Why do peanuts blossom above and bear fruit underground?

Peanut is a leguminous plant, and its flower organ structure is butterfly-shaped like other leguminous plants, but the difference is that after fertilization, some cells at the base of the ovary begin to divide and elongate, forming the so-called ovary stalk; Ovary stalk is rapidly elongated due to its cell meristem. Ovary is located at the top of ovary stalk, and its outer layer is lignified epidermal cells, forming a conical protective cap, so people call it "fruit needle" figuratively. Fruit needles and roots have the same geotropism. It stops elongation after being stuck in the soil for 3- 10 cm, and then the ovary begins to expand and grow into a pod.

So does peanut needle have to be cultivated to bear fruit? Someone has done such an experiment: insert a fruit needle into a transparent bottle, even if there is nutrient solution in the bottle; The ovary does not swell, but if the above experiment is replaced by a light-proof bottle, the ovary can swell and bear fruit, indicating that darkness is the most important factor affecting the result. It is also common in production that immature fruit needles can only be elongated continuously, but they do not bear pods. This is why the enlarged ovary and even the developing young pods will stop growing if exposed to the ground, and can only continue to grow after being covered with soil.

Mechanical stimulation and soil water, fertilizer, gas and heat conditions also affect pod development. Fruit needles can not only meet the requirements of mechanical stimulation, but also have the function similar to roots, and can absorb water and nutrients, especially calcium, from soil. According to the measurement, the calcium absorbed by the root system is rarely transported to the pod, and the calcium required by the pod horn is mainly taken up from the soil by the fruit needle and the pod itself. If the soil layer of seed pod lacks calcium, the development of seed will be hindered, and empty fruit, broken fruit and even rotten fruit will easily appear.