They are in contact via plasmodesmata. Sieve tubes in the phloem of angiosperms are flanked by one or several plasma-rich, nucleated companion cells, which do not occur in gymnosperms. Much less is known about the phylogeny of sieve elements than about that of xylem members. One cause are the cell walls of sieve elements, which are strengthened entirely by cellulose and are therefore not as resistant as lignified ones, another is the transitory working order of the single sieve element. After loss of function are the cells reorganized and lose their typical structural properties.
Good fossils are thus rare.
Specialized sieve areas are concentrated in the final walls. A gradual change of orientation of these final walls from very sloping to right angles occurs. The change is followed by a stepwise transition from compound to simple sieve plates. The activity of the sieve areas in the side walls is reduced. This leads to canalizing of the flow of assimilates in longitudinal direction and the long-distance transport rates are enhanced.