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. i), 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 stramonimn 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 infrec|uently a vessel will have annular thick- 

 enings at one end and spiral tliickenings at the other. Such 

 vessels are found in the pumpkin stem (Plate 40, Fig. i). 



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, sclarifonn, pitted, and pitted 

 with bordered pores. 



SPIEAL 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 t}pes of spiral vessels: those with one (Plate 41, Fig. i), 

 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 



