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Plant Basics If you're not a microbe and you're not an animal, chances are you are a plant. There are loads of species of plants on Earth. Just as there is a system of classification for animals, there is also a system of classification for plants. Because plants adapt so well to any climate, scientists need a way to organize the hundreds of thousands of species. What Makes a Plant? What do they all have in common? The big thing that connects plants is photosynthesis. Photosynthesis is the process that allows plants to take energy from the Sun and create sugars. Not all plants go through the process of photosynthesis. As with all of biology, there are exceptions and you may learn about plant species that are parasites. Plants also have cell walls. In the cells tutorials we explained that all cells have a membrane. Only plants have an additional cell wall made from cellulose. Let's look at photosynthesis. Plants are able to turn sunlight into energy but not directly. Plants are actually able to store energy in some chemical bonds that can be used later. Before we get into details, we'll explain that there are two processes on Earth: Photosynthesis and Respiration. Photosynthesis stores the energy and respiration releases that energy. It all starts with the Sun. Check out the tutorial on photosynthesis. Learning from Plants Not only do you see plants everywhere in the real world, but they are also all over the scientific world. Scientists use them for studies in genetics. A guy named Gregor Mendel used pea pods and their flowers to come up with some of the first ideas on how traits are passed from one generation of organism to another (genetics). We also use plants for food. Scientists are constantly developing new plants that are more resistant to disease and insects. Scientists also help create plants that grow faster and make more food. A GENERAL PLANT STRUCTURE We're going to look at plant structure in this section. The plants we discuss will be vascular plants that have systems of tubes (xylem and phloem) for the transport of nutrients and water. Remember that there is a wide variety of plants on Earth and even a whole group that doesn't have vascular systems. Mosses and liverworts may still have photosynthesis, but they do not have that 'classic' plant structure. Then you will find species such as cacti that don't have leaves. They conduct photosynthesis in their stems. Anyway, just remember that there are many other possibilities in the plant kingdom. ALIKE BUT DIFFERENT We just told you about the many exceptions to the basic plant structure, so let's look at some similarities. An easy similarity is on a cellular level. Plants conduct photosynthesis. This process of converting the Sun's energy into molecular energy happens in chloroplasts with the help of chlorophyll molecules and a variety of enzymes. Vascular plants share a similar set of structures called roots, stems, and leaves. Many plants have specialized versions, but the basics are there. One specialization might be the petals of a flower. Those flower petals are specialized leaves that surround the reproductive structures of the plant. THE ROOTS BELOW GROUND We'll start at the bottom with the roots. These structures are designed to pull water and minerals from whatever material the plant sits on. For water plants, the roots may be in the water. For traditional trees, the roots go deep into the soil. There are even plants called epiphytes that live in trees and their root system clings to branches. Humans often capitalize on the roots of plants for food. Carrots are just one big orange root. Root systems also provide support for plants in the form of an anchor in the soil. If the wind blows hard, those roots keep the plant from falling over. Some plant species have roots above ground that provide support for the entire plant. Roots are further broken down into the primary root and lateral roots that each has apical meristem at their tips. Root hairs are also a common structure on roots. They make the roots look fuzzy and help in the absorption of water and nutrients. SHOOTS ABOVE GROUND Sure we said that there are some roots above the surface, but the majority of the plant you see is made up of stems and leaves. Think about a tree. The stems are the trunks and branches. Leaves are self-explanatory. Stems are all about transporting food and water and acting as support structures. Leaves are all about photosynthesis, creating food molecules and absorbing carbon dioxide for the plant. These parts are connected by the vascular system of xylem and phloem that spreads through the entire plant. The tip (terminal bud) of the main stem has a specialized structure that is the source of new growth for plants. You will find the apical meristem that develops into young leaves (primodium). There are other points of growth at each node where leaves and branches develop on the stems. Those branching points are home to axillary buds that can also develop into new branches. PLANTS VASCULAR SYSTEMS Xylem and phloem make up the big transportation system of vascular plants. As you get bigger, it is more difficult to transport nutrients, water, and sugars around your body. You have a circulatory system if you want to keep growing. As plants evolved to be larger, they also developed their own kind of circulatory systems. The main parts you will hear a lot about are called xylem and phloem. It all starts with a top and a bottom. Logically, it makes sense. Trees and other vascular plants have a top and a bottom. The top has a trunk, branches, leaves, or needles. The bottom is a system of roots. Each needs the other to survive. The roots hold the plant steady and grab moisture and nutrients from the soil. The top is in the light, conducting photosynthesis and helping the plant reproduce. You have to connect the two parts. That's where xylem and phloem come in. ZIPPY XYLEM The xylem of a plant is the system of tubes and transport cells that circulates water and dissolved minerals. As a plant, you have roots to help you absorb water. If your leaves need water and they are 100 feet above the ground, it is time to put the xylem into action! Xylem is made of vessels that are connected end to end for the maximum speed to move water around. They also have a secondary function of support. When someone cuts an old tree down, they reveal a set of rings. Those rings are the remains of old xylem tissue, one ring for every year the tree was alive. PHLOEM FUN The fun never stops in the plant's circulatory system. Most plants have green leaves, where the photosynthesis happens. When those sugars are made, they need to be given to every cell in the plant for energy. Enter phloem. The phloem cells are laid out end-to-end throughout the entire plant, transporting the sugars and other molecules created by the plant. Phloem is always alive. Xylem tissue dies after one year and then develops anew (rings in the tree trunk). What is the best way to think about phloem? Think about sap coming out of a tree. That dripping sap usually comes from the phloem. PLANT REPRODUCTION - THEY'LL MAKE MORE If you are an organism, you will need to reproduce. Otherwise, there will be no more of your species and the species will die off. You may have heard of endangered animals. There are also endangered plants. These endangered species have very few individuals left and scientists/naturalists are working together to make sure the species don't become extinct. We talked a little about reproduction when we discussed meiosis in the cells tutorials. Reproduction is one of two things. (1) One cell can split into two, giving you two identical cells. That type is asexual reproduction. (2) The second type is when two cells, each with half of the DNA needed, combine and create a living cell. That type is sexual reproduction. When plants hit a point in evolution, the second is the one that occurs more often. MAKING MORE MOSSES Sporophytes are the reproductive structures you will find in mosses. They are actually a phase of the moss life cycle that feeds off the green parent plant (the gametophyte). The sporophyte is a stalk that grows after the haploid sperm of one moss plant is able to mix with the haploid egg of a female moss plant. The resulting diploid cell grows into the sporophyte stalk. When ready, spores stored in the sporophyte are released and they grow into new moss plants. CONFIERS AND THEIR CONES While there are male and female mosses, conifers produce two types of cones on the same tree. One of the cone types gives off pollen (the staminate cone). The other type of cone catches the pollen if the wind is moving in the right direction. Better yet, the wind blows the pollen to another conifer of the same species, and a cone (called the ovulate cone) catches the pollen. Again, the pollen and megaspore (receiving haploid cell) are haploid and combine to form a diploid cell. That diploid cell grows into a zygote (baby conifer) that eventually lives in a seed. FLOWERS AND POLLEN The most advanced of the plants have their own way of sexually reproducing. It is a very fancy and very complex process. Plants that rely on flowers for reproduction are also very dependent on outside help such as insects and animals. While conifers have the two structures on one tree, flowering plants went one step further and put the devices that make and receive pollen in the same structure. How does that help? A bee might go to one flower and get a little pollen on its back. If it goes to another flower of the same species, that pollen can land on the stigma. From that point, one haploid male nucleus combines with a female nucleus and the other haploid male nucleus combines with a polar nucleus. If successful, an embryo and seed/fruit develop respectively. DIFFERENT PARTS = DIFFERENT ADVANTAGES Obviously, not all plants look the same. They have different flowers, stems, and even root structures. Extreme examples have given some plants big advantages. These advantages have let them settle in new environments and become more successful. SPECIALIZED LEAVES What kinds of leaves are there? What kinds aren't there? There are thick ones for storing water as in succulents. There are long twisting vine-like leaves that can wrap around and dig in for support as in grapes. There are also thorns. Nothing says, "Don't eat me" like a bunch of sharp thorns on your branches. FLOWERS Flowers have developed such a wide variety. That variety is often dependent on what kind of creature helps out with the pollination. If I am a big insect, I will be looking at plants with big flowers. If I am a tiny little bug, I might live my whole life inside a flower. There are also a variety of colors that attract different insects and animals. STEMS Stems are a good place to store water. It's very efficient to develop a big protected area. Think about a barrel in hot areas where water is scarce. Enter a cactus. All stem and trunk. No leaves. Having no leaves means very little evaporation on hot days. Other extremes are plants with no stem. They could grow one, being vascular plants, but they have found it to be an advantage to stay near the ground. Vines are another extreme. The bark of a tree or plant can also perform a specific function. Corks in wine bottles are actually from the bark of a tree (cork tree). Some bark has been designed to peel away as the tree grows. Other types of bark are very thick to protect the plant from animals and insects. EPIPHYTES - SPECIALIZED ROOTS Not all plants even live in the ground. Some specialized plants called epiphytes actually live on the side of other trees or on rocks. They are able to collect water themselves but do not use roots to gather it up. Their roots have been specialized to dig in or grab on to the larger object. They don't always hurt the trees; they just hang out on the outside. Epiphytes can include some seedless species, bromeliads, and orchids. There are also epiphyte species that can grow very large and even break tree limbs. They can suck nutrients away from the tree and weaken it over time. Several ficus species are killer parasitic epiphytes. GYMNOSPERMS - FIRST PLANTS WITH SEEDS So you've got a vascular system. What comes next? Seeds. Seeds let you send your offspring out into the world. Seeds provide a protective coat so that the embryo plant can develop when it finds a nice piece of soil. But remember this: gymnosperms have not developed the ability to make flowers. Flowers are an evolutionary advancement after seeds. So if you have a vascular system, seeds, and no flowers, what are you? A gymnosperm! Seeds are a protective structure that lets a plant embryo survive for long periods of time before it germinates. Seeds have food sources pre-packaged for plant embryos to provide for an embryo's needs in early growth. Seeds let plants spread their embryos over large areas. Some are even so lightweight that they are carried across the planet by strong winds. Seeds are an advantage if you want to be a plant that can grow anywhere. Seeds are da bom'! CYCADS IN THE TROPICS Looking like a fern. Looking like a palm tree. It's actually neither! It's a cycad. These are another favorite of landscape designers. These are sturdy little plants that can survive in harsh conditions. You won't find them in cold areas like the conifers. Cycads need warmer weather to survive. They have cone-like structures for reproduction. Instead of being on branches, their cones are in the center of the plant and can get really large. They also have big waxy fronds, and when it's time to reproduce, the female plants have a great fruit that grows in the middle of their stem. CONIFERS IN THE FORESTS Pine, cedar, redwood, and spruce. Sounds like we're at a hardware store buying lumber. Not so. We are listing off a bunch of trees that are called conifers. If you've ever gone skiing or to northern latitudes you have seen loads of conifers. The conifers most people think of are pine trees. Every year millions of trees are grown for Christmas and they are all conifers. They usually have needles and cones (thus the name CONifer). They are also evergreens: even in cold winter months they are able to keep their needles. That ability is one reason they do so well in northern latitudes. The ever-present needles allow conifers to take advantage of the Sun whenever it is around. They are also some of the tallest plants in the world. They are able to get very tall and strong because of heavy-duty xylem that hardens and makes them sturdy. That sturdiness is why these kinds of trees make good lumber - hard and strong wood. GINKGOES ON YOUR STREET Not every plant made it to the modern day. Fossil evidence shows what plants used to be alive in other geological eras. The Ginkgo is one of the ones that made it. Some people call it a "Maidenhair Tree". It's the last one of its kind. It has needles that have combined to form very sturdy leaf-like structures. You need to remember they are not like leaves in the traditional sense. You've probably seen these all over. Landscape designers love to use them because they look very nice and are very resistant to pollution. They are great for cities. Being able to resist insects and disease has let this species survive beyond all of its close relatives.