Final answer:
Sucrose is actively transported from source cells into companion cells and then into the sieve-tube elements (STE) in the phloem. The high concentration of sugar in the phloem causes water to move by osmosis from the adjacent xylem, resulting in positive pressure and bulk flow of phloem sap from source to sink. These experimental observations support the understanding of how sugars are transported within plants.
Step-by-step explanation:
The experimental evidence for the phloem transporting sucrose is supported by multiple findings in plant physiology. Sucrose is actively transported from source cells into companion cells and then into the sieve-tube elements (STE). This process reduces the water potential, causing water to enter the phloem through osmosis from the adjacent xylem. The resulting positive pressure forces the sucrose-water mixture down toward the roots, where sucrose is unloaded.
Additionally, phloem sap contains up to 30 percent sugar, minerals, amino acids, and plant growth regulators. The high concentration of sugar in the phloem decreases the total water potential and leads to an osmotic movement of water into the phloem from the xylem. This increase in water potential causes bulk flow of phloem sap from source to sink.
Overall, these experimental observations provide strong evidence for the active transport and translocation of sucrose in the phloem, supporting the understanding of how sugars are transported within plants.