How can I kill a plant by overwatering it, yet propagations of the same plant will grow in water?
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Roots, like all plant tissues need oxygen and wet soil does not have enough oxygen in it for plant roots to survive. When some root tissue dies from lack of oxygen, microbes can infect the root system and cause additional rot.
So why is there enough oxygen in a glass of water, but not in wet soil? Well, oxygen can dissolve in water, but it does so pretty slowly. If the oxygen is used up slowly, the water will absorb enough new oxygen from the air above to keep the roots of the cutting alive. In soil the roots have to compete with microbes for that oxygen. The microbes in the soil have plenty of food (soil organic matter). Because the microbes food, they can use up more oxygen. This meas that roots in the soil get outcompeted and die due to lack of oxygen.
Plant cuttings will be happiest if you give them fresh water regularly. In hydroponics, air bubblers are used to make sure there is plenty of oxygen for the roots.
Plants that grow in wetlands have special adaptations that help the roots get enough oxygen. Rice for example has air passages in the roots called aerenchyma that act as snorkels and bring oxygen down to the roots.
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