The explanation that best explains this phenomenon is: the roots grow towards the force of gravity and the stem goes away
The parts of a plant can be divided into shoot and roots. The root of the plant is the part that grows below the ground surface in order to anchor the plant.
When a seed germinates, the root part of that seed grows towards gravity i.e exhibits geotropism while the shoot part grows away from gravity i.e. upwards.
Hence, if a seed is planted upside down, it will still grow into an erect mature plant because the root will grow downwards towards gravity (positive geotropism) while the stem grows upward away from gravity (negative geotropism).
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