1. Phloem translocates sugars at very high concentrations (very negative ψ_p). Water flows down the water gradient into phloem cells, increasing their ψ_p enormously (2 MPa). Breaks in phloem cells would bleed sap profusely unless blocked rapidly. P-proteins act like fibrinogen in blood, rushing to block any holes, and forming a network on which callose may be synthesised to scab over the hole.
2. Sieve tubes lack a nucleus and most of their other organelles. Companion cells are needed to regulate the metabolic activity of the sieve tube element.
3. Pressure flow hypothesis:
   • Sieve plates unobstructed: observe under electron microscopy.
   • Bidirectional transport cannot occur: use radiotracers.
   • ATP is not required: use cyanide.
   • Turgor in sources > turgor in sinks: use micromanometers.
   • Phloem transport rates: look at rates of radiotracer translocation.
4. Polymer trapping hypothesis:
   • Sucrose more concentrated in mesophyll than phloem, vice versa for raffinose.
   • Raffinose synthesis enzymes located in intermediary cells.
   • Plasmodesmata between mesophyll and intermediary cells should exclude raffinose but not sucrose.