112 PHARMACEUTICAL BOTANY 



other tubes forming anastomosing network. These vessels occur 

 in the following families: Composites, Papaveracea, Campanulacea, 

 Convolvulacea, Euphorbiacea, Aracea, Oleacea, Geraniacea, and 

 Musacea. 



Secretory cells with a latex-like content are probably of secondary 

 oiigin in plants. They resemble in many respects latex cells and are 

 seen in various species of the Celastracea, Urticacea, Tiliacece, and 

 Oleacea families. 



All laticiferous elements contain a colorless, milky- white, or 

 otherwise colored emulsion of gum-resins, fat,, wax, coautchouc and 

 in some cases, alkaloids, tannins, salts, ferments, etc. This emulsion 

 is called "latex." 



SIEVE (LEPTOME OR CRIBIFORM) TISSUE 



This tissue found in the phloem (rarely in the xylem) region of 

 nbro-vascular bundles consists of superimposed, elongated, tubular 

 cells whose longitudinal walls are thin and composed of cellulose and 

 whose transveise walls, called "sieve plates," are perforated, per- 

 mitting of the passage of proteids from one cell to another. Occa- 

 sionally sieve plates are formed on the longitudinal walls. Sieve 

 tubes are usually accompanied by companion cells excepting in 

 Pteridophytes and Gymnosperms. Both companion cells and sieve 

 tubes arise by the division of the same mother-cell. The companion 

 cells may be distinguished from the sieve tubes by their abundant 

 protoplasmic contents, and also by the fact that they retain their 

 nuclei after complete maturation. Besides sieve tubes, companion 

 cells, and bast fibers, parenchyma cells are often found in the 

 phloem. 



TRACHEARY TISSUE 



The tracheary tissue of plants comprises two kinds of elements, the 

 trachea (ducts or vessels) and tracheids. Both of these conduct 

 crude sap (water with mineral salts in solution). The trachea are 

 very long tubes of a cylindrical or prismatic shape which are formed 

 by the disintegration of the transverse walls between certain groups 

 of superimposed cells, during the growth of the plant. The tubes 

 frequently retain some of, their transverse walls. The longitudinal 



