One of the biggest differences between the plants we grow in our gardens and the ones we see in nature are the leaves. Go out into the woods or fields, and every plant you meet will have green leaves, right? Big leaves, small leaves, needle leaves; dark-green, light-green; blue-green, sure. But always green. Walk around the average garden and not only will there be plants with red leaves or golden ones, there will be all those leaves with both yellow and green on the same leaf, maybe as a border around the edges, or in stripes, spots, and other patterns. These patterns are all described by the term variegation, which just means ‘variety’, ‘not all the same’, so it’s a handy word to cover any differences in the surface appearance of a leaf. These patterns are actually surprisingly common, and one study found variegations in 1,710 different plant species, from a total of 78 plant families. That might make you think it isn’t rare, until you realize that number is less than 0.1% of the estimated total number of flowering plants (about 250,000 species). Variegation is a rare thing outside of our gardens.
So what’s going on? What is it with these variegated leaves, and how did they get that way?
This is a question that has occurred to botanists and plant scientists too, and they have some answers – let’s take a look at some of them.
Before doing that, let’s look at an obvious side question here. Why are these plants, with a few very rare exceptions, found only in gardens, and not growing wild? The answer is that the things that make these plants look as they do are rare events, so you can’t expect to see them very often, or hardly at all. Perhaps more importantly, these plants are almost always ‘less fit’. The reduced growth most show means that in nature they would soon be outgrown by their more vigorous brothers and sisters, and they need the help of you and me to keep going. I bet you never before thought of your garden as a ‘care home’ for the misfits and less-able members of the plant world!
A Bit of Basic Leaf Anatomy
To understand the issue of variegation better, it helps to have a basic idea of the structure of a leaf. Really, a leaf is like a sandwich. On the top and bottom are two thin layers of cells (the epidermis) covered with a protective and waterproof coating (the cuticle). These cells contain no chlorophyll, and are almost always as clear as water. Between these ‘slices of bread’ is the filling, at the top a layer of roughly cylindrical, close-packed cells, arranged upright, like a shelf of drink cans. These contain most of the chlorophyll, inside small green structures called chloroplasts. Beneath that a more open area, of loosely-packed cells, like sponge, with lots of air spaces and cells that also contain green chloroplasts.
Of course these text-book diagrams often don’t match the reality of the much more complex organization found inside specific leaves, but they give us a rough idea.
Plant Meristems and Leaf Primordia
Another part of the plant very relevant to the development of variegation patterns is the meristem. A meristem is a cluster of cells which function as the source of the parts of the plant, while also continuing as more of themselves. As the cells divide, some become stem or leaf, while others become the continuing meristem. It’s a system not unlike human stem cells.
Those that grow of to become leaves are called leaf primordia (singular, primordium)
In this diagram SAM is the shoot apical meristem, while LP is a leaf primordium. P1 and P2 are emerging leaf primordia (color enhanced for clarity).
There is a coherence to this development, so almost always the outer edges of the primordia remain as the outer edges of the leaf, and the inner layers of it remain in the same orientation as they become the inner layers of the leaf. That’s important in the development of variegation.
Different Types of Variegation
Just as the patterns we see and call ‘variegated’ are enormously varied, so the structural and chemical causes are varied too. Most botanists recognize four different basic causes, or types, but among garden plants it is the first – cells lacking chloroplasts – that is the most common.
Chlorophyll type Variegation
Patterning of this kind is certainly the most common type of variegation found in garden plants grown for the patterns of white, cream, yellow or gold that decorate their leaves. The pale color is because the cells in that region of the leaf don’t have the functioning chloroplasts they should have. [Worth noting here that each plant cell is stuck with the chloroplasts its starts out with. It can’t make replacements because chloroplasts are quasi-independent bodies, with their own genetic system.]
Since the meristem that is making stems is so internally organized, a mutation or error in just one cell in a specific region will produce the same repeating pattern in every leaf primordium that it produces. So no chloroplasts in the ones that become the outer parts of the leaf mean a leaf with a white margin – marginal variegation. If they make the central part of the leaf, then the white will be in the central part – central variegation. If it should be a bit more scattered in the primordia, then random variegation patterns can be the result. You can see from this array of leaves that most patterns we see are garden plants can be placed in one or other of these groups.
If you are wondering why plant ‘g’ in this array is shown as random variegation, when it looks at first glance a lot like plants ‘i’ and ‘j’ in the marginal group, look again. See how there are areas where the green is darker and others where it is lighter? That’s because those areas are a mix of green and white cells, randomly mixed together.
Why are Some Variegations White and Others Yellow?
Looking at variegated plants you soon notice that sometimes the ‘un-green’ part is white, and sometimes gold. There are also lots of plants that have yellow rather than green leaves, but where the whole leaf is yellow, without any patterning. Yellow is due to a lower number of chloroplasts that normal, while white indicated they are more or less completely absent. So an all-white plant would quickly perish, but yellow ones survive, growing more slowly.
Another Fundamental Cause of Variegation Pattern Differences
There is another fundamental characteristic of plants that leads to something else that’s very interesting in the variegation patterns of our garden plants. You may already know that plants are divided into two big groups. One group are called Dicots, and the other are called Monocots.
Dicots – this name is short-hand for di-cotyledon. Cotyledons are the very first leaves that a seed produces when it germinates. If you have ever grown vegetables or flowers from seed you might have noticed that often a little pair of round leaves sit atop a short stalk when the seedling first appears.
Monocots – short-hand for mono-cotyledon. If you have grown vegetables, you might have sown onion seeds and noticed instead long, thin, single seedling leaf.
The differences between these two groups doesn’t stop there, though. There are fundamental differences in basic aspects of the structure and form of plants in this group. In fact, it’s a long-standing traditional division in the flowering-plant kingdom to place plants in one or other of these groups. This table shows all the important differences, but only the last one is significant when it comes to patterns of variegation
|Onions, lilies, grasses, hostas||Roses, maples, geraniums, petunias|
|Flower parts in multiples of three||Flower parts in multiples of four or five|
|Stem vascular bundles scattered||Stem vascular bundles in a ring|
|All roots are adventitious||Roots develop from a single radicle|
|Presence of a cambium under the bark||No cambium in stems|
|Major leaf veins parallel||Major leaf veins reticulate|
What the last row is saying is that if you look at the veins on the leaf of a grass, you will see that the veins run in straight lines along the leaf, from base to tip. They also do that in hosta, although the broadness of the leaf causes curving, so it takes a moment to see the ‘parallel’ nature of those veins. Now look at a rose leaf. You can see a long central vein, with branching veins spreading outwards from it. Look at that on a maple, or a geranium. There the veins spread out like fingers on a hand, but again they are net-like (= reticulate), dividing and splitting as they cover the leaf surface.
Variegation in Grasses and Iris
If you have variegated grasses in your garden, or some of the lovely variegated Iris, you will notice that many have a different patterning to their variegation. Often there are alternating bands of green and white running the whole length of the leaf. Only occasionally do we see just a green center with white margins, or the reverse, although they exist. Of course if you think about it, that patterning is just a simplified vertical banding anyway.
You can see very clearly in these close-ups of variegated grass leaves how the leaf primordia in the meristem grow in straight lines, carrying the lack of chloroplasts along the whole length of the leaf. See the same thing in the picture of the variegated Japanese iris, also a monocot – you can see that because the flower parts are in threes.
Although monocots, the patterning in hosta resembles more closely the types seen in dicots because the broad leaf confuses the picture a bit. It is because of this ‘dicot’ leaf-shape on a monocot plant that there is such an abundance of different patterns in hosta leaves.
Virus Induced Variegation
Usually the cause of variegation can be traced back to mutations in the cells, causing the absence or reduction of chloroplasts. But not always. A second cause can be the presence of viruses. This microscopic ‘things’ hovering in a region between living and dead, can affect cells, but not enough to cause disease. The most well-known is seen in the flowering maple, Abutilon pictum, and also in Abutilon megapotanicum, which is often called trailing abutilon.
It is also possible to have ordinary chlorophyll type variegation in Abutilon too, which you can see for comparison in the right hand plant, Abutilon ‘Souvenir de Bonne’. The plant on the left is very different looking and is Abutilon pictum ‘Thompsonii’. The bold splashes and spots of yellow are areas with reduced chloroplasts, which has been caused by the Abutilon mosaic virus. This virus, harmless to the plant, people, animals or anything else, does reduce the growth rate, but these vigorous plants still grow rapidly. The virus is naturally transmitted by a small sap-sucking insect called the sweetpotato whitefly, Bemisia tabaci. Adults fly from plant to plant, carrying the virus with them. If you have a plain-green flowering maple, you can turn it yellow by rubbing some sap from a yellow plant into a cut on the stem. It is more often done by grafting a small piece of the variegated plant low-down on the stem of a green one. In a few weeks the whole plant turns golden.
Other Causes of Variegation Patterns
This next picture shows variegation in geranium leaves, which you see, as dicots, are quite different from those in the grasses.
You can see here typical marginal variegation in the middle picture, but the outer ones segue us nicely into the next cause of variegation.
Pigment type Variegation
In the center of every plant cell is a big bag of water, called the vacuole, containing minerals that keeps the cell ‘turgid’, blown up and rigid, like a balloon full of water instead of air. This liquid is usually clear, but sometimes it has pigments in it. That’s why carrots, or beetroots are the color they are, and not white like most roots. The same can happen in leaves – think of all the shrubs we grow that have red leaves. That is due to pigments in the vacuole, and their intense color obscures or modifies the basic green of the leaf.
What happens when mutations switch on or off the formation of pigments in the leaf primordia? You can see what happens then, in these geranium leaves. The ones on the left and right have the regular chlorophyll type marginal variegation, with a yellow margin around a green center. But in addition there is a pigment variegation pattern that affects the middle zone of the leaf. You can clearly see how it is overlayed on the chlorophyll variegation, and independent of it. Here are other examples in Elephant Ears, Caladium on the left, and Coleus on the right.
Airspace type variegation
We see in some plants variegation patterns that have a distinct character and don’t appear to be caused by pigments or the absence of chlorophyll. This type of patterning often has a silvery appearance, and not a distinctive coloring. Leaves are very variable inside, and in some plants there can be areas, often along the veins, where spaces occur between the epidermis (outer layer of the leaf) and the inner tissue. We see this type of variegation in many beautiful begonias. With this type of variegation, all the cells in the leaf have their normal quantity of chloroplasts, so, different from what we see in chlorophyll type variegation, there is little or no cost to the plant in terms of growth and energy from this type of variegation. (Nevertheless, these types of begonias can be tricky to grow, as many houseplant lovers will admit.)
Epidermis type Variegation
This last recognized type of variegation is where changes only in the thin skin – the epidermis – of the leaf are involved. It is much rarer, and hasn’t been studied very much either, but one example that gardeners can relate to is seen in the prayer plant, Maranta leuconeura. We see many different variegations in this plant. Some are obviously pigment type. In the variety shown here, though, called ‘Kerchoveana’, a study has shown that the different colors are caused simply by changes in the shape of the cells of the epidermis. In the whiter areas, the cells are flat, and there is also a second layer, called a hypodermis, of larger cells underneath them. In the greener parts the cells are smaller, and have small bumps called papillae on their surface. These difference clearly cause increased or decreased light reflection from the leaf, causing us to see the leaf as different colors.
Does Variegation Have any Value to Plants?
Although most variegation is found in garden plants, there are some that are seen naturally. These are often of the kind that creates white patches or spots. Very little work has been done on what advantage this might give wild plants, but one idea has been tested a little, and could be a reason. White spots on leaves can be the result of bird droppings – we have all seen this beneath areas used by birds to roost. Some studies have shown that plant-eating animals avoid eating leaves with droppings on them, and also avoid plants that look like they have droppings on them. So we could be seeing plants practicing mimicry, to protect themselves from being eaten – pretty smart trick! Looking back at the air-space type variegation in Begonia, which is ‘no-cost’ in terms of energy, it’s easy to imagine that some of this could be mimicry of droppings.
On the other hand we have already noted that most variegation causes reduced growth, so is counter-productive and unlikely to have natural value. We should also recognize this reduced ability when growing variegated plants. Typically they grow more slowly, so when you buy a hosta with large areas of white on the leaves, it will take longer to become big and strong than a similar plain-green variety will do. This doesn’t mean they are weaklings or runts – those would be quickly discarded by breeders. No, they just need a little more time to reach their full beauty.