Suggest why only a small fraction of the phosphate ions taken up are moved to the growing points.
Gossypium hirsutum is the plant species most commonly cultivated worldwide for cotton production. Scientists carried out an investigation to trace the route taken by phosphate ions from the leaves of cotton plants into the stems. The scientists used a radioactive isotope of phosphorus ($^{32}\text{P}$) to track the route of phosphate ions. Some cotton plants were split into two groups: A and B. In group A, the scientists: • placed impermeable waxed paper between the xylem and phloem in the stem below a leaf on each plant • injected a solution containing phosphate ions labelled with $^{32}\text{P}$ (labelled phosphate ions) into a vein in each leaf, as shown in Fig. 5.1.