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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."- 



.f 



PIG. 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; /, /, sclerenchyma 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: 



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) 

 f Bast fibers 

 I Wood fibers 



Sclerenchyma fibers 



Stone cells 



Collenchyma 



I. CONDUCTING TISSUES 



II. PROTECTIVE TISSUES 



III. MECHANICAL TISSUES 



