120 
PHARMACEUTICAL BOTANY 
canals. Occasionally they are named according to the nature of 
their contents—resin or oil canal or reservoir, etc. They are gen¬ 
erally lined with a layer of cells, usually more or less flattened, which 
are characterized by possessing large nuclei. To this layer has been 
assigned the name “epithelium.” 
Fig. 56.—Resin duct (secretion reservoir) in leaf of Pinus silvestris, in cross 
section at A, and in longitudinal section at B; h, cavity surrounded by the secret¬ 
ing cells; /, /, selerenchyma fibers surrounding and protecting the duct. ( Stevens , 
after Haberlandt. 
Classification of Tissues According to Function. —According to 
their particular function, tissues may be classified as follows: 
I. Conducting Tissues 
II. Protective Tissues 
III. Mechanical Tissues 
Parenchyma (fundamental tissue) 
Medullary rays 
Xylem cells (wood parenchyma) 
Tracheae (ducts) 
Phloem cells 
Sieve tubes 
Companion cells 
f Epidermis (outer cell walls cutinized) 
| Cork (suberized tissue) 
Bast fibers 
Wood fibers 
Selerenchyma fibers 
Stone cells 
Collenchyma 
