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A TEXT-BOOK OF BOTANY. 



strand developed on the inner face of the hadrome ; thus each 

 mestome strand carries two strands of leptome (Fig. 197, C). In 

 the CONCENTRIC mestome strands the leptome may encircle the 

 hadrome, as in the Ferns (Fig. 56), or the hadrome may partly 



Mt 



M. 



FIG. 196. The outer bark and part of the inner bark of Rhamnus Purshianus in trans- 

 verse, radial-longitudinal, and tangential-longitudinal sections. Me, transverse section of 

 inner bark; Mt, tangential-longitudinal section of inner bark; Mr, radial-longitudinal section 

 of inner bark; Sc, transverse section of stone cell area; St, tangential-longitudinal section 

 of stone cell area; Sr, radial-longitudinal section of stone cell area; He, transverse section 

 of outer layers of cortex; Hr, radial-longitudinal section of outer layers of cortex. Kc, Kt, 

 Kr, transverse, tangential-longitudinal, and radial-longitudinal sections of cork; b, bast 

 fibers; f, crystal fibers; p, parenchyma; e, sieve; sk, stone cells; m, medullary ray cells; 

 c, collenchyma. 



(as in the rhizomes of many Monocotyledons) surround the lep- 

 tome (Fig. 195). While thus the collateral type of strand or 

 bundle occurs in both Monocotyledons (Fig. 195) and Dicotyle- 

 dons (Figs. 192, 193), etc., the presence of a cambium is found 

 only in the Dicotyledons and occurs extremely seldom in the Mono- 



