Plants probably began as multicellular algae. Plants are autotrophic, which means that they make their own food. They make food through a process called photosynthesis. In photosynthesis plants use carbon dioxide, water and sunlight to make food. The food that they make is glucose.
The molecular formula for glucose is C6H12O6. Carbon dioxide is CO2. Water is H2O. Six molecules of carbon dioxide result in six carbons and 12 oxygens. Six molecules of water result in 12 hydrogens and six oxygens. Therefore, six molecules of carbon dioxide and six molecules of water result in a net of six carbons, 12 hydrogens and 18 oxygens. The 12 extra oxygens are given off as six O2. Animals such as ourselves breath this O2. Before the evolution of photosynthetic organisms such as plants, there was no O2. All the oxygen which existed on earth was bonded to other elements such as hydrogen.
Plant life began in water. Plants which live in water have everything that they need to live. Carbon dioxide can be obtained from water, as most naturally occurring water has some carbonic acid. Carbonic acid is carbon dioxide dissolved in water.
As land was formed plants moved to land. To live on land plants had to have a way to get water to their leaves for photosynthesis. To do this plants became vascular.
Vascular means that they have vessels. The vascular tissue in plants are xylem, which transports water and minerals up to the leaves, and phloem, vessels which take food down the plant to feed the stems and roots. To be successful on land, plants also needed a way to stand up to get more sunlight.
Sclernchyma are the cells in a plant which form fiber. These tough fibers allow plants to stand up.
On the bottom of leaves is a hole called a stomata. This hole is guarded by two bean-shaped cells called guard cells. Guard cells open and close the stomata. The stomata is there for transpiration. Transpiration is the excretion of extra water which is not needed by the plant. If there is no extra water, the guard cells close up to preserve water. Desert plants have fewer stomata.
The main mode of reproduction in land plants is sexual reproduction. The plant forms haploid cells by meiosis. Haploid means that the cells have only one half the normal number of chromosomes. Through fertilization, two haploid cells combine to form a diploid cell. Diploid cells have the full number of chromosomes.
Sex cells are called gametes. For many plants the male gametes are called pollen
and the female gametes are seeds. Both of these sex cells are haploid. When
fertilization occurs they form an diploid embryo.
Multicellular algae are classified as being plants. However, they lack many of
the features of other plants. They have no vascular tissue and no stomata. There
are three phyla of multicellular algae:
1. Rhodophyta, commonly called red algae, uses a red chemical called phycobilin for photosynthesis. Red algae are used to grow bacteria. The red algae and bacteria are combined together to make synthetic chocolate milk. This synthetic chocolate milk is thicker than real chocolate milk and is cheaper to produce. This same substance is also used to make ice cream thicker so that it will not easily melt.
2. Phaeophyta, commonly called brown algae, is brown because in addition to chlorophyll, which is green, they have the chemical xanthophyll to assist in photosynthesis. Xanthophyll is yellow and they combine to make a golden-brown color. Examples of brown algae are kelp and sea weed. Kelp is very useful. It is used as a thickening agent for cosmetics. Kelp has complex carbohydrates and protein, so it can be good food. The scientific name for sea weed is Sargassum. An area in the middle of the Atlantic ocean is called the sargassum sea. The sargassum sea is an area so thick with sea weed that it makes sea travel through it difficult. Small boats can be stopped by it. All ocean going vessels try to avoid it.
3. Chlorophyta, commonly called green algae has a chemical called carotenoid, a substance which is believed to reduce the risk of cancer.
Many land plants have cuticles. Cuticles are waxy coats which usually appear on only one side of leaf. Cuticles protect the leaves from moisture loss and from predators. Other adaptations, such as stickers, bristles and fuzz, make it harder for insects to eat the leaves.
In botany the words division and phyla are interchangeable. The first division of land plants is bryophyta. Bryophyta include mosses, liverworts and hornworts. Bryophytes are true mosses and are found as ground cover that looks like a green carpet surrounding trees. The Spanish moss, which we have in Louisiana, is not a moss at all, but a bromiliad that is related to the pineapple plant.
Many plants reproduce by forming seeds. There are two kinds of seed plants: gymnosperms--non-flowering plants, and angiosperms--plants which produce flowers.
There are many plants in the division of gymnosperms. The ephedra is a gymnosperm which looks like a pencil sticking in the ground. It contains the chemical ephedrine which is used to make antihistamines, pain killing drugs and stimulant drugs. The juniper tree is a gymnosperm used to make the alcoholic beverage called gin. The pine tree is also a gymnosperm.
There are over 250,000 species of angiosperms. The smallest angiosperm is duck weed, a plant abundant in the waters surrounding St. John parish. The largest angiosperm is the eucalyptus tree, which can grow to 300 feet tall.
There are two groups of flowering plants, the monocotyledons and the dicotyledons. Dicots have two thick seed leaves. The seed leaves are the first leaves to sprout from the seed. The seeds of dicots have two halves, such as peanuts and beans. The seed leaves contain starter food for the plant.
Monocots have only one seed leaf. The seeds of monocots are a one piece unit,
such as corn. Monocots have flowers whose petals come in multiples of three,
whereas dicot flowers have petals that come in multiples of four or five. The
veins of the leaves of monocots are parallel. The veins in the leaves of dicots
branch out like a tree. The xylem and phloem of monocots are random so that the
trunk or stalk has no rings. The stalk or trunk of dicots have xylem and phloem
in vascular bundles that are in rings or layers. Therefore, the cut trunk or
stalk of dicots will usually reveal rings. Examples of monocots are lilies,
orchids, irises, and grasses such as rice, corn, wheat, barley, oats and bamboo,
the largest grass. Many monocots are food. Examples of dicots are pecan trees, walnut trees, cherry trees, beech and pear trees.