Leaves are so often taken for granted, but without them we would not be able to live.
They come in all shapes and sizes and are the part of a plant that makes the "food" for the rest of the plant. They can be simple, which means a single leaf blade. The leaf is then connected to the stem by a petiole. Examples of simple leaves are the oak and the maple. Fortunately for us, many leaves are breathtakingly beautiful and are a source of wonder at the amazing nature of, well, nature!
A compound leaf on the other hand is when a leaf is made up of a number of smaller leaflets. These compound leaves can stop you in your tracks and often demand our attention simply because of their intrinsic, complex beauty. The leaflets are attached to the stem just like a simple leaf - by the petiole. Ash trees have compound leaves. Some leaves do not have petioles at all and are attached directly to the stem. This kind of leaf is known as a sessile leaf.
Although leaves come in all shapes and sizes, they all have the same job! They are produced by a plant to catch light! They have openings so that water and air can come and go. On the outer surface of the leaf is a coating. This is called the cuticle and it protects the leaf, making it waterproof effectively! The cuticle also prevents water loss and protects the epidermis of the leaf.
The food making process of leaves is called photosynthesis. This is when four things come together - water, carbon dioxide, light energy and the green pigment or coloring of the plant which is called chlorophyll. The leaf gets is food from this process, glucose, but we get something very valuable to us. Oxygen! The air that you are breathing right now was brought to you with the help of a green plant somewhere!
Of course, plant organs such as fruit are much more nutritious than leaves, but many animals including humans eat them. We have to be careful, though, as some can be poisonous. Some animals eat only leaves and they are known as folivores. Obviously a plant doesn't much like having bits of it ripped off and chewed up by some dopey looking folivore, so many have evolved protection against this.
Many leaves contain tannin, a chemical which makes the digestion of protein very difficult and which can taste pretty nasty! Some species of animals use leaves to hide from their predators. Some caterpillars will fold a leaf over themselves and use it as a hiding place. Other insects have adapted to look like leaves so they won't be noticed by their enemies. One, the katydid even sways from side to side, mimicking the movement of a leaf in the breeze!
Whatever protection the leaf creates for itself, though, there are going to be some creatures that can't resist a good nibble! The poplar leaf above has been ravaged by a hungry visitor, who looks as if it was so taken with the leaf it didn't know which part to eat next!
As well as coming in simple and compound the base of a leaf can come in many different shapes. They can be wedge shaped, otherwise known as cuneate or cordate, which means that they are heart shaped. When the leaf comes to a sharp point which is long, it is known as acuminate. When the tip is pointed but not very long then it is known as acute. There are many other names for the base of the leaf, depending on its shape.
Some leaves get "hairy". It isn't a sign of plant adolescence, however, but these "hairs" which are properly known as trichomes can give a leaf more than eight degree of hairiness! These are known by a myriad of names (which can sometimes overlap) but various forms of trichomes are known as pubescent (short and erect!) scabrous (rough) and tomentose. Tomentose is when a leaf is densely pubescent, which is really quite a scary thought. Many leaves are, however, glabrous which sounds like a Shakespearian insult but means that they have no trichomes at all!
The mesophyll is the interior of the leaf and is sandwiched between the upper and lower layers of the epidermis. This is where most of the photosynthesis will go on.
The four leafed clover above shows off its venation perfectly - these veins are situated in the mesophyll and are the important conduits of important stuff! They are made up of xylem, which are tubes that bring water and minerals up from the roots.
The other part of this amazing structure is the phloem which the sugar and sap out of the leaf after photosynthesis. The phloem is usually below the xylem and both are embedded in a tissue known as pith. Very pithy, I'm sure!
A question that children ask all the time is why do leaves on trees change color in the Fall? Inside each leaf are millions of packages of color. Chlorophyll is contained in the green packages, xanthophyll in the yellow and the orange packages are carotene.
During the summer the green packages are at their busiest. The chlorophyll catches sunlight so it can be used as energy. With this energy the plant can take water from the ground and carbon dioxide from the air. It uses a process to combine the two and turns it in to glucose. The glucose is food for the tree. The summer period is the warmest and as the chlorophyll is at its most active then, its green color effectively hides all the other colors.
Water travels up the trunk and in to the leaves. When the weather becomes colder the tree knows that winter is on its way. It has to get ready to survive the cold period. A layer of cells grows over the water tubes. This closes them up and it means that no more water gets in to the leaf.
The chlorophyll needs to water to produce energy. Without any it starts to disappear. The packages of xanthophyll and carotene come to the fore. So, in the fall these packages can finally be seen by them. So, what this means is that leaves don't turn color in the Fall. What really happens is that they simply lose their green.
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So, the xanthophyll and carotene explain the orange and yellow color, but why do some leaves go purple and red? Is the tree in an incredibly bad mood? When the cells in the leaf build that wall to keep water out there is sometimes another effect. The same tubes are used to carry the glucose around the tree so every part is fed, not only the leaves. Sometimes when the cells close up the leaf some of the glucose is trapped in the leaf. It can cause the sap to go purple or red and this is reflected in what we see.
Without water the leaf eventually dies. The wonderful skeleton of the leaf exposes all the tubes that used to be so active. No need to worry though - the Spring will mark the return of new leaves and the cycle will begin again!
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