136 HISTOLOGY OF MEDICINAL PLANTS 
may be either circular or angled in outline, a given form being 
constant to the plant in which it occurs. The pits vary from 
oval to circular. Pitted vessels with bordered pores occur in 
belladonna and aconite stems. 
Vessels and tracheids lose their living-cell contents when 
fully developed. In the vessels the cell contents disappear at 
the period of dissolution of the cell wall. 
The walls of vessels and tracheids are composed of lignin, 
a substance which prevents the collapsing of the walls when 
the surrounding cells press upon them, and which also prevents 
the tearing apart of the wall when the vessel is filled with ascend- 
ing liquids under great pressure. Lignin thus enables the 
vessel to resist successively compression and tearing forces. 
Tracheids are formed from superimposed cells with oblique 
perforated end walls. The side walls of tracheids are thickened 
in a manner similar to those of vessels. The tracheids in golden 
seal are of a bright-yellow color, and groups of these short 
tracheids scattered throughout the field form the most char- 
acteristic part of the powdered drug. In ipecac root the tracheids 
are of a porcelain-white, translucent appearance, and they are 
much longer than are the tracheids of golden seal. 
The cellulose walls of parenchyma cells are stained blue 
with hematoxylin and by chlorzinciodide. Cellulose is com- 
pletely soluble in a fresh copper ammonia solution. 
SIEVE TUBES 
Sieve tubes are downward-conducting cells. They conduct 
downward proteid food material. This fact is easily demon- 
strated by adding iodine to a section containing sieve tubes, in 
which case the sieve tubes are turned yellow. 
Developing sieve tubes have all the parts common to a living 
cell; but when fully mature, however, the nucleus becomes 
disorganized, but a layer of protoplasm continues to line the 
cell wall. . 
Sieve tubes (Plate 46, Fig. 1) are composed of a great number 
of superimposed cells with perforated end walls and with non- 
porous cellulose side walls. The end walls of two adjoining 
cells are greatly thickened and the pores pass through both 
