134 PHARMACEUTICAL BOTANY 
lining its walls and a large central vacuole but no nucleus. 
The tubes are arranged in longitudinal rows and the protoplasts 
of each series are connected by means of protoplasmic strands 
which extend from one sieve. tube to 
another through perforations in the sieve # 
plates. The cell sap found in the vacuole sees 
of sieve tubes contains protein substances o| 
often of slimy character. When fixed assed 
990 
939 
° 
3 
9° 
t) 
Fic. 73. Fic. 74. 
Fic. 73.—Stages in the development of sieve tubes, companion cells, and 
phloem parenchyma. A. a and 6, Two rows of plerome cells; in c and d, a has 
divided longitudinally and ¢ is to become companion cells; d, a sieve tube, and 
b, phloem parenchyma. B. c, Companion cells, and d, a beginning sieve tube 
from ¢ and d, respectively in A. The cross-walls in d are pitted; 6, phloem paren- 
chyma grown larger than in A. C. The same as B with the pits in the cross-walls 
of the sieve tubes become perforations, and the nuclei gone from the cells composing 
the tube. (From Stevens.) 
Fic. 74.—Vascular elements. A, annular tracheal tube; B, spiral tracheal 
tube; C, reticulated tracheal tube; D, pitted tracheal tube; E, cross-section through 
plate of sieve tube, and adjoining companion cell; F, length-wise section of sieve 
tube; G, portions of two companion cells. (A, B, C, D, Robbins; E, F, and G, after 
Strasburger.) 
and stained sections are examined, coagulated masses of 
protein will often be seen filling the pores of the sieve plates. 
These are called slime blugs. 
_ Sieve tubes are usually accompanied by companion cells 
excepting in Pteridophytes and many Gymnosperms. Both 
