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Lesson 6: Plant Reproduction

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Lesson 6: Plant Reproduction LESSON 6: PLANT REPRODUCTION LEVEL ONE Like every living thing on earth, plants need to make more of themselves. Biological structures wear out over time and need to be replaced with new ones. We’ve already looked at how non-vascular plants reproduce (mosses and liverworts) so now it’s time to look at vascular plants. If you look back at the chart on page 17, you will see that vascular plants are divided into two main categories: plants that produce seeds and plants that don’t produce seeds. The vascular plants that do not make seeds are basically the ferns. There are a few other smaller categories such as “horse tails” and club mosses, but if you just remember the ferns, that’s fine. So let’s take a look at how ferns make more ferns. The leaves of ferns are called fronds, and brand new leaves that have not yet totally uncoiled are called fiddleheads because they look like the scroll-shaped end of a violin. Technically, the entire frond is a leaf. What looks like a stem is actually the fern’s equivalent of a petiole. (Botanists call it a stipe.) The stem of a fern plant runs under the ground and is called a rhizome. Ferns also have roots, like all other vascular plants. The roots grow out from the bottom of the rhizome. Ferns produce spores, just like mosses do. At certain times of the year, the backside of some fern fronds will be covered with little dots called sori. Sori is the plural form, meaning more than one of them. Just one is called a sorus. Sorus is a Greek word meaning a “bunch or heap.” If you look at a sorus under a magnifier it does indeed look like a heap of little dots. This is how one sorus looks under a magnifier. The individual bumps on a sorus are called sporangia. Inside each sporangium are many microscopic spores. The sporangia break open when they are “ripe” and release their spores into the air. When the spores fall to the ground they grow into a heart-shaped thing called a prothallus (very similar to a moss thallus). The prothallus then produces male and female structures that make sperm and eggs. When it rains, the sperm can swim through the water to reach the eggs. When egg and sperm join, they will make a baby fern plant that grows up as a frond. 51 The fern displays the same alternation of generations pattern that the mosses do. The fronds produce spores that produce a prothallus that makes male and female cells, which grow into a sporophyte that produces spores. Each part of the cycle has certain advantages. The advantage of making spores is that the plant can make millions of them, increasing the odds that the plant will successfully reproduce, with some of the offspring perhaps at quite a distance from the parent plant. The advantage of having a prothallus that makes male and female cells is that the female cells can be fertilized by male cells from another plant, thus increasing the variety of its DNA. If a fern’s egg cells were always fertilized by its own male cells, the baby plants would always be clones of the parent — identical in every way. If every single plant in a species is identical, the species is more vulnerable to extinction. Some ferns might have DNA that make them more able to survive drought or to survive an attack by a fungus. By swapping male cells, stronger plants can “share” their DNA with weaker plants, so to speak, thus strengthening the species as a whole. Sometimes a fern prothallus will produce only female parts so that it can’t possibly fertilize itself. Then, when it rains, the female parts produce a chemical that attracts male cells. If there is enough water, male cells from neighboring ferns will be able to swim over and literally “follow the scent” to find the female cells. (This sounds like something Carl Linnaeus got in trouble for talking about!) Ferns also have a “Plan B.” Relying completely on spores and on male cells finding female cells would limit ferns in their ability to survive. The weather isn’t always cooperative, after all. Periods of rainfall might not come when the fern’s life cycle needs them. Ferns, like most plants, can do something called vegetative reproduction. The vegetative parts of a plant are the ones that are not involved in these complicated reproductive processes. The vegetative parts are the leaves, stems, and roots. Vegetative reproduction means that new plants can grow from leaves, stems and roots. (Gardeners will often take “cuttings” from plants — pieces of leaves, stems or roots — and then replant them.) Fern rhizomes will grow longer and longer under the ground, putting up new fronds along the way. If you cut off a section of rhizome and replant it somewhere else, you’ll have a new fern plant. Technically, you can also cut off a frond, plant it, and it will grow a rhizome. But this method is tricky and doesn’t always work, even for professional gardeners. Plants that make seeds are the plants you are most familiar with. They fill our yards and gardens. Look back to page 17 again and you will see that seed-bearing plants are divided into two categories: the gymnosperms and the angiosperms. (“Sperm” just means “seed.”) Let’s look at the gymnosperms first. The gymnosperms are mostly pine trees and fir trees. The word “gymno” is Greek for “naked.” (Greeks ran naked in their gymnasiums.) Yes, that little seed inside a pine cone doesn’t have any fruit covering it! It’s shocking but true. The seed is botanically naked. The reproductive structure of a gymnosperm is called a cone. No great surprise there — you already knew that. What you may not know is that cones can be male or female. What you think of as a pine cone is actually a female cone. Male cones are smaller than female cones and they are usually more fuzzy. They appear just for a few weeks at certain times of the year. The male cones produce pollen. Each tiny grain of pollen contains two sperm cells. The male cones pollen grains are so small you need a magnifier to see them. 52 The male cones release their pollen into the air. The pollen grains of pine trees have two little “balloons” attached to them that help them fl oat upward in currents of air. Being airborne as long as possible is an advan- tage for the pollen. It increases the odds of fi nding female cones. Even so, many pollen grains don’t make it very far. That’s why a tree has to release millions of pollen grains. (If you are allergic to pollen, this isn’t good news!) Most of the pollen ends up on the ground, on other plants, or all over your car — places where it won’t do any good. But a small portion of the pollen will land on the developing female cones. The pollen grains that land on the female cones be- come trapped in a sticky sub- stance at the base of each scale. Each scale contains two female parts, called ovules. Inside the ovules are eggs. After a pollen grain gets stuck on the end of an ovule, it starts growing a tube. This tube keeps growing until it reaches the egg. Pine tree sperm can’t swim like fern sperm can, so the pollen tube has to take them directly to the egg. Only one of the two sperm can enter the egg. Like it or not, the natural world is very competitive. The winner enters the egg and the loser disintegrates and disappears. (The story is a bit happier for angiosperms, though. There is a runner-up prize for the other sperm.) When the sperm and egg join together, a zygote is formed. A zygote is a single cell. As this single cell begins to divide and develop, we no longer call it a zygote, but an embryo. The ovule around the embryo turns into a hard protective covering but remains attached to the “seed wing.” It is now offi cially a seed. When the seeds are ready, the female cone opens up and the seeds drop out. This whole process, from when the pollen grains fi rst stick to the scale until the day the mature cone opens up and sheds the fi nished seeds, takes two years. Green pine cones are female cones that are only one year old. They will need another season to fi nish their growth. The seeds don’t have to start growing immediately. They can sit and wait, sometimes for a very long time. The most extreme case of pine seeds waiting to grow is a species of pine tree that requires a forest fi re to open its female cones. The cones will wait for years and years until there is a fi re. The fi re will destroy the parent tree, but it will also force the female cones to open and release the seeds. Since all the trees in the area were destroyed by the fi re, the new seedlings will have plenty of wide open space in which to grow! And now... the angiosperms. Okay, it’s intermission time. Get a snack, take a walk, maybe check out some of the fern videos on the Botany playlist. When you get back, we’ll begin our study of fl owers.
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