering because once they do, the taste we grow them for is lost.

So, whether we grow plants for flowers or want to prevent flowering, we often adopt a specific feeding and pruning regime to encourage whichever outcome we seek.

But, taking the case of ornamental plants for the moment, it is very often the flower colour, size and shape that dictates whether and where we include a plant in our garden.

Part of the great creative art of gardening lies in combining plants with complementary colours that flower at the same time or planning the succession of flowering plants, so that a garden might be full of colour for a whole growing season.

Finally, flowers provide gardeners with the wonderful gift of seeds. If you have the patience, it is hard to beat the satisfaction of looking at a well-grown mature plant that started its life as a seed in your own hand.

Conclusion

These basic facts about how plants grow and about the functions of the various main parts of plants are the foundational knowledge for almost everything else in gardening that is connected with raising plants.

For example, when we prune plants we usually do so to restrict or promote growth or to restrict or promote flowering or fruit production.

Understanding the relationship between the plant’s main stem, its buds and branches enable us to carry out these tasks to achieve the ends we seek.

There is a lot here to understand I know (and there is also a lot more that I haven’t included). But once you know this stuff, it will give you a solid foundation to your gardening knowledge.

In the next step in this series for the beginner gardener, I’ll look at how to assess your own garden, in the light of what you now know about the needs of plants.