From Seed to Adult Plant and Back

Transcription

1 W ISCONSIN FAST P LANTS: BEGINNING C YCLE From Seed to Adult Plant and Back These tiny Wisconsin Fast Plants seeds look rather insignificant compared to this bean seed. But the plants that result from each type of seed are quite impressive, not to mention nutritious. Take a look at some Fast Plants seeds. They are so tiny! They re hard and appear to be dry. In fact, they don t even look like they are alive. How does one of these small seeds turn into a plant? The process by which a seed becomes a plant is called germination. Because germination normally takes place underground, we do not see it happen. All we see is the exciting result a tiny green plant emerging from the soil. But suppose you had x-ray vision and could follow each step of what happens underneath the soil as a seed germinates. Here is what you would see. Right after planting, a bean seed would look pretty much like it did before you placed it in the soil. That s because the seed is in a dor- mant or inactive stage. Protected by its tough seed coat, the seed can withstand extremely hot and cold temperatures. Some varieties of seeds can remain dormant for years. A dormant seed may look lifeless, but inside, it s a different story. Inside the seed is an embryo, which contains the tiny beginnings of a root, a stem, and leaves. These will become the major parts of the mature plant. An important food-storing tissue called endosperm surrounds the embryo. Also contained in the seed are one or more seed leaves, called cotyledons. The seeds of some plants only have one cotyledon. These plants are called monocotyledons, or monocots for short. Other plants, whose seeds contain two cotyledons, are called dicotyledons, or dicots. (continued) STC/MS O RGANISMS FROM M ACRO M ICRO 59

2 Monocot seeds, which include cereal grains, wheat, rice, and corn, use energy from the endosperm to nourish the developing embryo and germinating seedling. Their cotyledon never emerges from the soil. Dicot seeds, such as bean and Fast Plants seeds, transfer energy from the endosperm to the cotyledons. These cotyledons emerge from the soil with the stem and continue to provide energy for the developing plant. After the bean seed has been underground for a while, it begins to absorb water and gets larger. This happens faster when the ground is warm. Finally, the process of germination begins. A root emerges from the seed. This root, called the primary root, grows downward. It forms an anchor for the developing plant. Tiny root hairs and secondary roots form. Root hairs are microscopic, fingerlike extensions of the outer cells of the roots that give the root more surface area through which water and minerals may enter the plant. Meanwhile, there is further activity in the seed. Water, which comes in through the root hairs, causes the tightly packed cells in the tiny young stem to elongate. This causes the stem to push upward. The cotyledons, which are at the top of the stem, are dragged upward through the soil until they poke through its surface. As the seed leaves sense light, they expand. 60 STC/MS O R G A N I S M S F R O M M A C R O Leaves Embryo Seed coat Cotyledons (seed leaves) Notice the seed coat, cotyledons, and embryo of this bean seed. No matter what position the seed is in, the roots will still grow downward.

3 In bean plants, the cotyledons provide the energy for germination and emergence from the soil. As soon as the true leaves form and begin to function, the cotyledons wither and die. In Fast Plants, the cotyledons also supply energy for germination and emergence, but upon emergence, they expand, turn green, and gather energy from the sun for further growth until true leaves form to take on that role. As the bean plant grows, it develops more leaves and its root system matures. The plant now is able to get the nutrients it needs from the soil, water, and air. It can also manufacture its own food in its leaves in a process called photosynthesis. The plant eventually matures and flowers. The flowers are pollinated, fertilization occurs, and seeds develop. As the flowers wither, a fruit develops. The fruit provides a protective covering, which supports the development of the seeds, and aids in their dispersal. When these fruits decay, their seeds are left behind. Many are eaten by animals; some mold. But others survive, and given proper conditions, will germinate and grow and the life cycle begins again.! Three stages in the development of a young bean plant 1998 WILLY A. VERHEULPEN If you plant a seed and it does not germinate, what could be the reason? Sometimes, when the soil is over-watered, the seed does not get enough oxygen. Occasionally, a seed doesn t germinate because it is too cold or it doesn t get enough water. More often than not, though, if a seed does not germinate, a fungus is the culprit, as you can see in this photograph. STC/MS O R G A N I S M S F R O M M A C R O 61

4 The bean plant is looking more like an adult each day. Soon, the seeds will fall from the ripened seed pods and the cycle will begin again. 62 STC/MS O R G A N I S M S F R O M M A C R O

5 W ISCONSIN FAST P LANTS: BEGINNING C YCLE Table 5.1 Wisconsin Fast Plants Maintenance Chart Directions This table gives directions for tasks that your group must perform during the development of your Fast Plants in addition to ensuring that the plants receive a constant supply of light and nutrient solution. Day Sow Fast Plants Plants should seeds. have sprouted. If not, start over Thin growing Flower buds system to six begin to appear. plants Begin Continue Continue Continue Last day for pollinating Fast pollinating pollinating pollinating pollinating Plants flowers. flowers. flowers. flowers. flowers. Cut off unopened buds Remove nutrient solution from reservoirs Harvest your seeds. Set them up for germination Observe the stem and leaf color of the growing sprouts. STC/MS O RGANISMS FROM M ACRO M ICRO 63