92 



HISTOLOGY AND PUYSIOLOOY. 



fa-t^ pros. pa.r. 



if. sp. 



ascend from the lowest to the highest, we find a gradual 

 modification of the tissue for special purposes. The outer 

 cells form a boundary tissue, and, together with their ap- 

 pendages, constitute the epidermal system. This in- 

 cludes the epidermis, trichomes, and stomates. Certain other 

 cells, or rows of cells, become modified into tubes or ducts, 

 and form the string-like masses, or form fibers in the stems 

 of the higher plants. These are the fibro-vascular bun- 

 dles. They contain a woody portion (xylem), bast por- 

 tion (^phloem), and cambium ; from the last, the first two 

 are formed. All the other tissue, unmodified, or slightly 

 modified, is designated by the terra fundamental tissue. 



Fig. 209. Diagrammatic longitudinal section through tracheary tissue ; P, pith ; 

 xy, wood : C, cambium ; Pk, bast ; Cx, cortex ; par, parenchyma ; pros, prosen- 

 chyma r sie, sieve-tubes ; pit, pitted ; ^<7r, bordered pits ; sea, scalariform; ret, re- 

 ticulated , an, annular ; sp, spiral. 



