Botanical Term

Endogenous Dormancy

Anther, Ovary, Style

Embryo, Endosperm, Seed Coat

Endogenous Dormancy
Endogenous dormancy occurs when special conditions within the embryo prevent germination from occurring. This class of dormancy is divided into three major classes – physiological dormancy, morphological dormancy and combined dormancy.

Physiological dormancy occurs when chemicals or enzymes are present within the embryo prevent germination from occurring. Germination will not occur until these chemicals or enzymes are broken down. Many seeds, when released from the plant, contain too much moisture. Simply drying this seed will allow it to germinate. Most grasses and grains like wheat and corn fall into this category.

Other seeds exhibit photodormancy and must either be exposed to light or have light excluded for the seeds to germinate. Begonia, impatiens and snapdragons and other species with very small seeds are examples of seeds which exhibit photodormancy properties and must be sown on the surface of the medium for them to germinate.

Thermodormancy is a third major division of physiological dormancy. This type of dormancy requires certain temperature requirements to be met before germination to occur. Some types of plants need extremely hot temperatures to germinate. These include most species of cacti and amaranth seeds. Others, like lettuce, celery, and the cabbage species, need cool temperatures to germinate. Many seeds of wildflowers, trees and shrubs need a period of cold, then a period of warmth before they will germinate. This type of germination is broken by using a process called stratification. The seeds are planted in the soil medium, moistened and covered with plastic and then stored in a refrigerator for a period of time before being placed in the germination area.

Combined dormancy is when the seed displays a combination of different types of dormancy. It may need stratification and then exposure to light, for instance.

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Botanical Term

Exogenous Dormancy

Anther, Ovary, Style

Exogenous Dormancy

Embryo, Endosperm, Seed Coat

Exogenous Dormancy

Exogenous Dormancy refers to dormancy induced by conditions outside the seed (embryo) and the term usually means conditions in the seed coat. Exogenous dormancy can be of two forms, chemical or physical with physical dormancy making up the vast majority of seeds with exogenous dormancy. These seeds are sometimes called “hard seeds.” This type of dormancy may be broken by a process called scarification. Treatment with sulfuric acid can also be used but this is not recommended for the home gardener. Using boiling water may also be used to break exogenous dormancy. Bring the water to a boil, drop the seed in and remove the water from the heat source. Do not leave the seed in the water very long or you risk killing the seed.

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Botanical Term

Anther, Ovary, Style

Stratification – Seed

Embryo, Endosperm, Seed Coat

Stratification – Seed

Stratification refers to the process some seeds need to undergo before they will germinate. There is both cold and warm stratification.

Cold stratification is a period of cool temperatures needed before the seed will germinate. For cold stratification the seed is placed in a planting medium, watered, and placed under refrigeration for a specified period of time. Usually the seed container is covered with plastic before refrigeration to conserve moisture. The seeds usually don’t need watering during the stratification period, but
periodic checking of the moisture level of the planting medium should be done.

Warm stratification is needed by some seeds before germination will occur. The same procedure is followed as for cold stratification, only the seed medium is placed in a warm environment

Some plants require both warm and cold stratification, generally the warm first, then the cold. Plant seed which requires stratification usually have different requirements. Sometimes gardeners just plant the seeds in an outdoor bed, letting nature do the work. Others, wanting more precise control, will store the seeds in a refrigerator, placing them in the stratification medium on a scheduled basis. Refrigeration of seeds in seed packets, dry, does not satisfy the stratification requirements, as the seeds must have moisture available to them during the stratification process.

There are special requirements for some seeds. Some seeds need to be frozen for a period of time in the moistened planting mediums, followed by cold stratification. Annual plants usually do not
need any form of stratification, just remove from the seed packet and plant in warm, moist conditions. Some perennials and trees need the specialized process of stratification. So if you have had problems germinating some perennial and tree seeds, possibly you have not fulfilled the stratification requirements of the seed.

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Botanical Term

Thermodormancy

Anther, Ovary, Style

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Embryo, Endosperm, Seed Coat

Thermodormancy
Thermodormancy is a type of dormancy in which temperature plays a role in breaking dormancy and beginning germination.
Many plants, such as ablemoschus, amaranth, cocklebur and most cactus species, need high temperatures to germinate. These seeds will not germinate until soil temperatures have reached high levels.
Other plants, such as lettuce, pansy and radish, need cool temperatures to germinate properly and many will simply not germinate in high soil temperatures. Some may germinate, but at lower levels or the resulting seedlings may lack vigor.

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Photodormancy

Anther, Ovary, Style

Embryo, Endosperm, Seed Coat

Photodormancy
Photodormancy is a type of dormancy exhibited by many types of seed which need exposure to light to germinate. This type of dormancy is advantageous to plants by allowing the seeds to remain dormant until favorable growing conditions exist. Many types of woodland plants exhibit photodormancy which prevents the seed from germinating until spring before the leaf cover develops and soil and weather conditions are congenial for growth. Plants which require full sun may also exhibit photodormancy. The seeds may be prevented from germinating while in the shade of competing vegetation.
Common garden seeds which have the photodormancy trait are lettuce, begonia, impatiens and snapdragon. There are many others. These seeds, when planted, should not be covered when seeded in. They should be planted and lightly misted, then exposed to warmth and light. Most seeds which are photodormant are very fine seeds, almost like dust.

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Botanical Term

Scarification

Anther, Ovary, Style

Embryo, Endosperm, Seed Coat

Scarification
Many seeds have very hard seed coats which make them impervious to soaking up water, preventing seed germination. In the natural world many of these seeds must pass through the digestive systems of an animal or bird before it will germinate, naturally scarifying them by exposing them to high acid stomach fluids or the gizzards of the birds. This is advantageous to the plant because it allows its seed to be disseminated over a larger area because of the bird or animal’s wanderings. Other seeds from prairie or desert areas need extremely high temperatures to germinate. These temperatures are caused by wildfires which may rage over an area, clearing the way for new plants to grow. Once the seed coats have been weakened by the high temperatures of a fire, the seed will germinate and grow in a freshly cleared environment.
Seeds with these hard seed coats can be difficult for the planter to germinate. A process called scarification is used to abrade or crack the seed coat and allow water to penetrate to the embryo, allowing it to germinate.
Scarification methods range from physical scarification to chemical scarification. On a small scale for the home gardener a rasp, file or coarse sandpaper may be used to abrade the seed coat, thinning it and cutting away the hard outer coat. Larger nursery operations use mild acids and other solutions to break down the seed coat.
Once the hard seed coat is thinned or even partly cut through the seed is placed in soil media and germinated according to the needs of the species it belongs to.

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Botanical Terms Used
scarification
photodormancy

Thermodormancy
stratification
Exogenous Dormancy
Endogenous Dormancy

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After the seed is formed it enters a dormant state in which little cellular activity takes place. The advantage of dormancy is ensuring that germination of the seed will not occur until the seed has been disbursed. Breaking this seed dormancy for most plants grown in the garden is simple. All you have to do is supply the proper conditions for growth and the seed, if it is viable, will germinate and grow.

Many plant seeds display more complex types of seed dormancy, though, and if the requirements needed to break this dormancy are not met the seed will not germinate even if proper germination conditions are met. The advantages of these more complex types of seed dormancy for plants is that they allow the plant to survive conditions in which the seedling would not survive the conditions in which it is produced. Plants have devised many forms of dormancy to help them survive the often harsh environments in which they exist.

Types of Seed Dormancy
No hard and fast classification rules have been established for seed dormancy but two main classes of seed dormancy types are generally recognized. These major divisions are exogenous and endogenous dormancy.

Exogenous Dormancy
This type of dormancy refers to dormancy induced by conditions outside the seed’s interior, mainly in the seed coat. This dormancy type is usually broken down into two divisions, physical and chemical.

Physical dormancy usually denotes a seed coat which is impervious to water. This type of seed coat will not allow moisture to pass to the embryo, so germination will not occur. Breaking this type of dormancy usually involves cracking or chipping of the seed coat to allow water to penetrate. This process is called scarification. This can be a tedious operation, usually done with a file or rasp. Care must be taken so you do not scarify your fingers while rasping the seed. In nature this process may be done by alternate freezing and thawing of the seed. The advantage for the plant is that the seed will not germinate until the rigors of winter have cracked the seed coat, allowing the plant to germinate in the spring.

Some seed coats contains germination inhibiting chemicals or enzymes which will not allow germination to occur. This type of dormancy is referred to as chemical dormancy. Once these chemicals or enzymes are leached from the seed coat, germination will occur. The advantage of this type of dormancy for a plant is that the seed will not germinate until there is sufficient moisture present to supply the young plant until its root system is established. Breaking this type of dormancy is simple. Just soak the seed in water for a period of time, supply the proper germinating conditions for the seed and it should germinate.

Endogenous dormancy occurs when special conditions within the embryo prevent germination from occurring. This class of dormancy is divided into three major classes – physiological dormancy, morphological dormancy and combined dormancy.

Physiological dormancy occurs when chemicals or enzymes are present within the embryo prevent germination from occurring. Germination will not occur until these chemicals or enzymes are broken down. Many seeds, when released from the plant, contain too much moisture. Simply drying this seed will allow it to germinate. Most grasses and grains like wheat and corn fall into this category.

Other seeds exhibit photodormancy and must either be exposed to light or have light excluded for the seeds to germinate. Begonia, impatiens and snapdragons and other species with very small seeds are examples of seeds which exhibit photodormancy properties and must be sown on the surface of the medium for them to germinate.

Thermodormancy is a third major division of physiological dormancy. This type of dormancy requires certain temperature requirements to be met before germination to occur. Some types of plants need extremely hot temperatures to germinate. These include most species of cacti and amaranth seeds. Others, like lettuce, celery, and the cabbage species, need cool temperatures to germinate. Many seeds of wildflowers, trees and shrubs need a period of cold, then a period of warmth before they will germinate. This type of germination is broken by using a process called stratification. The seeds are planted in the soil medium, moistened and covered with plastic and then stored in a refrigerator for a period of time before being placed in the germination area.

Combined dormancy is when the seed displays a combination of different types of dormancy. It may need stratification and then exposure to light, for instance.

Seed dormancy can be a very complicated process to break and there is not general guide to breaking seed dormancy for all seeds. Plants have evolved to survive many different types of environments and have different needs based upon those environments. A study of the plant and where it grows may give clues to the methods of breaking the dormancy of any individual seed.
Plants are wondrous things which have evolved many processes which enable them to survive, seed dormancy being just one of these processes.

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Botanical Term

Zygote

Anther, Ovary, Style

Embryo, Endosperm, Seed Coat

Zygote
The zygote is the cell that is formed after the sperm fertilizes the egg. It is a fertilized egg. The word is derived from the Greek word “zygotos” which means “yoked” or united. The zygote is a single cell which soon begins to divide and grow, forming the embroyo.

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Botanical Term

Sperm

Anther, Ovary, Style

Embryo, Endosperm, Seed Coat

Sperm
The sperm is the male sex cell necessary for sexual reproduction. It is produced in the anther of the flower and transferred in the pollen grains to the stigma.

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