=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 to bring the different series of bands in focus. The series of bands are usually of unequal width and length. [Illustration: PLATE 40 ANNULAR AND SPIRAL VESSELS 1. Pumpkin stem (_Cucurbita pepo_, L.). 2. Two characteristic views of spiral vessels. 3. (_A_) Upper part of spiral vessel in focus. (_B_) Under part of spiral vessel in focus. 4. Spiral vessel of the disk petal matricaria (_Matricaria chamomilla_, L.).] [Illustration: PLATE 41 SPIRAL VESSELS 1. Single spiral vessel of pumpkin stem (_Cucurbita pepo_, L.). 2. Double spiral vessel of squill bulb (_Urginea maritima_, [L.] Baker). 3. Triple spiral vessel of eucalyptus leaf (_Eucalyptus globulus_, Labill).] Sclariform vessels occur in male fern (Plate 42, Fig. 2), calamus, tonga root (Plate 42, Fig. 3), and sarsaparilla (Plate 42, Fig. 1). In each they are characteristic. Sclariform vessels, with these few exceptions, do not occur in drug plants. In fact, drugs derived from dicotyledones rarely have sclariform vessels. They occur chiefly in the ferns and drugs derived from monocotyledenous plants. Their presence or absence should, therefore, be noted when studying powdered drugs. RETICULATE VESSELS =Reticulate vessels= are of common occurrence in medicinal plants. In fact, they occur more frequently than any other type of vessel. The basic structure of reticulate vessels (Plate 43, Fig. 1) occurring in different plants is similar, but they vary in a recognizable way in different plants (Plate 43, Fig. 2). The walls of reticulate vessels are thickened to a greater extent than are the walls of spirally thickened vessels. PITTED VESSELS =Pitted vessels= are met with most frequently in woods and wood-stemmed herbs. There are two distinct types of pitted vessels--_i.e._, simple pitted vessels and pitted vessels with bordered pores. The pitted vessel represents the highest type of cell-wall thickening. The entire wall of the vessel is thickened, with the exception of the places where the pits occur. The number and size of the pits vary greatly in different drugs. In quassia (Plate 44, Fig. 1) the pits are numerous and very small, and the openings are nearly circular in outline. In white sandalwood (Plate 44, Fig. 3). the pits are few in number, but when they do occur they are much larger than are the pits of quassia. PITTED VESSELS WITH BORDERED PORES