This is a really good question and something a lot of people usually don't think about. The answer is that all plant cells need oxygen to live, because without oxygen they can't perform aerobic respiration (respiration is the process of breaking down food to get energy). Of course you probably know that when plants perform
photosynthesis, they combine water, carbon
dioxide, and the sun's energy to produce sugar and
oxygen. So the cells in the green parts of the
plant, where photosynthesis is taking place, get
all the oxygen they need from the oxygen produced
by photosynthesis. So cells in the leaves and
stems are okay. The trick is the cells down in the roots, where there is no photosynthesis.
In most plants, these cells get their oxygen from air in the spaces between dirt particles in the soil (you'd be surprised how much empty space there is in the soil -- mostly because earthworms are always moving around, churning up the dirt).
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