THE FIBRO-VASCULAR SYSTEM. 



Ill 



destroyed shortly after their formation, thus forming a 

 wide canal (Fig. 99; t, spiral, and r, annular vessels 

 on the border of the canal). Immediately in front of or 

 outside of the canal is a considerable mass of sieve tissue, 

 made up of true sieve tubes and the nearly allied cambiform 

 or latticed cells 

 (s, Fig. 99). 

 Eight and left of 

 the sieve tissue 

 lie a few annular 

 and reticulated 

 vessels (g, g, Fig. 

 99). Exterior to 

 all the bundles 

 (in this species) 

 is a cellular lay- 

 er, which has re- 

 ceived the name 

 of bundle sheath, 

 but which, prob- 

 ably, has no rela- 

 tion to the lay- 

 er so named that 

 surrounds each 

 fibro - vascular 

 bundle of some 

 plants. 



140. The 



Structure of the Fig. 100. Cross-section of theatem of Selctffinellaincegui- 

 v Ji c*7 /0/i, showing three bundles; in each bundle the inner 

 bundle in bUlCigi- thicker walled tissue is composed of scalariform vessels, 

 with a few narrow spiral vessels on each extreme margin; 

 surrounding the scalar-form tissue is the thinner walled 

 sieve tissue, and around this again is a layer of cells, which 

 may be called the bundle sheath ; I, I, intercellular spaces 

 able resemblance surrounding the bundles. X 150.-After Sachs. 



to that of Pteris aquilina. There is in each bundle a 

 central plate of tracheary tissue, consisting of a few narrow 

 spiral vessels in its two edges and a remaining mass of scala- 

 riform vessels (Fig. 100). The tracheary portion is sur- 

 rounded by a tissue of elongated, thin-walled tissue which 

 is, at least in part, a sieve tissue. In this and allied species 



