ROOTS. 31 



(c) The woody cylinder which forms the central portion of the root. 



The distinction between (6) and (c) is more evident when the section 

 has been exposed to the air for a few minutes and changed somewhat in 

 color. It is a good plan to look with the low power first at a thick 

 section, viewed as an opaque object, and then at a very thin one mounted 

 in water or glycerine, and viewed as a transparent object. 



Observe the cut-off ends of the ducts, or vessels, which serve as pas- 

 sages for air and water to travel through ; these appear as holes in the 



w 

 Fig. 19. — Magnified Cross-Section of a very Young Exogenous Root. 

 w, root-hairs with bits of sand adhering j r, parenchyma cells of the bark ; e, inner-, 

 most layer of the bark ; 6, bast ; h, vessels ; c, c\ cambium. 



section, and are much more abundant relatively in the young than in the 

 older and larger portions of the root. Sketch one section of each kind. 



Examine with a higher power (100 to 200 diameters), and note the 

 ends of the thick-walled wood-cells. Compare these with Fig. 19. 



Notice the many thinner-walled cells composing stripes radiating away 

 from the centre of the root. These bands are the medullary rays, whose 

 mode of origin is shown in Tig. 52. Moisten some of the sections with 

 iodine solution,^ and note where the blue color shows the presence of 

 starch. Split some portions of the root through the middle, cut thin 



^ If the roots are in their winter condition. 



