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Why does auxin slow growth on the lower side of a root?
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Plant development
The life cycle of the fern. (1) Clusters (sori) of sporangia (spore cases) grow on the undersurface of mature fern leaves. (2) Released from its spore case, the haploid spore is carried to the ground, where it germinates into a tiny, usually heart-shaped, gametophyte (gamete-producing structure), anchored to the ground by rhizoids (rootlike projections). (3) Under moist conditions, mature sperm are released from the antheridia and swim to the egg-producing archegonia that have formed on the gametophyte's lower surface. (4) When fertilization occurs, a zygote forms and develops into an embryo within the archegonium. (5) The embryo eventually grows larger than the gametophyte and becomes a sporophyte.
In the context of plant development, the downward growth of roots is associated with an accumulation of auxin along the lower side of the root. While auxin typically promotes cell elongation in stems, it has an inhibitory effect on root growth.123
This means that when more auxin accumulates on the lower side of a root, it suppresses the growth of those cells compared to the upper side. Consequently, the cells on the upper side continue to grow or elongate more effectively, causing the root to curve downward. Although this inhibitory action is well-documented, the precise biological mechanism by which auxin inhibits root growth is not yet fully understood.12