SUNFLOWER STEM 67 



The layer of thin- walled cells abutting directly on the thick- 

 walled sclerenchyma fibres (yellow with chlor-zinc-iodine, com- 

 pare p. 38) show in their radial walls the characters of a bundle- 

 sheath, i.e. (i.) they are coloured brown with chlor-zinc-iodine ; 

 (ii.) they resist the action of sulphuric acid ; (iii.) they have the 

 characteristic dark dot (see p. 76). This layer may sometimes 

 be traced as continuous round the ring of bundles, but this is 

 difficult, owing to divisions in the cells of the bundle- sheath, 

 similar to those above noticed in the cortical tissue and 

 epidermis. 



Treat some thin sections with sulphuric acid. The bundle- 

 sheath and cuticle resist its action, and since they retain theii 

 sharp contour, they are thus brought into prominence. Compare 

 the above reactions of walls of the bundle-sheath with those of 

 corky walls, p. 39. 



Within this are 



4. The vascular bundles. Select one of the 

 largest of these for more minute examination : it will 

 be found to consist of two well-marked masses of 

 thick-walled tissue, peripheral and central as regards 

 the stem, with a transparent thin-walled portion be- 

 tween them. Further, on examining the latter more 

 carefully, it will be seen that the external part of it 

 has thicker walls, and is less regularly arranged than the 

 central portion, and must accordingly be distinguished 

 from it. We have thus four portions of the bundle 

 which, taking them in succession from the periphery 

 to the centre, are named as follows : 



(i.) Sclerenchyma. 

 (ii.) Soft Bast or Phloem. 

 (iii.) Cambium. 

 (iv.) Xylem. 



F 2 



