I34 GENERAL BIOLOGY 



and 2) the best fibres, situated nearer the periphery of the 

 bundle. The remainder of the bundle is parenchyma, little 



FIG. 83. Vascular bundle of the fern (Pteris) after Sedgwick and Wil- 

 son, a, longitudinal section; b, cross section; / p, common paren- 

 chyma; b s, bundle sheath, p s. phloem sheath of parenchyma; b /, 

 best fibres; 5 t, sieve tubes; p p, phloem parenchyma; i, tracheids 

 (wood tubes); w p, wood parenchyma; s v, spiral vessel. 



differentiated.. It constitutes the packing, so to speak, 



