Let’s germinate some seeds!
Germination is defined as the process by which an organism grows from a seed or similar structure that leads to the formation of a seedling.
The seed of the vascular cannabis plant is produced in the female flower after the union of male and female reproductive cells. The majority of fully developed seeds contain an embryo and in most plant species, some form of food reserve wrapped in a seed coat.
The seed coat is the outer covering of a seed that helps protect the embryo from injury and from drying out.
The stages of germination of a cannabis plant.
C. Primary Root
D. Secondary Root
H. Tap Root
The radicle is the first organ to appear when a seed germinates. It grows downward into the soil anchoring the seedling, forming what becomes a primary root, or the taproot, which is larger in diameter than the secondary lateral roots and grows downward.
Secondary roots, called lateral roots, grow out the sides of primary roots.
Tertiary roots grow out the side of secondary roots, and help anchor the plant into the growing medium, keeping it upright and holding it in place. They work together as a whole with the secondary and primary roots to uptake water and nutrients from the growing medium.
Cotyledons are the first small, round leaves produced by plants. They are not considered true leaves and are sometimes referred to as "seed leaves," because they are actually part of the seed or embryo of the plant.
In cannabis plants, cotyledons contain the stored food reserves of the seed. As these reserves are used up, the cotyledons may turn green (beginning photosynthesis,) or may wither as the first true leaves take over food production for the seedling.
The plumule is the part of a seed embryo that develops into the shoot bearing the first true leaves of a plant. Growth of the plumule does not occur until the cotyledons have grown above ground.
Look at this 2004 study of sexual differentiation of cannabis:
Scientists identify fragments of gene sequencing that determine the sex for both male and female plants. They also identified certain gene fragments which may play a role in the development of hermaphrodite plants.
In any case, the results of the study show that the genetics of the plant play a role in determining the sex. The commitment to a specific sex is thought to take place as soon as the leaves of the fourth node emerge.
Very early on, stress and other environmental factors can also influence the sex determination by the plant (often leaning towards males when they get stressed out.)
Dormant seeds are viable seeds that don’t germinate because they require stimulation from specific internal or external (environment) to start growth. Under proper conditions, the seeds begin to germinate and the embryo resumes growth, developing into a seedling.
Seed dormancy can originate in different parts of the seed, such as the embryo (internal) or the seed coat (external.) Breaking dormancy often involves changes in the membranes which is initiated by dormancy-breaking signals. This generally occurs only within hydrated seeds.
Factors affecting seed dormancy include the presence of certain plant hormones, most notably abscisic acid (inhibits or hinders germination) and gibberellin (plant hormones that help end seed dormancy and aides in cell elongation, which makes branches longer.) Seeds can be treated with gibberellic acid to help ensure even germination.
Some plants produce varying numbers of seeds that lack embryos; these are empty seeds which never germinate.
Germination rate describes how many seeds in a particular plant species, variety or seed lot are likely to germinate over a given period. It is a measurement of germination and is usually expressed as a percentage, so for example a 75% germination rate would indicate that about 75 out of 100 seeds will likely germinate under optimal conditions over the course of the germination period.
Seed germination rate is determined by the seed genetic composition, morphological features (physical form and external structure, like leaf area, node density, bud number, etc) and environmental factors.
Germination rate is useful for calculating the number of seeds needed for a given area or desired number of plants. For seed physiologists and seed scientists "germination rate" is the reciprocal of time taken for the process of germination to complete starting from time of sowing.
Germination capacity is the number of seeds able to complete germination in a population (like a seed lot.
Seed germination depends on both internal and external conditions.
Various plants require different variables for successful seed germination. Often this depends on the individual seed variety and is closely linked to the ecological conditions of a plant's natural habitat. For some seeds, their future germination response is affected by environmental conditions during seed formation; most often these responses are types of seed dormancy.
Water is required for germination. Mature seeds are often extremely dry and need to take in significant amounts of water, relative to the dry weight of the seed, before cellular metabolism and growth can resume. Most seeds need enough water to moisten the seeds but not enough to soak them.
The uptake of water by seeds is called imbibition, which leads to the swelling and the breaking of the seed coat. When seeds are formed, most plants store a food reserve with the seed, such as starch, proteins, or oils. This food reserve provides nourishment to the growing embryo. When the seed imbibes water, hydrolytic enzymes are activated which break down these stored food resources into metabolically useful chemicals.
After the seedling emerges from the seed coat and starts growing roots and leaves, the seedling's food reserves are typically exhausted; at this point photosynthesis provides the energy needed for continued growth and the seedling now requires a continuous supply of water, nutrients, and light.
Oxygen is required by the germinating seed for metabolism. Oxygen is used in aerobic respiration, the main source of the seedling's energy until it grows leaves. Oxygen is an atmospheric gas that is found in soil pore spaces; if a seed is buried too deeply within the soil or the soil is waterlogged, the seed can be oxygen starved. Some seeds have impermeable seed coats that prevent oxygen from entering the seed, causing a type of physical dormancy which is broken when the seed coat is worn away enough to allow gas exchange and water uptake from the environment.
Temperature affects cellular metabolic and growth rates.
Seeds from different species and even seeds from the same plant germinate over a wide range of temperatures. Seeds often have a temperature range within which they will germinate, and they will not do so above or below this range. Many seeds germinate at temperatures slightly above 60-75 F (16-24 C) [room-temperature in centrally heated houses], while others germinate just above freezing and others germinate only in response to alternations in temperature between warm and cool. Some seeds germinate when the soil is cool 28-40 F (-2 - 4 C), and some when the soil is warm 76-90 F (24-32 C). Some seeds require exposure to cold temperatures (vernalization) to break dormancy. Some seeds in a dormant state will not germinate even if conditions are favorable. Seeds that are dependent on temperature to end dormancy have a type of physiological dormancy. For example, seeds requiring the cold of winter are inhibited from germinating until they take in water in the fall and experience cooler temperatures. Cold stratification is a process that induces the dormancy breaking prior to light emission that promotes germination . Four degrees Celsius is cool enough to end dormancy for most cool dormant seeds, but some groups, especially within the family Ranunculaceae and others, need conditions cooler than -5 C. Some seeds will only germinate after hot temperatures during a forest fire which cracks their seed coats; this is a type of physical dormancy.
Most common annual vegetables have optimal germination temperatures between 75-90 F (24-32 C), though many species (e.g. radishes or spinach) can germinate at significantly lower temperatures, as low as 40 F (4 C), thus allowing them to be grown from seeds in cooler climates. Suboptimal temperatures lead to lower success rates and longer germination periods.
Light or darkness can be an environmental trigger for germination and is a type of physiological dormancy. Most seeds are not affected by light or darkness, but many seeds, including species found in forest settings, will not germinate until an opening in the canopy allows sufficient light for growth of the seedling
Scarification mimics natural processes that weaken the seed coat before germination.
In nature, some seeds require particular conditions to germinate, such as soaking in a body of water for a long period of time.
The appearance of the radicle typically marks the end of germination and the beginning of the establishment of being a seedling, a period that utilizes the food reserves stored in the seed.
Germination and establishment as an independent organism are critical phases in the life of a plant when they are the most vulnerable to injury, disease, and water stress
The germination index can be used as an indicator of phytotoxicity in soils. The mortality between dispersal of seeds and completion of establishment can be so high that many species have adapted to produce large numbers of seeds.