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 



