Lesson 1 - Page 1


The life cycle of a plant varies depending on the individual species. There are, however, certain requirements for life that most plants need. The growth of a plant is dependent upon light, water, oxygen, carbon dioxide, minerals in the soil, temperature and microbes in the soil. Light, water, and carbon dioxide are needed for photosynthesis which produces food for the plant. Oxygen is needed when it is dark, because the plant then needs oxygen to maintain itself. The correct temperature, soil, and minerals are all needed when the plant first germinates and subsequently grows. Soil helps bind the roots so the plant can anchor itself. Microbes in the soil include a number that are beneficial to plants. Microbial activity helps bring about the decay of organic material (dead plant material and animals) necessary for the production of soil. Temperature or light intensity varies for each type of plant, and this helps explain global plant distribution; light intensity or temperature also effects the rate of photosynthesis in plants; the time at which a plant flowers and the rate at which water loss occurs in a plant (transpiration.)

When these requirements are static for a seed, it will begin to grow or germinate. Sufficient food and minerals are stored in almost all seeds, so that these factors do not limit germination. As water is absorbed by a seed, the inner tissue swells more rapidly than the seed coat. The penetration of water allows the tissues to become hydrated and enzyme activity increases. The food that is stored in the cotyledons or the endosperms are now digested and used. 

ll plants need light, water, air, moderate temperatures and most need soil. Some plants, such as mistletoe and duckweed, do not require soil for growth and life but they do not constitute the majority of plants. There are wide variations in the amount of light and water that plants require. A mature Joshua tree, for example can store enough water to last three years or until another rainy season.

Most plants, however, need water on a more regular schedule. Some plants require a full day's hot sun, and others cannot be taken out of deep shade. Temperature tolerance also varies tremendously. Tundra and lichens can survive near the Arctic, but many tropical plants cannot survive being carried from the store to a car when it is near freezing.

Many plants can reproduce either sexually (seeds) or vegetatively (asexual), utilizing other plant parts. Whole plants can be grown from stems, leaves or roots, if the right plant is chosen. The following are some suggestions that can be used to illustrate vegetative reproduction.

Stems: Ivy, potato tubers, bamboo and iris rhizome, bulbs of various kinds (bulbs are actually modified shoots), crocus or gladiolus, corn, Philodendron, Monstera (split-leaf philodendron), strawberry and spider plant offsets. (Many others will grow, even hardwoods, but they take a lot of time and effort.)

Roots: Japanese anemone, Oriental poppy, trumpet creeper, blackberry, raspberry, lily of the Nile, and any other plant that produces sprouts from roots. (The roots you plant will show no visible growth buds, the buds develop after the root cutting is planted.)

Leaves: Begonias, African violets, various succulents, sansevieria, piggy back plant (if leaf has plantlet), and Bryophyllum. 

Vascular plants are plants that have specialized conducting tissue and are usually grouped as tracheophytes and include the ferns, horsetails, angiosperms (flowering plants) and gymnosperms (pine-like trees). Thallophytes (water type plants) and bryophytes (mosses) do not have true roots, stems, and leaves and possess no specialized system for the conduction of food and water from one part of the plant to another. Plants that have a vascular system are larger and able to cope with a "land situation." There are no plants with a vascular system in a total water environment because the water provides the nutrients the plants require, so they do not have to "conduct" these substances.

Woody stems are mostly secondary xylem (wood) surrounded by bark. The xylem may include heart-wood and sap-wood. Heart-wood is dead and non-functional. The sap wood is functional and has living parenchymal cells. 

Cell walls are made primarily of cellulose but there are also other substances like hemicelluloses, minerals, tannins, resins, pigments, proteins, mucilages, and gums that can be found in plant cell walls. Cell walls are porous, allowing an exchange with substances outside the cell wall. Cellulose has a straight chain structure, forming strong fibers that are ideal cell-wall structural materials and useful in making products.

Cellulose is used commercially in making paper, rayon, explosives, cellophane, buttons, and many other materials. Cellulose comprises approximately 80 per cent of the dry weight of wood, and forests are the source of many valuable articles in addition to lumber. Plant fibers are twisted together to manufacture thread or yarn from which fabrics are woven. The fibers are obtained primarily from the cotton, flax, or hemp plants.

Lumber is mainly cellulose. There are two zones of wood on a tree, the sapwood, a light-colored outer zone and the heartwood, surrounding a generally darker-colored zone. The sapwood functions in sap conduction and food storage, the heartwood is used for mechanical support. Sapwood will eventually turn into heartwood. Heartwood is the more durable portion of lumber. 

Plants form the basic food staple for all life forms. They are the major source of food and oxygen on earth, since no animal can supply these necessary components without plants. The cattle we eat as beef, feed on grasses and the fish we eat, consume algae and are therefore dependent on plants for well being. Other important uses of plants include, providing shelter for animals, providing materials for clothing (cotton fibers), paper products, medicines and other chemicals, producing coal from once living plant material, reducing wind speed and noise levels, and reducing soil erosion and water runoff.

There are many different types of cash crops that produce money for farmers. Olive oil comes from olives, corn oil comes from corn, and peanut oil comes from peanuts. Typical agricultural products like corn, wheat, rye, and rice are all considered cash crops. Coffee plants produce beans that are used to make coffee; coca plants give us chocolate; vanilla plants grow long thin beans that are used to produce vanilla flavoring. Many drinks and beverages, like cola and tea, come from plants. Rubber from trees is also a cash crop, as is lumber, fruit, vegetables, and cotton.

Plants are also used in agriculture to help reduce wind speed. Planting trees in a row prevents the wind from blowing away the valuable topsoil. In the forest, trees act as shelter for many organisms.

Plants are also important for the overall ecology of an area. Roots help to stabilize soil and prevent erosion by water run off (soil conservation). Plants are also important in our atmosphere because they use carbon dioxide and give off oxygen while they undergo photosynthesis.

Plants are also used in the urban setting to reduce noise, produce shade, and to beautify an area. Trees add value to homes and communities. 



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