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.
- 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...
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