Thursday, October 11, 2007

Evolution of Plant Kingdom

For 3 billion years the earth's terrestrial surface was lifeless. This means there was no plants or animals on the continents or the world. The 1st photosynthetic organinisms (orgainisms that use photosynthesis for food) were aquatic green algae.

Around 500 million years ago was when the first land plants evoloved. To survive on land these plants needed special adaptions. They needed these adaptations to protect them from drying out, to obtain and move water/nutrients, ways to exchange gasses and how to protect embryos. To protect themselfs from drying over time plants devloped a waxy cuticle. This would help them retain moisture better int he hot sunlight. Holes in a plant's leaves made gas exchange easy. Guard ceels would open and close the holes at approprate times to let gas eather in or out. To transport nutrients and water a series of pipes was devloped over times. These pipes are called xylem & phloem. For millions of years the embryos were unprotected. Over time the embryo was eventually protected in seeds.

Animals and plants have different life cylces. Animals spend most of their life as a diplod mutlicellular organisms. To reproduce animals as well as plants make gametes. This is where things start to differ between animal reproducion and plant reproduction. Through meiosis animals make haploid unicellular gametes. On the other hand plants use meiosis to make spores. From there they use mitosis to make a haploid multicellular gametophyte. A gametophyte is a plant that makes gametes through mitosis. Then fertilization occurs and the cycle starts over again. This is a disadvantage for plants because plants spend most of their life as a haploid. This means they only have one strand of DNA with no copy. If theres a mutation it could cause the plant to not function and die, where as diploids have two stands of DNA acting as an "insurance policy". If there is a mutation there is a good chance the other strand of DNA doesn't have it and the cell can still function proberly.

The first land plants were Bryophytes. More commenally known as mosses.

Mosses are non vascular; meaning they don't have pipes to transport water and nutrients. Because of this moss is usually small. Moss also have swimming sperm. To reproduce moss needs to be locaed near water so the sperm can fertilize an egg. The fuzzy moss you see is a haploid. The spores needed for reproduction sprout from haploids to for gametophytes. This is the onyl part of moss that is a diploid.

The first vasucalr plants were Pteridophytes, better known as ferns. The fern is a very leafy plant. It modled the first system of pipes. These are known as xylem and phloem. They also had as system of roots and leaves. Unfortunatly the fern still had swimming sperm so they still needed to live near water. Most of the ferns life is spent in the sporophyte stage. The leafy fern we are all familiar with is a diploid. The spores that are used for reproduction are located nder some of the leaves. To reproduce ferns alter their generations. The use differnt plants to make gametophytes. Their gametopytes are small haploid plants which produce gametes.
These small plants are homospory. This means there is a male and female on the same plant. The fern is millions of years old. Forests of seedless plants decaded into seposites of fossil fules such as coal & oil.

The first seed plants were Gymnosperms, also known as conifers. THese new plants were vascular and heterospory. This means the have male and female gametophytes. The seeds they had were naked seeds. This means they arent protected by fruits. The seeds were spread by the wind. This meant that plants could finally reproduce away from water. The seeds were fertilized by pollen.Pollen eliminated the requirement for water fertilization. The pollen contained teh male gametophyte, while the seed had the female gametophyte. The seeds protected the embryo from drought and UV radiation.

The first flowering plants were called Angiosperm. They are diploids, vascular, were heterospory, they flowered of corse. The floweres enabled the plants to have seeds within fruit. This added protection as well as it made new modes of transportaion possible. An animal could coem along eat the fruit and the seeds would survive and pass out sometime later in a different place. Most flowering plants are trees and bushes. Examples of these are apples and strawberries. These plants also use pollen to fertilize seeds. The gametophytes (the haploid stage) are now microscopic.

The Flower came from modified leaves. The flower has a modified shoot with four rings of modified leavescalled petals. Under the leaves where the stem meets are the sepals. At the center of the petals are teh reproductive organs: the stamen- the male organ and the carpel- the female organ.

What are teh advantages of a seed? A seed offers protection for the embryo. It also has stored nutrients for the growth of the embryo.The first keaves of a new plant are in the seed. They are called cotyledons. A embryo could have be monocot (one leave) or dicot (two leaves). Most dicots are woody plants, trees, shrubs, or beans. Thel eaves hvae a network with veins. Most monocots are grasses palms and lilies. They contain leaves with parallel veins.

Well thats just about it. Tomarrows shurpa is going to be jesse.

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