54 PRACTICAL BOTANY. 



action of sulphuric acid ; (iii.), they have the character- 

 istic black dot (see p. 63). 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 their 

 sharp contour, they are thus brought into prominence. 



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 thus 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 : 



f (i.) Sclerenchyma. 



A. Phloem. ^ ]'/. f 



{ (n.) Soft Bast. 



B. (iii.) Cambium. 



C. (iv.) Xylem. 



Taking first (A) the Phloem examine 

 (i.) The Sclerenchyma. This appears as a half- 

 moon shaped mass of tissue consisting of elements with 

 rounded cavity, in which may be recognised the 

 remnants of protoplasmic contents. The walls are 



