In the previous lecture, we learned about different cells and the tissues that make up a plant. While there are more than 200,000 different types of flowering plants, their general structure is remarkably similar. All plants have two main systems, a root system below the ground and a shoot system above the ground. The root system has the main job of absorbing water and giving support for the plant, while the chute system absorbs the light, does photosynthesis, and leads to flowering seeds' fruits. All plants have five types of organs, roots, stems, leaves, buds, and flowers. While all plants have the same organs, there's a great diversity in their structure. But among this incredible diversity in structure, there is an order, and this can be seen in the two families of flowering plants. These are called the monocots and the dicots. The difference between these plant families is seen in the embryos of their seeds. For example, a monocotyledon in the seed, such of that of corn, has an embryo which has a single cotyledon, while a seed from a dicotyledonous plant, such as, for example, a pea plant, has an embryo with two cotyledons. These are the names of the families, monocots from monocotyledon and dicot for dicotyledon. The leaves of the two families are also quite different. Leaves of dicots, such as a maple leaf, have a petiole which connects the leaf to the stem. If we look at the structure of the veins in the leaf, the veins in a maple leaf are formed like network of structure. They diverge one from each other forming a net, or a web of leaves. This is the xylem and phloem of the leaf. When we look at a monocot leaf, such as the leaf of a corn plant, they don't have a petiole. The whole leaf itself just wraps the stem. And their veins are arranged in a parallel type of structure. They're not diverging one from the other, but all of the veins, the vascular tissue, the xylem, the florum are arranged in a parallel type of structure. If we're talking about the vascular tissue, we see a complete difference in the structure of the vascular tissue within the stems of monocots and dicots. In dicots the vascular structure is arranged in rings, and this is what allows trees to grow in width, in girth. Whereas in monocots, again such as in corn, the vascular tissue is arranged randomly through out the stem. The roots really show a huge divergence between monocots and dicots. The root system of dicot plants have a single main taproot, which goes very deep into the ground. And from this taproot, you have lateral roots which diverge from it. This type of root structure's very good for getting to very deep ground water and giving support for large trees. Monocots usually have a shallower, fibrous root system, such as found in the roots of grass. These type of roots are very good for holding the topsoil and inhibiting erosion, and they're also very good at absorbing water that becomes quickly available, such as run off water, or from a short rain. So you can see that each root system has a different role ecologically. And the final difference is seen in their flowers. Monocot flowers have petals that usually come in multiples of threes, three petals or six petals, whereas dicot flowers usually have parts that come in multiples of four or five. But as opposed to animals, plants grow throughout their life. Now, I know some of you are thinking, well, I'm growing throughout my life. Some of us, unfortunately, are growing in a certain direction. But in general, animals, humans, we grow to a certain size and then stop growing. But plants, all plants, continue growing throughout their entire life cycle. Doesn't mean that plants don't die. Actually, we could talk about three different types of plants. There are annual plants that germinate, grow, make flowers and seeds, all within one year of a life cycle. But until they're done making seeds, they keep growing. There are biennial plants. These are plants that take two years to go through a complete life cycle from germination through to making seeds. For example, such as carrots. But once they finally do make their seeds, they die. And there are perenial plants, such as shrubs and trees, which grow and add new growth each year. So what causes perineal plants to die? Well, trees usually die not from old age, but from either some type of disease or from some type of natural disaster, such as a fire. But until they die, all plants continue to grow. So, the big question is, how do plants maintain growth throughout their entire lifecycle? How do they keep growing new tissues each and every day? And to answer this question, we need to differentiate between two types of growth, growth which I'm gonna define as determinant growth and growth which is indeterminate growth.
