The root hairs are where most of the nutrients and water are absorbed. The root tip produces new cells on a daily basis, and thus also root hairs. When the plant is short of water and/or nutrients, it will devote more assimilates (photosynthesis energy) to producing more cells in the root tip. This also generates more root hairs until the root has found what the plant needs (more water or nutrients). The oldest root hairs will then die off.
In practice, as the medium gets drier, the root starts looking for water and will produce more cells, and thus more root hairs. Absorption capacity increases, because more root hairs are produced. But the youngest root hairs will enter even more “moist” soil. The plant can still take up water and sometimes even more! This is why the general advice is to grow on the dry side: when you water, some of the root hairs become redundant. To limit the energy-loss (dissimulation energy), the oldest root hairs will die off.
If you give the plant too much water, all the root hairs will die off. Effectively, the roots drown and it takes at least three days before a root tip has produced enough new cells with root hairs. The dying off of root hairs also happens after re-potting the plants or after moving them very roughly. So when this is the case, go gently on your climate control the first 3 days.
And pay good attention to the watering: don’t overdo it but on the other hand, don’t let the plant dry out too much.
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