Lesson 3 - Page 1



Woody plants are hard with a thick, wood-like covering on their stems or trunk. The are generally long lived. Examples of woody plants are trees and shrubs. The major parts of a wood plant are the leaves, branch, trunk, and roots. The leaves are an outgrowth of the stem and can be thin, flat, needle or scale-like, and green in color due to the presence of chlorophyll during the growing season. Woody plants have cambium (the bark area) which is a substance that gives a tree support so it can grow tall. Leaves provide the surface area necessary for absorption of sunlight which begins the process of photosynthesis.

A branch is a secondary wood limb growing from the trunk of a plant. It helps transport materials from the tree trunk to the leaves. The trunk is a massive primary stem of a tree located between the roots and upper tree canopy. The trunk provides upright support to trees and transports nutrients and water from the roots to the leaves of a tree. The roots are usually not green and are found beneath the soil and are greatly branched or netted in appearance. Roots absorb water and minerals from the soil and anchor the plant to the ground and store food materials.

Herbaceous plants or small flowers are soft and green with little or no wood stems. These plants are generally short lived. Examples include grasses, garden vegetables, and small plants. Petals are usually brightly colored with distinctive colors and are used mainly to attract insects for the purpose of cross pollination (i.e., exchange of pollen from one flower to another). Although most herbaceous plants are noted for their flowers, many trees also produce flowers.

The leaf has the same description as that for wood plants with the exception of being needle-like or scale-like in structure. The function of a leaf is the same as in wood plants.

The stem is the structure between the roots and the leaves, and may be woody or non-woody. The stem transports water and minerals from rocks to other parts of the plant and provides for the support of leaves.

The roots have the same description and function as in wood plants.

Flowering plants are normally divided into roots, stems, leaves, flowers, and fruit. It is also useful to discuss buds (flower, leaf and stem), seeds, trunks and branches. Roots serve to anchor plants and absorb water and minerals from the soil and some roots function in food storage. Stems may be found above or below ground and provide support for the plant and transport fluids. Some stems have been so specialized for food storage and reproduction that they no longer look like a normal stem (for example potato tuber, iris rhizome, crocus or gladiolus corn). They can be distinguished from roots by the presence of buds and leaf scars. Both stems and roots may have small roots on them, but only the stems will have buds.

Leaves absorb sunlight and produce the plant's food through photosynthesis. Flowers are the main reproductive organs, producing pollen and eggs which unite in the ovary to produce seeds. The ovary turns into a fruit. Seeds may also be found below ground (peanut).

There is no scientific distinction between a fruit and a vegetable. To a botanist, a fruit is the plant part produced from the ovary of a flower; the fertilized eggs develop into seeds. Thus tomatoes, green peppers, avocados, squash, cucumbers, corn and other "vegetables" are really fruits. The grains, such as rice, wheat, barley, and oats, are also considered fruits. It is probably best just to explain that the way we define plant parts does not correspond well with the way we usually see the terms "fruit" and vegetable." If young children can understand that a fruit is a plant part with seeds, it will be a good start.

The basic parts of flowering plants include the roots, stems, leaves, and flowers. Stems are important in water and food transportation, and provide support for the leaves, flowers and fruit. Some plants store food (potatoes) or water (cactus) in stems. Flowers are necessary for seed production, without which many species would die out.

The main parts of a flower are drawn on the right. A plant with colored petals and/or a pleasant scent is usually pollinated by insects or birds. Wind or self-pollinated plants usually have rudimentary, inconspicuous flowers. Seeds develop in the ovary, and the ovary may develop into a fruit. Flowers that we eat include broccoli, cauliflower and artichokes, although the last one is actually the flower bud.

Tree trunks are a special kind of stem, and the age of a tree can be determined by counting the rings of a tree trunk. A ring is actually the new water-conducting tissue that is made each year. Thick rings mean that the tree grew a lot in the spring and summer. Thin rings reflect bad growing conditions, like drought, or just the fact that the tree grows slowly. Concentric rings mean that the tree grew straight up, and rings that are off-center, with one side wider than the other, mean that the tree grew on a slope (look at diagram to the left). The downhill side has wider rings. Seasonal variations in conditions may cause color variations inside a ring.

There are many different kinds of stems. They may be underground or above-ground, woody or soft, stiff or flexible. Stems that we eat include asparagus and potatoes. (Potatoes are very specialized stems that help to reproduce the plant. Small semi-circular leaf scars near the eyes show that they are stems.) Brussels sprouts are stem buds.

The plant kingdom includes seed plants, ferns and mosses. Plants can have one cell or many cells. Plants make their own food by producing simple sugars through a chemical process called photosynthesis. Most of the food produced by plants is made in the leaves. The leaves provide ample surface area for the absorption of light energy and contain many chloroplasts and it is in the chloroplasts where photosynthesis occurs.

Leaves are often modified to help the plant in other ways. Cactus leaves, for example, have spikes to protect the plant, and most of the food production occurs in the stem. Some leaves are waxy to help conserve water, and some are hairy, making them less palatable to animals. Venus fly trap leaves help capture insects, which provide nitrogen to the plant.

A leaf can be considered a plant organ, since it is made up of different tissue layers. The main function of a leaf is to produce food for the plant. The leaves are the sites where photosynthesis mainly takes place.

All during spring and summer the leaves manufacture the food needed for plant growth, especially in trees. This food-making process takes place in the leaf in numerous cells containing the pigment chlorophyll, which gives the leaf its green color. Along with the green pigment leaves also contain yellow or orange carotenoids which, for example, give the carrot its familiar color. Most of the year these yellowish colors are masked by the greater amount of green coloring. But in the fall, partly because of changes in the period of daylight and changes in temperature, the leaves stop their food-making process. The chlorophyll breaks down. The green color disappears, and the yellowish colors become visible and give the leaves their changing color.
Roots help anchor the plant to a substrate and draw water and minerals from the soil. Some roots form a shallow network underneath the soil surface, while others have tap roots that can go quite far down to find water and anchor the plant.

We eat a number of different kinds of leaves and roots. We eat the leaf blades of spinach and lettuce, and the stalks of celery leaves. Onions are layers of leaf bases (the lower part of the blade) that have been modified to store food for the plant. Carrots and radishes are examples of roots we eat.

1. Go over the parts of a flower. 

1. stamen = the pollen producing organ of a flower consisting of a long filament and an anther
2. petal = usually brightly colored with a distinct odor which serves to attract insects for cross-pollination (the transfer of pollen from one plant to another of the same type)
3. pistil = the seed-bearing female reproductive organ
4. sepal = one of the green segments forming the outer protective covering of a flower

2. Go over the parts of a tree, which is a complete plant with many specialized organs and tissues. The picture on the worksheet represents a cross section of the woody tissue.

1. bark = the wood covering of roots, stems and main trunks of trees and other wood plants
2. phloem = the pathways through which food material is conducted throughout the plant (moves food)
3. pith = the soft central portion of the branches and stems of plants
4. annual growth rings = alternative light and dark concentric rings in tree branches and stems due to periods of growth between summer and winter
5. xylem = the pathways through which water is conducted throughout the plant

Seeds come in many different shapes and sizes. A seed is the "baby" plant in waiting.

The purpose of the seed coat is primarily to protect the embryo (or baby) plant against such hazards as excessive drying, mechanical injury, and the digestive juices of animals (if it is eaten). Many seed coats are impermeable to water or oxygen or are hard which helps it remain "asleep" or dormant for a long time. When the conditions are right the dormant seed will grow. Seed coats are frequently specialized and may facilitate dispersal by wind or animals. The winged seeds of some species are familiar examples of wind-dispersed types. In some plants, the seed coat is adhesive (as in mistletoe) or becomes gummy when wet (as in mustard seeds) and adheres to the feet of birds. Hairy seeds may cling to the bodies of animals and be transported for long distances.

Seeds are used by other organisms as food, because seeds have food stored in them. Seeds furnish humans with great proportions of food. A large part of the world relies on the grains of wheat, rice, soybeans, corn, rye, and barley. Oils and fats are produced from the seeds of coconut, corn, cotton, flax, castor bean, sesame, peanut, and soybean. Oil from the cotton seed and peanut is utilized in the manufacture of various products such as butter and lard substitutes and soap. Linseed oil from flax seed is used in the manufacture of paints, varnishes, artificial leather, oilcloth, and linoleum.

Plants are an important food source for animals. Plants, like all other organisms, have developed unique strategies for reproduction. Most plants produce seeds, which are plants waiting to grow but which have the capacity to wait a long time before they begin the growth cycle.

Seeds may be scattered by wind, water, animals, or propulsion. Animals may spread seeds by a variety of ways such as by eating hard seeds which pass through the animal's digestive system unharmed or by picking up seeds on their coats and feathers. The propulsion method results when the seed covering opens in such a way that the seed shoots out. In many cases it is possible to look at a seed and figure out which method is used. For instance, if a seed has feathery extensions (like dandelions), then it can be sail through the wind, looking for a suitable place to germinate (grow).

Seeds insure that plants will continue to live on this Earth. It may take years before a seed will germinate, but this is a survival strategy. Plants have developed different methods to make sure their seeds find a suitable location to grow. Since plants themselves are not mobile they must have a mechanism to disperse, otherwise, all plants would grow in one area.

There are four basic methods of seed dispersal including by wind, by attachment to fur or feathers, by passing through an animal's gut, or by animals moving the seeds. Wind dispersal allows seeds to travel with the wind. Sometimes the distance that seeds travel can be long. For instance, if a seed gets into the upper atmosphere it can travel along the jet stream and travel hundreds, if not thousands of kilometers. Attachment to the fur of mammals or the feathers of birds helps seeds to "walk" or "fly" to a new location. The seeds may drop and fall into a suitable location to grow. When an animal eats a seed, sometimes its stomach cannot digest the outer portion of the seed and the seed is passed through the animal’s digestive system intact. Some animals move seeds purposely for storage and later consumption. Animals can drop seeds by mistake resulting in germination at that spot.

The first step in seed germination is the absorption of water through a small opening called the micropyle. The introduction of water through the pore causes the seed to swell. Placing a bit of candle wax over the pore will demonstrate that the seed will not swell when introduced into water. Many seeds will swell dramatically as the water enters, and you will notice a sweet, almost fermenting odor in the water after seeds have been soaked overnight (this is from enzyme action).

Ferns reproduce through primitive "seeds" called spores which are produced by the small pumps on the back of older leaves. The dark brown, tiny spores are single cells that will develop into plants if they land in the perfect environment. Seeds are more likely to produce plants in environments that are less than perfect. Spore producing plants produce tremendous numbers of spores in order to have just a few develop into plants.



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