Question 1
What are the cross walls left between sieve tube cells called?
The right answer is the one marked above.
Sieve plates. The cross walls dissolve incompletely, leaving a perforated plate at every join along the sieve tube.
Intercalated discs are between cardiac muscle cells, basement membranes are under epithelium, and nodes of Ranvier are gaps in a myelin sheath. Four boundaries, four different tissues.