The different size of plates as well as size and number of pores have a major impact on the resistance inside the tube. It is yet unclear why the structure of sieve plates is so variable. However, since sieve plates represent a major obstruction in the tube, they are ideal targets for tube occlusion mechanisms. Sieve tube occlusion is as important for the plant, just as blood coagulation is for humans. Mechanical injury of a sieve tube by chewing insects, mammals etc. constantly occurs. Without a sophisticated protection system, plants would bleed to death. There are two known mechanisms. Callose formation on the sieve plate and forisome reaction