112 
PHARMACEUTICAL BOTANY 
other tubes forming anastomosing network. These vessels occur 
in the following families: Composites, Papaveraceae, Campanulacea, 
Convolvulaceae, Euphorbiacece, Araceae, Oleaceae, Geraniaceae, and 
Musacece. 
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 Celastracece , Urticaceae, Tiliaceae , and 
Oleacece 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 
fibro-vascular bundles consists of superimposed, elongated, tubular 
cells whose longitudinal walls are thin and composed of cellulose and 
whose transverse 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 
tracheae (ducts or vessels) and tracheids. Both of these conduct 
crude sap (water with mineral salts in solution). The tracheae 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 
