CH. IV, 3] ANATOMY OF STEMS 131 



matching, the xylem strands. The larger cells are crossed 

 here and there by the perforated plates which show them 

 to be SIEVE-TUBES; and they are the protein-conducting 

 parts of the bundles, precisely as in leaves (page 31). Inter- 

 mingled with the sieve-tubes are other slender cells, COM- 

 PANION CELLS, which have something to do with the function 

 of the sieve-tubes, and BAST PARENCHYMA cells, in which 



FIG. 86. Generalized drawing of an exogenous stem, to show the 

 typical anatomy of the cellular elements ; highly magnified. From left to 

 right, the cork, the cortical parenchyma, starch sheath, bast fibers, phloem 

 parenchyma, sieve-tube, cambium, ducts with xylem parenchyma, and pith. 

 (From Kerner.) 



carbohydrates are conducted, and which, therefore, along 

 with the wood parenchyma, take the place in stems of the 

 conducting bundle sheath of leaves (page 30). Often the 

 phloem contains in addition very long and thick-walled but 

 flexible fibers, called BAST FIBERS (Fig. 86), which give stiff- 

 ness to the stem when sclerenchyma is wanting. It is these 

 bast fibers which in the flax plant yield us our linen, and 

 in some trees provide tough strands utilized by savage tribes 



