What is the reason why the roots of plants always grow downward and the stems always grow upwards?

Due to the different functions of roots and stems, the growth position is different, and the roots mainly absorb water and nutrients, fixing the plant. The leaves are exposed to sunlight, and they grow in the direction of the sun.

The nutrient content of the air is relatively high. The nutrient content of the soil is not very high, so it causes the roots of the plant to grow down the stem upwards.

Because the roots have to absorb water from the soil, the leaves have to photosynthesize, and it has to grow towards the sun.

Due to the action of gravity, the concentration of auxin on the near-ground side is higher than that on the far-ground side Roots are most sensitive to auxin, so the concentration of auxin on the near-ground side is high, which inhibits growth, while the auxin concentration on the far-ground side is low, which promotes growth, so the roots bend and grow downward

The auxin concentration on the near-ground side is higher than that on the far-ground side, and the auxin concentration on the far-ground side is low, the promoting effect is weak, and the growth is slow, while the auxin concentration on the near-ground side is high, the promoting effect is strong, and the growth is fast, so the stem grows upwards in a curved way, that is, the stem has back-to-ground characteristics

This up-to-earth growth of the root reflects the duality of auxin.

Since the optimal concentration of auxin to promote stem growth is higher than that of roots, for stems, a higher concentration of auxin on the near-ground side just promotes its growth, so that the near-ground side grows faster than the dorsal side, and the stem bends upward, so the growth of the stem is dorsimatic; The optimal concentration of auxin to promote growth in roots is much lower than that in stems, so a slightly higher concentration of auxin on the near-ground side of roots inhibits its growth, so the near-ground side of roots grows slowly, and the stem bends downward, so roots show the characteristics of ground-oriented growth

Therefore, the answer is as follows: under the action of gravity, the concentration of auxin in the near-ground side is higher than that in the far-ground side, the stem is not sensitive to auxin, the high concentration of auxin in the near-ground side grows fast, the stem bends upwards and the root is sensitive to auxin, and the growth is inhibited by the high concentration of auxin in the near-ground side, the growth is slow, and the roots bend downward

caused by uneven distribution of auxin.

Because this is the nature of plants, the roots of plants are terridogenic, water-oriented, and fertilizing (i.e., they grow to the ground, to water, and to fertile places), while the stems of plants are phototropic, that is, they grow in the direction of light. Thus the roots grow downwards and the stems upwards.

If you cover the plant in a black box and make a hole in the side of the box to let in the light, the stem will grow towards the hole in the box.

1. The geotropism of plant roots.

Due to the gravity factor, the concentration of auxin in the ground side of the root was higher than that on the dorsal side, and the sensitivity of the root to the auxin segment branch was higher, that is, the high concentration of auxin in the root side inhibited the growth of the root-side root, and the concentration of auxin in the root on the back side of the root promoted the growth of the root, so the ground tropism of the root appeared.

2. The backing of plant stems.

Due to the gravity factor, the concentration of auxin on the ground side of the stem is higher than that on the dorsal side, and the sensitivity of the stem to auxin is lower, so the auxin concentration on both sides promotes growth (both are in the augmentation segment of the image), because the concentration of auxin on the ground side of the stem is higher than that on the dorsal side, so the growth of the ground side is faster, so the backsoil nature of the stem appears.

The unidirectional action of roots and stems on gravity, which occurs in the direction of the ground or the growth of the ground, is called "ground-tropism". If a plant is left horizontally, over a period of several days, the roots of the plant will bend downward (positively) and the stems will bend upwards (negatively).

Auxin is a plant hormone that promotes growth at low concentrations and inhibits growth at high concentrations. Root and stem growth respond differently to auxin concentrations: low auxin concentrations promote root growth, high concentrations inhibit root growth, but promote stem growth, and higher concentrations inhibit stem growth.

When the plant is laid flat, due to the effect of gravity, auxin moves downward, and the root of the stem plant and the lower side of the lotus have a high concentration of auxin, and the growth is faster than that of the upper side, so that the stem tip bends upward. The high concentration of auxin on the lower side of the root produces an inhibition effect, and the growth is slower than that of the upper side, causing the root tip to bend downward.

Since the optimal concentration of auxin to promote stem growth is higher than that of roots, for stems, a higher concentration of auxin on the near-ground side just promotes its growth, so that the near-ground side grows faster than the dorsal side, and the stem bends upward, so the growth of the stem is dorsimatic; The optimal concentration of auxin to promote the growth of roots is much lower than that of stems, so the concentration of auxin on the near-ground side of the root is slightly higher to inhibit its growth, so the growth of the near-ground side of the root is slow, and the stem bends downward, so the root shows the characteristics of groundward growth Therefore, the answer is: under the action of gravity, the concentration of auxin on the near-ground side is higher than that on the far-ground side, the stem is not sensitive to auxin, the concentration of auxin on the near-ground side is high and the growth is fast, and the stem bends upwards The root is sensitive to auxin, and the growth of the hyperterranic side is inhibited, the growth is slow, and the root bends downward