BACKGROUND:
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.
PROCEDURE:
- Instruct students to list some
uses of plants: food, drink (cola, wine), lumber, clothes (cotton),
medicine (aspirin) and paper (trees). Plants also release oxygen into
our atmosphere which is then used by animals for respiration.
- Ask students to name some plants.
There are hundreds of thousands of species of plants throughout the
world. There are small plants and large plants; plants that live in
water or land; and even plants that can survive and live in environments
without soil.
- Plants utilize seeds for
reproduction. The seed is dormant until the right conditions allow it to
grow. Review the diagram of the seed on the preceding page and discuss
once more the requirements for plant growth.
- On the worksheet, have the
students examine the different germinating seeds, including the squash,
garden bean, pea seed, and corn grain. Instruct students to compare the
4 seeds and write down the differences and similarities that they
observe.
In the squash, the cotyledons are
lifted out of the ground and function as green leaves for a brief
period. Garden beans have cotyledons which are lifted out of the soil,
but they are bulky storage organs and do not function as leaves. In pea
seeds the cotyledons remain in the soil. A germinating corn grain is
really a fruit in which the seed coat is fused with the ovary wall.
