Some single cell organisms and some plants do not reproduce sexually through meiosis. Rather, they use mitosis only to make exact copies of their cells. The major disadvantage of this approach is that all of the organisms in a population or all of the plants in an area will have the exact same DNA. If a new disease comes along every single organism or plant will be wiped out because if one organism or plant is unable to fight the disease that means none can fight the disease. Mitosis also does not produce any variation. So, a plant that reproduces through mitosis won't have the chance to produce offspring that might be better than the parent...☺
