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Germination  [ Botany - Biology ]
Derived form: Germinative

Dictionary of botanic terminology - index of names
     
  The germination is the initial growth phase of a seed, spore or pollen grain.  
     
The germination is the process of emergence of growth from a resting stage, as the activation of a mature dormant seed causing it to sprout and develop the first tiny shoot and root; also the production of a pollen tube by a pollen grain deposited on a stigma or the growth of a vegetative cell or hypha from a fungal or bacterial spore. In a more broad sense, germination can imply anything expanding into greater being from a small existence or germ, either animal or vegetable.


Left: Germination of a seed of Astrophytum caput-medusae
     
Germination of Seeds
The seed of a higher plant is a small package produced in a flowering plant or gymnosperm containing an embryo and stored food reserves. The seed looks apparently dead. In fact, even with biochemical tests for the metabolic processes we associate with life (respiration, etc.) the rate of these processes is so slow that it would be difficult to determine whether there really was anything alive in a seed. Germination is the resumption of growth of the dormant embryonic plant inside the seed; it implies complex physical and chemical changes that occur as the embryo begins to develop into a young shoot and root (seedling). The germinating seed sends its first root (radicle) into the soil and the first stem with the first leaves (cotyledon) toward the sunlight.
In some definitions, the appearance of the radicle marks the end of germination and the beginning of establishment, a period that ends when the seedling has exhausted the food reserves stored in the seed. These are critical phases in the life of a plant. The mortality between dispersal of seeds and completion of establishment can be so high, that many species survive only by producing huge numbers of seeds. If a seed is not allowed to germinate within some certain length of time, the embryo inside will die. Each species of seed has a certain length of viability, varying from a few weeks up to 2000 years.  
A seed will germinate, or sprout, when conditions are right for survival, some seeds require particular conditions before they will germinate, these conditions encompass adequate moisture, heat, and/or light, also some species of seed have particular needs such as the heat of a fire (e.g., many Australian native plants), or soaking in a body of water for a long period of time. When a mature seed is placed under favourable conditions and fails to germinate, it is said to be dormant.
The length of time plant seeds remain dormant can be reduced or eliminated by some simple seed treatments called stratification, vernalization, soaking (imbibition).
 
Germination in Dicotyledons
  1. The primary root emerges through the seed coats while the seed is still buried in the soil.
  2. The hypocotyl emerges from the seed coats and pushes its way up through the soil. The two cotyledons protect the epicotyl structures — the plumule — from mechanical damage.
  3. Once the hypocotyl emerges from the soil, it straightens out.
  4. The cotyledons spread apart exposing the epicotyl, consisting of the primary leaf (or leaves) and the apical meristem.
    In many dicots, the cotyledons not only supply their food reserve to the developing plant but also turn green and make more food by photosynthesis until they drop off.
Germination in Monocotyledons
  1. The primary root pierces the seed (caryopsis) coverings and grows down;
  2. the primary leaf of the plant grows up. It is protected as it pushes up through the soil by the coleoptile — a hollow, cylindrical structure.
  3. Once the seedling has grown above the surface, the coleoptile stops growing and the primary leaf emerges and expands.
   
Environmental Requirements for seed germination:
1) Temperature
2) Water: Water is always needed to allow vigorous metabolism to begin. It is also sometimes needed to leach away a germination inhibitor within the seed. This is especially common among desert annuals. The inhibitor is often abscisic acid (ABA) but also Phenolic Compounds
3) A preceding period of dormancy (often): The seeds of many temperate-climate angiosperms will germinate only after a prolonged period of cold. An inhibitor within the seed (probably abscisic acid ABA)) is gradually broken down at low temperatures until finally there is not enough to prevent germination when other conditions become favourable. This mechanism is of obvious survival value in preventing seeds from germinating during an unseasonably warm spell in the autumn.
4) Oxygen
5) Correct photoperiod (often).

 
category of Seeds dormancy
Seeds Lacking True Dormancy
  This kind of seeds are ready to sprout. All they need is some moisture to get their metabolism activated, and temperature warm enough to allow the chemistry of life to proceed. Seeds taken from the wild, however, are frequently endowed with deeper forms of dormancy. Common vegetable and garden seeds generally lack any kind of dormancy.
Seeds With Dormant Embryos, There are several mechanisms that permit seeds to be truly dormant.
1) Thick Seed Coat: Many kinds of seeds have very thick seed coats that keep water out of the seed, so the embryo cannot get the water needed to activate its metabolism and start growing. This seed need to nick in some way the thick seed coat to initiate germination.   (see: scarification)
2) Thin Seed Coat: A thin seed coat is so thin that it don’t represent a barrier to water. Some other kind of dormancy mechanism is needed. This seed can absorb light and respond biochemically to light signals. This is possible for the presence in the seed of a light sensitive pigment called phytochrome which is biologically very active and plays a role in all systems when a plant needs to know if the lights are "on" or "off."
3) Insufficient Development: If a seed's embryo is not completely developed, some additional maturation may be needed before the seed can sprout. This happens in seeds with little-to-no storage material invested in the seed. Examples include orchid seeds. Such a seed needs an symbiosis with fungi to feed the developing embryo until the embryo is mature enough to penetrate the seed coat.
4) Inhibitors Present: Many plant species invest chemicals in the developing seeds, and these chemicals inhibit the development of the embryos. They keep the embryos dormant. Obviously the seed must have some way to eliminate these chemicals before they can sprout.
   

 


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Holdfast roots  [ Botany  ]

Dictionary of botanic terminology - index of names
 
     
  Some species of climbing plants develop holdfast roots which help to support the vines on trees, walls, and rocks. By forcing their way into minute pores and crevices, they hold the plant firmly in place.  
     
Climbing plants, like the poison ivy (Toxicodendron radicans), Boston ivy (Parthenocissus tricuspidata), and trumpet creeper (Campsis radicans),  develop holdfast roots which help to support the vines on trees, walls, and rocks. By forcing their way into minute pores and crevices, they hold the plant firmly in place. Usually the Holdfast roots die at the end of the first season, but in some species they are perennial. In the tropics some of the large climbing plants have hold-fast roots by which they attach themselves, and long, cord-like roots that extend downward through the air and may lengthen and branch for several years until they strike the soil and become absorbent roots.

Major references and further lectures:
1) E. N. Transeau “General Botany” Discovery Publishing House, 1994
     

 

 

 

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