=Pitted vessels with bordered pores= are of common occurrence in the woody stems and stems of many herbaceous plants (Plate 45, Figs. 3 and 4). In such vessels the wall is unthickened for a short distance around the pits. This unthickened portion may be either circular or angled in outline, a given form being constant to the plant in which it occurs. The pits vary from oval to circular. Pitted vessels with bordered pores occur in belladonna and aconite stems. [Illustration: PLATE 42 SCLARIFORM VESSELS 1. Sarsaparilla root (_Smilax officinalis_, Kunth). 2. Male fern (_Dryopteris marginalis_, [L.] A. Gray). 3. Tonga root.] [Illustration: PLATE 43 RETICULATE VESSELS 1. Hydrastis rhizome (_Hydrastis canadensis_, L.). 2. Musk root (_Ferula sumbul_, [Kauffm.] Hook., f.).] [Illustration: PLATE 44 PITTED VESSELS 1. Quassia, low magnification (_Picræna excelsa_, [Swartz] Lindl.). 2. Quassia, high magnification. 3. White sandalwood (_Santalum album_, L.).] [Illustration: PLATE 45 VESSELS 1. Reticulate vessel of calumba root (_Jateorhiza palmata_, [Lam.] Miers). 2. Reticulate tracheid of hydrastis rhizome (_Hydrastis canadensis_, L.). 3. Pitted vessel with bordered pores of belladonna stem. 4. Pitted vessel with bordered pores of aconite stem (_Aconitum napellus_, L.).] Vessels and tracheids lose their living-cell contents when fully developed. In the vessels the cell contents disappear at the period of dissolution of the cell wall. The walls of vessels and tracheids are composed of lignin, a substance which prevents the collapsing of the walls when the surrounding cells press upon them, and which also prevents the tearing apart of the wall when the vessel is filled with ascending liquids under great pressure. Lignin thus enables the vessel to resist successively compression and tearing forces. Tracheids are formed from superimposed cells with oblique perforated end walls. The side walls of tracheids are thickened in a manner similar to those of vessels. The tracheids in golden seal are of a bright-yellow color, and groups of these short tracheids scattered throughout the field form the most characteristic part of the powdered drug. In ipecac root the tracheids are of a porcelain-white, translucent appearance, and they are much longer than are the tracheids of golden seal. The cellulose walls of parenchyma cells are stained blue with hæmatoxylin and by chlorzinciodide. Cellulose is completely soluble in a fresh copper ammonia solution. SIEVE TUBES =Sieve tubes= are downward-conducting cells. They conduct downward proteid food material. This fact is easily demonstrated by adding iodine to a section containing sieve tubes, in which case the sieve tubes are turned yellow.