A plant seed comes about as the result of pollination in which 2 gametes from same (self pollination) or different plants (cross pollination) unite to create a zygote. The zygote develops within the ovary to become an embryo – the beginnings of a new plants- and this embryo is protected inside a seed.
A plant seed is comprised of the tests , endosperm, the hypocotylswhich are the first leaves and radical which is the first root.
Inside every seed is everything needed for a new plant. There is genetic information, a food store and an embryonic plant.
The endosperm is in a structure called the cotyledon which provides food for the embryo and there may be one or two. Plants are classified according to the number of cotyledons they possess in their seeds (monocotyledon or dicotyledon respectively).
The hard seed coat or testa protects the seed and this may be thick such as in seeds like sweet pea or thin like that of begonia seeds. Similarly, the cotyledon may be tiny (begonia) or large (beans). The testa is interesting in many ways. It not only protects the seed inside but it may also protect the seed while it passes through the gut of an animal, protecting the delicate embryo from damage by enzymes or it may contain germination inhibitors such as giberellins which prevent the seed germinating as soon as it falls. This is particularly important in temperate zones where, if the seed germinated when it fell in the fall, it would mean the young plants faced the most severe weather. Instead, germination inhibitors fill pores inthe seed coats and are gradually washed away over time until spring when the water coming in, combined with the riseintemperatures and increased light, encourages germination.
Germination is triggered by imbibitions or water being absorbed into the testa, softening it and moistening the cell walls of the seed so the next stages can proceed.
The cells of the seed take water in by osmosis, often assuming twice the volume of the dry seed. The water triggers enzymes to react and respiration to increase (seed dormant not dead). For germination to proceed a continuous supply of water is needed but water logging must not occur as respiration is also needed.
Temperature is important for seeds, particularly for germination as it affects germination and the exact point of germination is often specific to the species or even to the cultivar. Temperature affects the rate at which enzymes react and also the rate of respiration. Some plants germinate only within narrow ranges e.g cucumber minimum temperature of 15 deg C , tomatoes 10 deg C. All seeds will be prevented from germination in temperatures above 40 deg Celsius.
Some seed will germinate throughout wide temperature ranges. Mustard, for example, will germinate at near freeqing right up to 40 deg C.
Seed also reacts to light and this affects germination in some species while others are indifferent. Seeds of rhododendron, phlox and veronica are inhibited by exposure to light while in celery, lettuce, most grasses, conifers and many flowering plants germination is slowed down when light is excluded.
Seed also react to different wavelengths of light. Far red light (invisible to humans) inhibits germination in some trees such as birch, while red light promotes it. A canopy of tall, deciduous trees filters out red light for photosynthesis so seeds of these species receive mainly far red light and are prevented from germinating. Clearing of a tree allows red light to reach ground and seeds germinate which means that while trees form a canopy, their own seeds do not germinate but if the tree falls, leaving a space through which red light reaches the ground, the seeds germinate so it prevents competition from it sown kind while the tree is in place but ensures seed germinate if a gap appearts – clever huh!?
Many seedds are adapted for dispersal by a rangeof mechanisms. Some seeds are contained in dehiscent frit, which split along a defined line. For example broom pods burst and scatter seeds nearby, others are contained within in fruit which is eaten by animals. This ensures the seed is dropped a long way from the parent plant.
Some seeds like. dandelion, have parachutes attached to which is single seed. Others have burrs or spines to attach to animals’ fur or coats. Others still can pass through gut of bird . Some seeds are in fruit and incredibly sticky )mistletoe for example) so they have to be rubbed off the beak – a simple way to ensure they are deposited in tree bark where they grow.
Some fine seeds are scattered on the soil surface near the parent plant. These need only a light covering of soil or even none at all to germinate. Some are large and need to be pushed into soil. Larger seeds are often intended to pass through the gut of animal in fruit or on their own. They would therefore naturally be deposited in mound of manure so covering them deeper and they have a ready made food supply so germinate in low light conditions. Their cotyledons tend to be large as they need a larger food supply to reach the surface.
Within the seed, the plumule(first leaf) and radicle (first root) have chemicals which react to gravity and light. The radicle will always grow down (geotropic) and the plumule up (geophobic) so it does not matter which way seeds fall.
Seeds are just one way plants propagate. The advantages are that they are cheap, produced in large numbers and generally are easily transported. They are, whichever way you look at them, an incredible feat of nature.