78 
PHARMACEUTICAL BOTANY 
cepting in Pteridophytes and Gymnosperms. Both companion cells 
and sieve tubes arise by the division of the same mother-cell. The 
companion cells may be distinguished from the sieve tubes by their 
. , abundant protoplasmic contents, and 
also by the fact that they retain their 
nuclei after complete maturation. Be- 
sides sieve tubes, companion cells, and 
bast fibers, parenchyma cells are often 
found in the phloem. 
TRACHEARY TISSUE 
Fig. 28. — Vascular ele- 
ments. A , annular tracheal 
tube; B, spiral trachea 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 Stras- 
burger.) 
The tracheary tissue of plants comprises 
two kinds of elements, the trachea (ducts 
or vessels) and tracheids. Both of these 
conduct crude sap (water with mineral 
salts in solution). The trachea are very 
long tubes of a cylindrical or prismatic 
shape which are formed by the disintegra- 
tion of the transverse walls between cer- 
tain groups of superimposed cells, during 
the growth of the plant. The tubes 
frequently retain some of their transverse 
walls. The longitudinal walls of these 
tubes are of varying thickness, usually, 
however, thinner than those of woody 
fibers. The thickness is due to an infil- 
tration of lignin upon the original cel- 
lulose wall. The walls show character- 
istic thickenings on their inner surfaces. 
Tracheae are classified according to 
their markings as follows: 
Annular, with ring-like thickenings. 
Spiral, with spiral thickenings. 
Reticulate, with reticulate thickenings. 
Porous or pitted with spherical or oblique slit pores. 
Annulo-spiral, with both ring and spiral thickenings. 
Scalariform, with ladder-like thickenings. 
