Germination



Untitled-2a seeds copy


Seeds
Seeds vary from plant to plant in size and shape. The majority of seeds have similar structures. Most seeds are surrounded by a hard outside called the testa. Just beneath the testa is the endosperm. The endosperm is the food stores of the seed. It contains protein, fats, and starches which provide energy for the developing seedling.










Untitled-3a seeds

The cotyledons provide a similar function. These are the seed leaves and they also contain storage of food for the developing plant. The cotyledons in some cases are major photosynthetic producers during germination.
















DSCN0270a copySometimes sandwiched between the cotyledons or pushed compactly against the cotyledon are the remaining components of the seed. The radical, the hypocotyls and the plumule are found in most seeds. The radical is the first structure to appear from the seed. It is the root emerging and growing downward into the soil.










DSCN0269a copyThe hypocotyl is part of the stem that will emerge from the seed and is located between the radical and the cotyledon. The plumule is the first bud of the germinating seed and is located at the top of the seedling.

















Dormancy


When a seed does not germinate immediately after leaving the parent plant, it goes into a period of dormancy or inactivity. Dormancy is sometimes required for a seed to be viable. A seed can sometimes stay in a state of dormancy for years. In order for a seed to come out of this dormancy state, conditions have to be ideal. Ideal conditions depend of the seed type but they include factors like: moisture, temperature, light and in some cases even fire. For example, the seeds of the lodgepole pine are only released from the cone after being exposed to fire. Some seeds need complete darkness to germinate, while others need to germinate in the light.
Dormant seeds contain less than 10% water making them very dry. This enables the seed to be preserved and survive to the time of germination. At the time of germination a seed begins to absorb water and it begins to swell. Also the seed will begin to use its own food storage contained in the endosperm. The seed coat will split and the radical will emerge.


Roots
The radical is the first part of the seed that emerges.
IMG_001a copy2It is the root and begins to grow downward into the soil as soon as it appears. Its primary purpose is to begin to absorb water and nutrients for the developing seedling. The radical is surrounded by a protective sheath called the root cap. This protects the fast growing root as it pushes through the soil. The portion at the tip of the root where growth takes place is called the apical meristem. Here, cells are elongated and rapidly dividing promoting growth. As the root develops and grows deeper, it is called a tap root and will produce lateral roots which are offshoots of the primary root. The tap root or primary root becomes a food storage area and it helps in anchoring the plant in place. The lateral roots increase the absorption of water for the plant. Also present are root hairs. These are tiny extensions off of the primary root and lateral roots that also help in absorbing water and nutrients. At the center of the root is the core or stele. Here is where the conducting “tubes” of the plant are located, called xylem and phloem. Water that is absorbed in the roots gets transported upward to the rest of the plant through the xylem tissue. Phloem transports sugars from the top of the plant that is produced during photosynthesis downward to the rest of the plant.





Grasses


Flowering plants fall under two categories or classes. The first class is monocots. The second class is called eudicots. Grasses are considered to be moncots. Monocots are called monocots because their seeds contain one cotyledon (mono means one). Eudicot seeds have two cotyledons (eu meaning true, di meaning two). Cotyledons are housed in the seed and are the embryonic seed leaves. Grasses are wind pollinated. Their flowers have no petals or sepals. They have three carpels that are fused together to one, and two separate stigmas. Usually in grasses, male and female flowers are located on the same plant. The majority of the grass seed is endosperm. Grasses are well adapted to continuous grazing by animals. This is because of the location of the shoot apical meristem in grasses. The shoot apical meristem is the actively growing region of the plant. Typically this region is located at the top of a plant, but in grasses it is located near the bottom of the leaves and stems. This is helpful because it allows for the top portion of the grass to be eaten and the shoot apical meristem is stimulated to grow.
There are about 8000 species of grasses. An interesting example of a grass is bamboo, which is used in for constructing various things like furniture. Examples of grasses that we eat are: wheat (
Triticum), Oats (Avena), corn (Zea), and rice (Oryza).









Sources:
Biology of Plants, 6th ed. Raven, Peter H., Everert, Ray F., Eichhorn, Susan E. Worth Publishing 1999.

Botany: An Introduction to Plant Biology. Mauseth, James D. Jones and Bartlett Publishers. Sudbury, Massachusetts 1998.

Eyewitness Visual Dictionaries: Visual Dictionary of Plants. Dorling Kindersley, Inc. New York 1992.

Plant Physiology 4th ed. Salisbury, Frank B., Ross, Cleon W. Wadsworth, Inc Belmont, California 1992.

Time Life Understanding Science and Nature: Plant Life. The Time Inc. Book Company 1993