A misty pattern forms in my mind
Of leaves dancing in the sunshine.
Leaves have been the subject of many a poem; around 500 of them, according to the website Poem Hunter. They also play a prominent role in our everyday language. One may, for instance, shake like a leaf or turn over a new leaf.
If neither fear nor a desire for change applies to you, you might take a leaf from someone's book who you admire or throw a fig leaf over something you'd rather nobody knew about you.
Who knew that leaves were far more useful than simply laying in their shade on a lazy summer day and listening to them rustle in the breeze?
Indeed, leaves of all shapes and sizes are the workhorses of nature. Besides providing food - not just for the plant they grow on but for a variety of herbivores and omnivores, leaves help clean the air and lower humidity. Even after their life is finished, leaves can be made into compost to improve the soil in gardens and farms.
That's just some of the work that leaves do. How a leaf functions, thanks to its meticulous design, is another matter altogether.
It's what Superprof talks with you about today.
The Parts of a Leaf
For such a seemingly simple appendage, leaves' inner structure is highly complex. We'll dive deeply into those aspects in a bit; for now, let's talk about the deceptive simplicity of what meets our eyes when we first take in a leaf.
Before we can get started, we have to correct a common misconception.
At first glance, leaves have two main parts: the flat, thin part that is veined and usually green, and the part that most people mistakenly call its stem. Its correct name is petiole and its purpose is to connect the leaf to its stem - the part the leaf and petiole grow out of.
The petiole also serves as a part of the plant's transport system. It's the last major leg of that system; it carries nutrients and water from the roots of the plant to the leaf and channels photosynthesized products from the leaf to feed the rest of the plant.
A third function for the petiole: keeping the leaf in position and connected to the plant.
Another surprise for the casual leaf admirer is the importance of the leaf base. That is the bottom ridge of the leaf that radiates out from either side of the petiole. It has to be especially tough to keep the leaf blade attached.
It also helps protect the axillary bud on the leaf's stem which, later, may develop into a flower or another branch or stem. Not every axillary bud will develop further but they still need protection as that is where the petiole connects to the stem.
Finally, we arrive at the main part of the leaf: the leaf blade. Also called the lamina, it is generally flat and thin, often green and divided into three parts: the apex, margin and veins.
We'll give a brief overview of those veins in the upcoming segment on the internal structure of leaves; for now, let's be happy with knowing that veins and the smaller venules are the last legs of plants' transport system.
You might wonder: "Why are the margin and apex so important?" The leaf's apex is where the midrib - that relatively thick spine that bisects the leaf terminates. Other than that, the apex is not terribly important, but it does help us recognise what type of leaf we're studying.
In some cases, it may also ward off predators.
Leaf margins serve an equally important/unimportant function. They may be rolled under (revolute margins) to make the leaf less accessible and accommodating to any insect that may want to snack on it. They may also roll upward (involute margins) to reduce the leaf's surface and trap moisture.
Other leaf margin forms include:
- repand: slightly, irregular waves
- sinuate: shallow margin indentations causing a slightly wavy leaf appearance
- crenulate: wave-like 'teeth'; somewhat like a dentate margin but with rounded teeth
- serrate: like a saw with its teeth pointing forward; nettles are a prime example of such
- dentate: a leaf with a toothed margin; chestnut trees have these leaves
- lobed: major lobes and indentations along the margins; oak leaves are a good example
- entire: the margin is completely smooth with no indentations, crenellations or incisions
There are, of course, other margin forms. As mentioned before, they don't necessarily serve a major purpose for the leaf or the plant that grows it but they certainly help us distinguish one plant from the next.
A Leaf By Any Other Name
Generally, when we think 'leaf', we envision lush green canopies concealing nests and other animal life. We generally don't think of cactus spines and pine needles as leaves but that's exactly what they are. Like any other leaf, they emerge from the apical bud on the stem of the plant and their purpose is roughly the same.
Cacti's spines operate a bit differently, though. During the day, they seal in the cactus' moisture while venting heat out and, at night, they open up to draw moisture from the cool night air. Also, these spines are far more effective in defending their plant from predators than the flat, green growths that trees and bushes bear.
The scales on lily bulbs and asparagus stalks are also other-formed leaves.
Biology challenge: which plant hormones control how a plant grows?
The Internal Structure of Leaves
Now well acquainted with leaves' appearance, let's look at a cross-section of leaf to talk about what's inside those shiny blades.
And, speaking of shine... A leaf's glossy upper comes from its topmost, waxy layer called a cuticle. It forms a protective barrier to the leaf to protect against water loss and makes it harder for predators to attack it, eat it and digest it.
Underneath the cuticle lies the epidermis. It is generally a very thin skin; only one cell layer deep. As with the cuticle layer, both the top and underside of the leaf has an epidermal layer. Within the lower epidermis are the stomata, each flanked with a pair of guard cells. These cells control the stomata's opening and closing. More on stomata in a mo...
The middle part of the leaf, where all the photosynthesis action takes place, is called the mesophyll. It is where the bulk of leaves' chloroplasts are found and it is made up of two types of parenchyma cells: palisade and spongy.
Directly under the upper epidermis, the palisade parenchyma (also known as palisade mesophyll) cells are vertical, column-like and tightly packed. They may be two or three layers thick, depending on the leaf's thickness. These cells are where photosynthesis primarily takes place.
Below them, loosely arranged and irregularly shaped, are the spongy parenchyma (spongy mesophyll). Their loose arrangement allows plenty of room for air; that is where the gas exchange takes place. The stomata, on the underside of the leaf, are instrumental to this process.
Below the mesophyll comes the stomata-riddled lower layer of epidermis and, sealing the leaf, another cuticle.
With all of that clear, we only need to lightly touch on the veins and venules. These vascular bundles are a part of the transport system but, as they also form a part of the leaf's structure, we should at least mention them, right?
These bundles are actually pair-routes called xylem and phloem. Each vascular bundle, no matter its size, carries such a pair.
Xylems transport water and minerals to the leaves via pathways called tracheids and vessels while the phloem carries the products of photosynthesis back to the plant's other parts.
You might say that every vascular bundle is a two-way traffic road.
The Function of Leaves
Like any other organism, feeding itself is a plant's prime directive. They draw nutrients from the soil they're rooted in but they gain the bulk of their sustenance through photosynthesis - the fundamental purpose and function of leaves.
Chlorophyll is a pigment that colours the leaves green; it promotes light absorption. That light energy is absorbed by chloroplasts in the central part of the leaf (mesophyll) and reacts with enzymes to break water down into its composite elements - hydrogen and oxygen.
During photosynthesis' enzymatic process, hydrogen combines with carbon dioxide to form the sugars that feed the plant. Oxygen becomes a waste product. The oxygen freed in this process is expelled through the leaf's stomata - the microscopic pores on the leaf's surface, as you surely remember.
Breathing is not the first thing one thinks of when considering plants but, just like any other living organism, it too needs air as well as food. It is through these pores that plants breathe. That is leaves' secondary function.
Now, learn more about photosynthesis and plant growth...