128 HISTOLOGY OF MEDICINAL PLANTS 
spirally thickened at first, but later, when mature, an increased 
amount of thickening occurs and the vessel becomes a reticulate 
or pitted vessel. Many mature vessels, however, are spirally 
thickened as indicated above. In herbaceous stems and in 
certain roots and leaves spiral vessels are associated with the 
sclariform reticulate and pitted type. In certain cases a single 
spiral band will branch as the vessel matures. 
There is a great variation in the amount of spiral thickening 
occurring in a vessel. In leaves, particularly, the spiral appears 
loosely coiled; while in squills and other rhizomes and roots 
the spiral appears as a series of rings. When viewed by high 
power only half of each spiral band is visible. At either side 
of the cell the exact size and form of the thickening appear in 
two parallel rows of dark circles or projections from the walls. 
This thickening of the wall is rendered visible from the fact 
that the light is retarded as it passes through that portion of the 
spiral extending from the upper to the under side of the spiral; 
while the light readily traverses the upper and lower cross bands 
of the vessel. 
It should be remembered that, when the upper part of the 
spiral vessel is in focus, the bands appear to bend in a direction 
away from the eye; while when the under side of the bands are 
in focus, the bands appear to bend toward the eye. These 
facts will show that it is necessary to focus on both the upper 
and lower walls in studying spiral vessels. In double spiral 
vessels the spirals are frequently coiled in opposite directions; 
therefore the bands appear to cross one another. In eucalyptus 
leaf the three bands are coiled in the same direction. In all 
cases the thickening occurs on all sides of the wall. Its appear- 
ance will, therefore, be the same no matter at what angle the 
vessel is viewed. 
SCLARIFORM VESSELS 
Sclariform vessels have interrupted bands of thickening on 
the inner walls. Two or more such bands occur between the 
two side walls. The series of bands are separated by uniformly 
thickened portions of the wall extending parallel to the length 
of the vessel. Sclariform vessels are usually quite broad, so 
that it is necessary to change the focus several times in order 
