54 



BOTANT. 



lariform tissue are masses of soft tissue (parenchyma) and 

 a few spiral vessels, the latter occurring near the foci of 

 the elliptical cross-section of the bundle {s). Surrounding 

 or partly surrounding the tracheary portion of the bundle 

 is a layer of sieve-tubes {sp), separated from the large sca- 



Fia. 36.— Part of a transverse section of the fibro-vasoular bundle of the 

 underground stem of the common Brake-fern (Pteris aquilina). s, spiral 

 vessel ; o, 9, scalariform vessels ; sp, sieve-tissue ; b, fibrous tissue ; sa. 

 bundle-sheath- 



lariform vessels by a layer of parenchyma. Outside of the 

 sieve- tissue is a mass of fibrous tissue (i), which is itself 

 bounded externally by another layer of parenchyma. The 

 whole bundle is surrounded by a bundle-sheath. 



91. A noticeable feature in the structure of this bundle 

 is that the tissues have a concentric arrangement : the tra- 



