472 THE BEAN PLANT. [CHAP. 



cut than the rest : mount in water and examine with 

 i inch obj. : note 



a. The medullary or pith-cavity in the centre of the 

 section. 



b. The pith-cells, around the central cavity: large 

 and more or less rounded (parenchyma) : some- 

 times with dotted walls from spots of local thin- 

 ness on them (pits). 



c. The fibro-vascular bundles arranged in a ring im- 

 mediately outside the pith 1 . Two of them how- 

 ever will be found separate from the ring in two 

 opposite corners of the stem. Commencing at 

 the side nearest the pith, note in each bundle 



a. The small openings formed by the transverse 

 sections of the spiral vessels (protoxylem). 



(3. The larger cavities of the pitted vessels. 



y. The small thick-walled wood-cells, wedged in 

 between the vessels. These three (a, /3 and y) 

 form the wood or xylem of the bundle. 



3. The cambium zone : granular-looking, and com- 

 posed of small angular thin-walled cells, ranged 

 in regular radial rows. 



e. The bast or phloem. It presents internally 

 thin-walled cells of various sizes, the bast 

 parenchyma and bast vessels or sieve tubes. 

 Externally it appears in cross section to be 

 composed of rounded cells with thickened walls; 

 the bast fibres or sclerenchyma. Draw the section. 



d. The cortex, consisting of several layers of large 

 rounded cells containing chlorophyll. Note that 

 at the four corners of the stem the walls of these 



1 a, 6, f, and e together constitute the central cylinder. 



