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ALGAE AND AVAILABLE FREQUENCIES
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Plants, like all living things, are descended from ancient organisms that evolved and developed in the oceans. The ocean-living plants that represent land plants ancestors are simple, so much so that they are often classified as Protistans - little multicellular complexity was needed to float in the light.
Even in the ocean, though, there are a number of different niches available for plants - in the shallows, many algae varieties exist that can attach to submerged objects, probably a feature that helped the land plants' direct ancestors to stick out of the water into the air. But niches also exist in the deeper water, where there is a functional connection between the photosynthetic pigments used and the nature of how light interacts with water.
The surface of the water reflects some light, but the frequencies that get through get absorbed - you already know that the deeper you go, the darker it gets. What you may not know is that not all frequencies get absorbed equally - the frequencies that "regular" green plants use don't get very far beneath the surface, but frequencies in the bluish and greenish ranges penetrate much further. This has spurred the evolution of several classes of various-colored algae with different chlorophylls, capable of using these deeper-reaching frequencies to grow where the green algaes can't. Red algaes especially can occupy greater depths than the others, although Golden algaes can occupy a bit of a middle zone between green and red. The color of Brown algaes seems more to do with other pigments and is apparently not really depth-related.
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Needs of Land Plants - Structure
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The eukaryote plants that evolved in the oceans had very few structural needs; the buoyancy of the water held them up near the light, and occasionally they needed to adhere to surfaces in the surf zones. They remained fairly simple collections of cells, which is why they are considered protists. The Kingdom Plantae are the land plants, showing adaptations to living in the air.
Early land plants may have just been plants that could withstand being left behind by the tide, coating the surfaces of the surf zone, but once such plants could function without being under water, spreading and supporting photosynthetic platforms became a very useful ability. The bryophytes are probably the best surviving example of early land plants. There are indications that some materials that evolved to resist the action of waves may have been critical to the structures useful in the air.
Bryophytes include liverworts, which spread on very moist surfaces in flat sheets. But mosses, also bryophytes, hold leaf-like surfaces on stem-like stalks ("true" leaves and stems have internal tubes not found in bryophytes). This support ability appears to have come from modifications of the cellulose material of the cell walls, both making it stiffer and producing related materials that more strongly reinforce the force holding cells together.
Small green plants often have a structure inside their cells, a central vacuole, with a water-based solution that can build up pressure to reinforce the shape of the cells. You've probably seen that plants, as they dry out, wilt as water is moved out of the vacuoles for photosynthesis and not replenished.
One main land-adapted structural component is a cuticle (also common in animals) whose main function is to provide a waterproof surface and reduce water loss to the air. A material that acts as a waterproofing agent but which became critical in stiffening land plants is lignin, found in increasing amounts in larger land plants. It has the additional advantage of being somewhat protective against the more powerful effects of direct sunlight that land organisms have to deal with. Later land plants evolved waxy materials as more efficient waterproofing - you can often tell how dry an environment a plant is from by the waxiness / shininess of its cuticle.
Many paleobotanists (who study plants of the distant past) believe that the Earth's coal and oil reserves stem from a time when lignin became a major component of land plants, but decomposers who could break it down had not yet evolved; plants sank into the mucky sediments, and their organic materials slowly converted to the hydrocarbon-rich fuels we use today.
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Needs of Land Plants - for Photosynthesis
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Photosynthesis requires light, carbon dioxide, and water. On the land, light is abundant from above, carbon dioxide is in the air, and water, with dissolved minerals, comes mostly from rain-soaked soil.
Light can be an issue, because as electromagnetic radiation it can interact with and damage cellular materials. There are actually different types of photosynthesis: plants from dry sunny areas use a somewhat different process to reduce damage from light and from high amounts of produced oxygen.
Getting carbon dioxide from the air seems simple, but carbon dioxide cannot be absorbed across a waterproofed surface; anywhere it comes in, water vapor will be lost. Water lost by plants into the air is called transpiration, and is a major contributor to the atmospheric water cycle. Early land plants, who had pores to absorb carbon dioxide, were somewhat restricted to water-rich, humid environments. For long periods of the distant past, they also benefited from higher levels of carbon dioxide in the atmosphere. The bryophytes and later forms, the tracheophytes (ferns and their relatives), are still restricted in their ecological potentials by the limitations of their pores. But later land plants, the gymnosperms (conifers and relatives) and angiosperms (flowering plants), developed pores that could be opened and closed. These are called stomates (or stomata), and with them plants could "breathe" in a controlled manner.
The special-photosynthesis plants from desert systems mentioned above tend to open their stomates only at night; they "charge" their leaves with carbon dioxide when it's cooler and transpiration is slower. That's also why they need some chemistry to resist the activity of high oxygen levels, since lots of oxygen accumulates in the tissues during the day when the stomates are closed.
Water movement, as plant structure allowed for taller plants, became a major challenge. For small plants, osmosis - movement of water from more-dilute situations (surfaces or in the soil) to less-dilute situations (inside the plants) could move water into and up a plant in a process called root pressure (even for plants without roots), but gravity counteracts that movement, making it useful only for fairly small (lower than your hip) plants.
For larger plants, special vascular (tube) structures were needed: the water-using tops of the plant essentially "suck" the water (and nutrients) up special "straws" where the tendency of water to hold onto itself (and the water-sticky linings of the tubes) could help pull it to treetop heights. These special straws are called xylem, and thinner tubes to take photosynthetic products to the lower / buried parts of plants are called phloem. The tracheophytes, gymnosperms, and angiosperms are all vascular plants; any large plant you see is in one of those groups. Tracheophytes have vascular tissue in their stems and leaves, while the other groups have it in stems, leaves, and roots (technically, to be called stems, leaves, or roots, there has to be vascular tissue present).
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Needs of Land Plants - for Reproduction
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Organisms that evolved in watery environments had a fairly easy medium through which sperm could reach egg cells: the water. The further from water and wet environments organisms moved on land, the less viable using the environment for a passage medium became.
But first -
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HOW CAN A PLANT BE MALE OR FEMALE?
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Chapter 1, in the discussion about sexual reproduction, it was mentioned that such reproduction does not require two separate parents or even male and female sources, although different genders (roles in sexual reproduction) are extremely common. As we've just discussed, mosses can be male and female; so can flower parts, pine cones, parts of a flatworm, and jellyfish.
You may think you know the basics of telling a male from a female, but you can't peek under a moss looking for noogies and a penis. What makes a form male?
Gender is determined by the type of gamete produced. A gamete is a sex cell - sperm is the male variety, ovum (we'll use the less-precise egg cell instead) is the female. The differences in these cells will ultimately tell you if an organism or a reproductive organ is male or female, and it is the differences between the cells that really leads to the larger differences between males and females in any species that has genders...
SPERM
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EGG CELLS
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...are produced in much much higher quantities.
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...are produced in comparatively lower quantities.
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...are much smaller than egg cells.
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...are much larger than sperm, since they contain food for the embryo.
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...are equipped to get from where they're made to where the egg cells are.
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...pretty much wait in one place for the sperm to reach them.
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...from each starting cell that divides by meiosis, four functional sperm are made.
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...from each starting cell that divides by meiosis, one functional egg cell is made, with 3 tiny polar bodies used to discard "extra" sets of chromosomes.
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Some things to remember: don't assume that things always work the way that they do in humans, or even in animals. In most gendered organisms, lots of egg cells are made (it's just that many more sperm are made), not just one (which is a commonly-known "fact" that isn't even really true for people). And although many sperm types can swim or crawl, many (like in pollen) can't, even though they have evolved ways to get where they need to go. And forget "XX" and "XY" chromosome mixes - gender is coded by chromosomes only in some animals, and most of the time there is no chromosomal difference between males and females. In addition, many types of adult organisms are both male and female at the same time, a condition that used to be called hermaphroditism but is now called being monoecious.
The toughest problem to solve living on land, for both plants and many animal groups, concerned sexual reproduction that used swimming sperm, and this was true for plants: long after the drying, support, and other problems were adapted to, the reproduction problem existed. As also happened in the first vascular plants, bryophytes developed an alternation of generations pattern, with a sexual phase, the gametophyte, alternating with an asexual phase, the sporophyte. Unlike the vascular plants, whose "main" (most obvious) form is the sporophyte, what we think of as "moss" is the gametophyte form. A moss sporophyte is a small attached form that makes, not surprisingly, spores.
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Moss spores, containing only one set of chromosomes (haploid), are carried by the wind from the parent sporophyte. If they land somewhere with the right moisture content (remember, bryophytes don't do well if it's too dry), the spore begins to divide and produce very algae-like filaments that cover the surface it's on. From that surface will sprout the gametophyte, with structures to anchor it called rhizoids (rhiz- is from a word for "root") and upward-growing "stems" with "leaves" that are only a single cell layer thick. (they are not truly these structures because the "real things" always have vascular structures inside them - that's why they are being called "stems" in quotes, etc.) The materials needed by the moss to grow are absorbed by the upper structures rather than the rhizoids, but this allows mosses to grow on fairly sparse surfaces.
Some of the moss gametophytes are male and some are female, and the very tips form sexual reproductive structures called antheridia (male) and archegonia (female). The antheridia swell up and burst, and when the plants are wet with rain or heavy dew, release sperm that are splashed off and swim through the water coating nearby plants to the archegonia, where they fertilize waiting egg cells. The fertilized egg, now a zygote containing two sets of chromosomes (diploid), grows into a small sporophyte. Spores are eventually produced, and the tip of the sporophyte opens, releasing the spores into the winds, where they can travel great distances and still sprout even decades later.
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FERNS- SEEDLESS VASCULAR PLANTS
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Although the early vascular plants adapted new and better ways to rise above the bryophytes, they still had not quite solved the reproduction-on-land problem. Because of this, the
tracheophytes, like the bryophytes, follow an alternation of generations life cycle. Unlike the bryophytes, however, the larger plant, the form we all think of when the word "fern" comes up, is the sporophyte. In ferns, the gametophyte is a relatively tiny thing, barely noticeable compared to the sporophyte.
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As we did with the mosses, we'll start with a spore settling in a properly-moist environment. The haploid spores (cells with only one set of chromosomes) sprout and grow into small gametophytes that look like tiny heart-shaped leaves; these gametophytes contain male
antheridia and female archegonia. Sometimes these are separate little plants, sometimes they're on the same small plant. When things get wet, such as in the rain, the antheridia release sperm that swim to and enter the archegonia, fertilizing the egg cells and forming diploid (2 sets of chromosomes) zygotes. The zygotes grow into the large, leafy sporophytes, which eventually generate spore-making packets called sori on the undersides of the leaves. In the sori, the haploid spores are produced by meiosis and released into the air.
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