26 SOUTH AFRICAN BOTANY 
lignified tube with specially thickened end wall. These 
end walls are perforated with numerous holes, and 
through them the contents of one cell communicates 
with the contents of the next. This structure resembles 
Fia. 15.—Sieve Tubes. 
A and B. Longitudinal section, OC. Surface view of sieve plate, c. 
Companion cell. ca. Plate of callus, s.p. Sieve plate. 
a sieve, hence the name of these vessels. After a while 
the sieve plates become covered with a plate of callus. 
The sieve tubes contain a lining of protoplasm, but 
no nucleus. Accompanying them is always found a 
small thin-walled CoMPANION CELL, containing proto- 
plasm and a large nucleus. Lastly, in many plants we 
find another kind of tube usually filled with a milky 
