Question

A horticulturalist notices that when Zinniaflowers are cut at dawn, a small drop of water collects at the surface of the rooted stump. However, when the flowers are cut at noon, no drop is observed. Suggest an explanation.

Short answer

According to the given scenario, when flowers are cut at dawn, a drop of water collects at the surface of the rooted stump, but no drop is observed during the same procedure at noon. This is because during dawn, due to root pressure, the xylem is under positive pressure.

Step-by-step solution

Zinnia flowers

Zinniaflowers are seasonal flowers that need full sunlight to grow and bloom.They grow well in fertile and drained soil high in organic matter. They are annual plants that bloom in the late summer months. These flowers cannot bloom in the winter.

Positive and negative root pressure

The force that helps push the fluids upwards in the xylem (water-conducting vessels) is called root pressure. It is generated due to osmotic pressure in root cells. The positive root pressure indicates the pressure that pushes water up to the xylem, and negative root pressure indicates tension in the xylem vessels.

Reason for collection of water drop around root stump at dawn but not at night

Root pressure develops at dawn but not at noon. This is because stomata are open in the morning than at night, which results in transpiration.

During dawn, the development of root pressure results in the generation of positive pressure of the xylem that results in the development of water drop from the rooted stump.

At noon, the rate of transpiration is high, due to which root pressure cannot develop. Thus, when flowers are cut at night, the negative pressure of the xylem is developed, due to which xylem sap is pulled back into its rooted stump.