1 ) 

active loading of sucrose into sieve elements via companion cells at the source, increased hydrostatic pressure, mass flow, unloading at the sink 

2 ) 

hydrolysis of storage compounds in sinks, lowered water potential in sink, unloading of sucrose from sieve elements down the water potential gradient, mass flow from the source 

3 ) 

lowered pressure in sieve elements at the source, movement of sucrose down the pressure gradient from companion cells, mass flow down a diffusion gradient to the sink 

4 ) 

mass flow of dissolved sucrose via companion cells into the sieve element at the source, lowered hydrostatic pressure, diffusion of sucrose down concentration gradient to sink, active unloading