Science of Leaves: Anatomy, Arrangement, & Adaptations

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As we go about our day, leaves are all around us, yet most of us don’t stop to take the time to consider how important they are for plants. They’re not just there for decoration, leaves are essential to the survival of the plant and they come in all shapes and sizes.

What is the Purpose of Plant Leaves?

Every part of a plant is functional. Nothing is there by chance, and that includes leaves. Leaves grow from the stem of a plant and are typically green in color, and this is due to a chemical known as chlorophyll.

Leaves serve a multitude of vital functions for plants, including the absorption of nutrients and water, as well as facilitating the process of respiration.

Photosynthesis

Photosynthesis is something that most people have heard of but many aren’t aware of what it actually is. However, this is one of the most important processes for a plant and, without it, the plant would surely die.

Plants use photosynthesis to produce food for themselves. Unlike humans and animals, plants are unable to move around and therefore cannot get food in a traditional manner. Instead, they convert carbon dioxide, water, and sunlight into nutrients.

The leaves of the plant are the main organ it uses for photosynthesis to occur. But how does this happen? Let’s take a look at the steps of the process in a little more detail:

1. In the leaves of the plant is a compound called chlorophyll which it uses to absorb sunlight.
2. Plants have xylem which is similar in function to human veins. They carry water and nutrients that the plant takes from the soil using its roots, to other parts of the plant, including the leaves.
3. Leaves have microscopic holes called stomata and it’s through these pores that carbon dioxide is able to enter. In turn, the plants ‘exhale’ oxygen; the complete opposite of how we breathe.
4. Now that the plant has all the elements it needs for photosynthesis, a chemical reaction begins through which things like glucose are produced.
5. The plant has another set of ‘veins’ called phloem, and these transport the freshly made food around the plant, delivering important nutrients.
6. The waste product produced by the plant is oxygen, so you could say that we have a symbiotic relationship with all plant species.

Transpiration

Plants have roots that they use to draw up moisture from the ground. As I have already discussed, this water is then transported around the plant via the xylem. However, sometimes, the plant will send too much water to the leaves, so any excess is evaporated in a process known as transpiration.

Those little holes in the leaves called the stoma, that we talked about earlier, are responsible for this process.

That said, while it is important for plants to get rid of excess moisture, it can sometimes be to their detriment. In cases where the stomata are open because photosynthesis is taking place, water can escape when it doesn’t need to. That’s why there are some desert plants that don’t have stomata or whose stomata remain closed to avoid losing precious water.

Guttation

Another way that plants lose water is through a process known as guttation. You may have seen this without realizing; for example, the drops on a plant or those little droplets on your house plants. However, it is important to remember that, while guttation largely happens overnight, it isn’t the same as dew, which is a collection of moisture from the surrounding air on the leaves.

Despite people seeing these droplets on their plants, guttation isn’t actually a bad thing and is the sign of a healthy plant. You see, during the day, when the plant is photosynthesizing, the stomata are open so transpiration can happen. However, these holes close at night, so the plant is unable to release excess water this way.

It compensates for this by using pressure to force moisture upwards, and it comes out as those little drops you see on the leaves.

Internal Leaf Structure

Leaves are pretty amazing things, but what we see from the outside is only part of the structure. From the outer waxy parts that prevent water loss to the leaf ‘veins’ that can be found much deeper, every part of the leaf plays a crucial role.

Waxy Cuticle

The waxy cuticle is the uppermost layer of the leaf and is found in almost all instances. This thin layer is there to prevent the leaf from losing too much water through the process of transpiration. The waxy cuticle will also stop external water sources from penetrating the leaf, which is why you typically see water beading on a leaf as opposed to being absorbed.

Upper Epidermis

The upper epidermis contains the first true layer of cells in the leaf and is another level of protection against water loss. The cells are very closely packed to prevent this and, depending on the climate, this layer may be much thicker. However, despite this thickness, the upper epidermis is still thin enough for sunlight to get through.

Palisade Mesophyll

Next up, we have the palisade mesophyll which is a layer that is much thicker and the cells have a more column-like structure. Sometimes, this layer is called the palisade parenchyma and it is bursting with chloroplasts as this is where all the action takes place for photosynthesis.

Spongy Mesophyll

Where the palisade mesophyll contains very tightly packed cells, the spongy mesophyll has quite loosely packed cells. What’s more, they have a much more irregular shape. Some chloroplasts can be found in this layer as some photosynthesis does take place in the spongy mesophyll but not as much as in the previous layer. One of the main functions of this layer is for gas exchange which is easy here because of the space between the cells.

Vascular Bundle

The vascular bundle actually sits alongside the spongy mesophyll and is made up of various types of tissues.

Firstly, there is the xylem which are tubular ‘veins’ that have an upwards flow. These tubes allow for the transportation of water and nutrients from the roots to the rest of the plant.

We then have the phloem which is another type of vascular structure. However, unlike the xylem, which only flows upwards, the phloem has both an upward and downward flow. Moreover, where xylem transports water from the roots, phloem transports the products of photosynthesis.

These two types of cells are surrounded by what is called the bundle sheath. In some, but not all plants, this sheath is able to perform a unique type of photosynthesis and this is something we often see in desert plants.

Lower Epidermis

The final layer of the leaf is the lower epidermis. Much like the upper epidermis, one of the main jobs of this layer is to stop the plant from losing moisture. It is within this layer that we find the stomata, or stoma if we are talking about them singularly. These allow the plant to breathe, and with guard cells on either side, the opening can close when necessary. For example, when it’s hot, the stomata will typically remain closed to prevent losing too much water.

Leaf Anatomy

Have you ever stopped to take a look at a leaf? You’ll be familiar with the concept that they’re all different shapes and sizes, but underneath it all, they have very similar structures. Let’s explore the anatomy of a leaf.

Apex

The apex refers to the very tip of the leaf, and this is where we can see some real differences between species. There are typically four types of apex, including those with a long tapered point, known as acuminate apexes, bristles, rounded, and acute. Many people use these differences as a way of identifying leaves from various plants and tree species.

Blade/Lamina

The blade often referred to as the lamina, is the whole outer surface of the leaf. This is the flat, green part of the leaf where you can often see the vascular system. The layout of these veins is known as venation and occurs in two different formations:

• Reticulate venation can be seen on plants like coriander, basil, and papaya, and see that the veins form an irregular pattern.
• Parallel venation, as the name suggests, is where the veins are laid out in a parallel pattern and can be seen on species like rice plants, iris, lilies, and various species of grasses.

The main purpose of the leaf blade is for photosynthesis as well as the process of transpiration.

Margin

The margin is the outer edge of the leaf, and just like the apex, this can come in many different forms, which allows us to identify different trees and plants.

Entire margins are smooth and uninterrupted whereas lobed margins have rounded sections all the way around. There are two different types of toothed margins; the serrated margin which has saw-like teeth and the dentate margin whose teeth are more triangular in shape.

Veins

Yes, just like humans and animals, leaves have veins. But instead of blood, it’s water, food, and nutrients that run through them. These veins can often be seen on the leaves and form various patterns, as mentioned above.

There is usually a main vein that comes from the leaf stem, with several other, smaller veins coming off this.

Midrib

The midrib is the main vein that can be seen running along the length of the lamina. This is typically connected to the petiole; another word for the leaf stem.

Base

Leaves are totally unique between plants so not only do they have differently shaped laminas and apexes, but the base of the leaf will also be unique to the species.

The base is the lowest part of the leaf and is where the petiole connects to the lamina. There can be wedge-shaped bases, those that are rounded or squared, and even leaf bases that are different on either side of the midrib.

Petiole

The petiole is something that we will all be familiar with, but not everyone is aware of its name. It’s usually just referred to as the leaf stalk and is that part of the leaf that connects the stem of the plant to the blade.

However, some plants have leaves that connect directly to the stem, and these are called sessile leaves.

Stipule

The stipule is a small leaf-like structure found at the base of a leaf stalk (petiole) in many plants. They can vary in shape and function, serving purposes such as protection to the emerging bud or leaf while it is developing as well as water conservation.

Leaf Type Arrangement

Leaves are arranged in one of two ways; simple and compound. Although there are various subtypes of compound leaf arrangement.

Simple Leaf

You will find a simple leaf arrangement on things like the banana plant, the mighty oak, and the maple tree. These are plants where there is a single blade per leaf with no indentations or cuts that reach the lamina or the midrib.

Each leaf is directly attached to the stem via the petiole, and while you may see some examples of simple leaves, like those on the oak tree, with slight divisions, they are not complete and do not make contact with the midrib.

Compound Leaf

You’ll find compound leaves on a variety of different species, including the clover, the rose, poison ivy, baobab trees, and the horse chestnut.

These leaves are divided into smaller leaflets. While they all connect to the same lamina, so may appear to be separate leaves, the indents and cuts between them are actually just gaps between one leaf.

However, this is where it gets even more confusing because depending on how the leaflets are arranged and the type of veins, there are two further subtypes of compound leaves.

Pinnately Compound Leaves

Pinnately compound leaves look almost like a feather, with the leaflets arranged along the length of the midrib. Some common examples of plants and trees with pinnately compound leaves are the ash, pecan, and walnut trees as well as the many species of ferns.

They have a symmetrical pattern along the midpoint and can be further categorized as follows:

• Imparipinnate leaves are laid out in pairs along the midrib but only have a single leaf at the very tip.
• Paripinnate leaves are laid in pairs along the midvein, and at the end of the vein, there is one final pair.

Palmately Compound Leaves

Plants that have palmately compound leaves are the holly, pear tree, silver birch, some species of lime trees, and the clover, as well as many others.

Palmate leaves feature several leaflets, but they all originate from the same point, which is usually the end of the leaf stem. There is clear differentiation between each leaflet, and there may be various numbers of leaflets, depending on the species.

Plants with one leaflet are called unifoliates, those with two are called bifoliates, and so on. Anything with more than five individual leaflets is called a multifoliate leaf.

Leaf Arrangement on Stem

When you look at the arrangement of leaves on a stem, you will notice that the placement is different between species. The arrangement of leaves on the stem is known as phyllotaxy.

Opposite

Ash trees, maple trees, and olive trees all have opposite leaf arrangements. This is where there is a single node in the middle of the stem from which two leaves arise; one on either side.

Alternate

You’ll see alternate phyllotaxy of plants like the black walnut, smoke bush, and barberry. This formation is where only one leaf comes from each node. The leaves grow alternately on either side of the midvein.

Whorled

The lemonwood, the blackboard tree, and the mountain laurel are all great examples of plants that have a whorled leaf arrangement. Here, we see as many as three leaves arising from each node, and they may be in an alternate or opposite pattern.

Leaf Shape Types

Earlier, we looked at the various shapes and designs of separate parts of the leaf. But when everything is combined, it’s not difficult to understand that there is great diversity in the overall shape of a leaf. Here are some of the various leaf shapes explained.

Acicular (Needle-Shaped)

These leaves are needle-shaped, and some examples include the Japanese cedar, and the jack pine.

Cordate (Heart-Shaped)

With a deep base notch, cordate leaves have a heart-like shape. They can be seen on pond lilies, dogbane, and the eastern redbud. 

Cuneate (Wedge-Shaped)

These leaves are wedged-shaped, narrowing towards the base and broadening towards the apex. Examples of plants with cuneate leaves include some species of oak such as the white oak, star magnolia, and the gray dogwood.

Elliptical (Ellipse-Shaped)

As you may guess from the name, the shape of these leaves resembles an ellipse, wide in the middle with tapered ends. You’ll see them on plants such as sage and mint.

Hastate (Two-Lobed Sagittate)

With an arrow-like shape and two outward basal lobes, hastate leaves grow on plants such as caladium.

Lanceolate (Lance-Shaped)

These lance-shaped leaves can be seen on various types of willow trees, including the weeping willow and the white willow.

Linear (Slender, Narrow, Straight)

These straight leaves tend to be a little broader than other types. They grow on plants such as the cornflower and many species of grasses.

Lyrate (Lyre-Shaped)

A lyre is an old-fashioned stringed instrument, and some leaves take on this shape. Examples of this are the ficus lyrata.

Oblique (Unequal Base)

Oblique leaves are unequal on either side of the lamina. You will see examples of this leaf type on the, begonia, common hackberry and the American elm.

Oblong (Rectangular)

Oblong leaves have parallel sides and are longer than they are wide. Plants that have oblong leaves include the holm oak and the cherry laurel.

Orbicular (Orbit-Shaped)

These leaves are circular in shape and are found on plants such as water lilies, and the nasturtium.

Ovate (Oval)

Ovate leaves are egg-shaped or oval. The apricot tree is an example of a plant that has this type of leaf.

Reniform (Kidney-Shaped)

Leaves that are described as reniform have a kidney-like shape and can be found on plants such as the geranium and the indian-pennywort.

Rhomboid (Diamond-Shaped)

These leaves have the shape of a rhomboid. A common example is the black poplar.

Sagittate (Arrowhead-Shaped)

These leaves are shaped like an arrowhead and you’ll find them on various species of vines as well as many aquatic plants.

Spatulate (Spoon-Shaped)

These leaves start off narrow and widen as you reach the apex. Examples of plants with spatulate leaves include the common dandelion, and certain species of succulents like the jade plant.

Examples of Evolutionary Leaf Type Adaptations

Trees and plants have been around for millions of years. The conditions over time have changed, and this has caused all living things to evolve in order to adapt to their surroundings. This is no different where leaves are concerned, and they have developed some interesting adaptations.

Needle-Shaped Leaves

Needle shaped leaves are seen on a lot of species of coniferous plant such as the spruce, pine, and fir. These trees do not drop their leaves during fall, and one of the reasons for this is that the leaves are important in keeping snow off the tree during the winter.

It’s also been observed by scientists that leaf size gets smaller the further north you travel. The leaves in tropical rainforests and around the equator are much larger compared to the needles seen on trees at the far reaches of the northern hemisphere. It’s thought that this is an adaptation to keep out the cold, especially when temperatures dip at night.

What’s more, having very thick cuticles, these fine leaves are able to resist transpiration far more easily, so are able to conserve water. Coupled with this, the leaves have much less prominent stomata to further prevent moisture loss.

The adaptation of resin within the needle-shaped leaves helps to protect the plant and ensure it does die. It does this in two ways. Primarily, this stops the leaves from freezing, which could lead to the death of the tree. Furthermore, this resin acts as a deterrent to would-be herbivores looking for a tasty snack.

Spine-Shaped Leaves

Take a look at various types of cacti, such as the Saguaro cacti, for example, and you’ll see that instead of leaves, they have spines. But this is actually a type of leaf, just one that has adapted for desert conditions.

Spine-shaped leaves project from the plant and are super sharp which is one of the ways the plant is able to protect itself from predators. What’s more, the leaves cast shade over the plant to protect it from the hot sun.

What’s more, with a smaller surface area and stomata that remain closed, spine-shaped leaves stop the plant from losing too much water. They’re a characteristic of things like the acacia, which also benefits from ants living in its spines in a very interesting symbiotic relationship.

Succulent Leaves

A succulent is a type of plant that has very thick and fleshy leaves. If you’ve ever seen the lithops, also known as the living stone, you’ll understand how thick these leaves can be. Other common examples include the aloe vera plant and the baby rubber plant.

Normally, these plants grow in arid areas with a hot climate. Their leaves allow them to store water for months or even years, in some cases. This means that they won’t succumb to the detrimental conditions but can still perform photosynthesis like any other plant.

Leaf Tendrils

Climbing plants climb up structures for added support as they are typically too weak to support themselves. You may grow these types of plants in your garden and a common example is the sweet pea.

These tendrils are threadlike modifications of the leaves that not only help to support the plant but also ensure it is well anchored. This prevents it from blowing around and becoming damaged in the wind, and any part of the leaf could turn into a tendril from the petiole to the entire leaf.

Insectivorous Leaves

Most plant species get their nutrients from photosynthesis, but there are some that live in environments where this isn’t possible. In areas where the soil is lacking in nutrients or there isn’t a lot of sunlight, plants have evolved to become meat eaters and they’re known as carnivorous plants.

The most well-known example of this type of plant is probably the venus fly trap which has sensitive leaves which, when touched, snap shut, trapping the insect inside. The plant can feel the insect due to sensory hairs that are all over the leaf. Contrary to popular belief, these plants don’t continuously feed on passing creepy crawlies, and it can take up to five days for them to digest a meal, sometimes going months in between feeding.

There are other carnivorous plants whose leaves have evolved into the shape of a pitcher, the pitcher plant being a prime example of this. Inside this cup is a digestive liquid, and once an insect falls inside, it’ll be ‘guzzled’ up by the plant.

The sundew plant could almost be called an active hunter since when an insect gets trapped in the sticky hairs, the plant wraps its tentacles around it so it cannot escape. The insect suffocates while the plant absorbs all those important nutrients.

Why Do Deciduous Trees Shed their Leaves During the Fall?

There are two types of trees in the world; coniferous ones that keep their foliage all year round and deciduous trees that lose their leaves in the fall.

The main reason that the tree does this is to conserve resources like nutrients and water during the seasons when they aren’t going to be as readily available. It’s all to do with a hormonal reaction within the plant that happens in response to cooler temperatures and lower levels of light. When the hormone, auxin, is released, it causes the leaf stems to weaken, and the leaf falls away from the tree.

What’s more, when the sun disappears and things get colder, the plant stops producing chlorophyll, which is what keeps the leaves green and aids in photosynthesis. A lack of this is what causes the leaves to lose their color. With chlorophyll out of the way, auxin has its time to shine.