Basically his experiments, which scientists came to know only after he was dead. His experiments, and of course, the result of this experiments that served as a proof of what Mendel afirmed, where one of the most importnat factors that made his ideas accepted.
They where also accepted because he discovered DNA, the basic laws of inheritance and chromossomes.
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Photosynthesis is one of the main processes that is basically responsible for the function of all living things. Without it carbon would not be cycled, oxygen would not be made and plants would not be able to creat food for themselves and would have to rely on another source.