When a seed first starts to grow and the tiny plant bursts forth from inside it, this is germination. A similar thing occurs with organisms that grow from spores, such as certain plants and fungi. A number of factors contribute to the germination of a seed or spore, and variations in these factors can keep seeds from growing.
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The evolutionary history of life on Earth begins with the most basic bacterial cells. Some of these developed into protists and simple plants with chloroplasts that turn sunlight, water and carbon dioxide into food. Others developed into fungi with chitin-based cell walls. Still others developed into the various other kinds of organisms we know today. Fungi, protists and many simple plants reproduce by releasing spores. This is a sign of their common evolutionary history.
The reason spores do not simply begin to sprout as soon as possible, but wait until conditions are right for germination, is because it does no good for a spore to start growing if the plant, protist or fungus it is going to sprout into cannot survive under the current circumstances. Due to their simplicity and rugged design, spores can survive through tougher conditions than newly sprouting adults can. This is why it is important for there to be certain necessary triggers for spore germination.
Unlike its larger and more advanced cousin the seed, a spore is always very small. Their small size allows adults to produce them in vast numbers. Being microscopic, most spores appear as dust, if they are visible at all. If you poke a mushroom with a stick or shake a fern, you can often see the spores falling from the adult's reproductive structures and drifting through the air. Humans often unknowingly inhale spores that are drifting in the air, which may cause allergic reactions and respiratory problems.
Despite their diminutive size, however, once spores germinate, they can often grow into adults many millions or billions of times their size. A single fungal spore, for instance, though nothing more than a speck on its own, can develop into an adult that fills many square feet of ground.
The main benefit of spores is their simplicity. Having the same basic capabilities of a seed, a spore is much smaller and can be produced on a much larger scale. Additionally, while many animals feed on the seeds of plants, virtually no animals feed on spores because they are so very small. Also, since they are carried so easily on the wind, spores frequently travel great distances before finally settling into the ground and beginning to germinate.
In fungi, most spores are asexual. This means that male and female spores do not need to combine in order to reproduce. Instead, the fungi resulting from spores are mere copies of the adult. However, some sexual reproduction does occur among fungi. Plants that reproduce with spores, such as ferns, almost always have sexual spores.
Requirements for spore germination differ from species to species. However, a few basic requirements do exist for all, to varying degrees. First, the environment must maintain a certain temperature range. This is because certain chemical changes and biological processes cannot occur if it is too hot or too cold. Second, a certain moisture level must be maintained. Generally, the higher the moisture content of the air, the better. Third, there must not be a significant amount of malevolent bacteria present. Though animals do not eat spores, bacteria sometimes do. For sexual spores, combination with another sexual spore is necessary for germination.