CONDUCTING TISSUE 127 
The combined length of the vessels is about equal to the height 
of the plant in which they occur. The length of the individual 
vessels varies from a fraction of a meter up to several meters. 
ANNULAR VESSELS 
The annular vessels are thickened at intervals in the form 
of rings (Plate 40, Fig. 1), which extend outward from and 
around the inner wall of the vessel. In fact, it is the inner wall 
which is thickened in all the different types of vessels. The 
ring-like thickening usually separates from the wall when the 
drug is powdered. Such separated rings occur frequently in 
powdered digitalis, belladonna, and stramonium leaves. An- 
nular vessels are not, however, of diagnostic importance, be- 
cause more characteristic cells are found in the plants in which 
they occur. Not infrequently a vessel will have annular thick- 
enings at one end and spiral thickenings at the other. Such 
vessels are found in the pumpkin stem (Plate 4o, Fig. 1). 
Vessels are distinguished from other cells by their arrange- 
ment, by their large size when seen in cross-section, and by the 
thickening of the wall when seen in longitudinal sections of the 
plant or in powders. ‘The side walls of vessels are thickened in 
a number of striking yet uniform ways. The chief types of 
thickening of the wall, beginning with one that is the least 
thickened, are annular, spiral, sclariform, pitted, and pitted 
with bordered pores. 
SPIRAL VESSELS 
In the spiral vessel the thickening occurs in the form of a 
spiral, which is readily separated from the side walls. This is 
particularly the case in powdered drugs, where the spiral thick- 
ening so frequently separates from the cell wall. There are 
three types of spiral vessels: those with one (Plate 41, Fig. 1), 
those with two, and those with three spirals. Single spirals 
occur in most leaves; double spirals occur in many plants 
(Plate 41, Fig. 2), but they are particularly striking in pow- 
dered squills. Triple spirals are characteristic of the eucalyptus 
leaf (Plate 41, Fig. 3); in fact, they form a diagnostic feature 
of the powder. Frequently a spirally thickened wall indicates 
a developmental stage of the vessel. Many such vessels are 
