472 THE BEAN PLANT. [CIIAP. 



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 {parefichyma) : 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. 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 i^essels (protoxylem). 



yS. The larger cavities of the pitted vessels. 



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

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

 form the wood or xylem of the bundle. 



8. The cambium zojie : granular-looking, and com- 

 posed of small angular thin-walled cells, ranged 

 in regular radial rows. 



€. The bast or phloem. It presents internally 

 thin-walled cells of various sizes, the bast 

 parenchyma and bast vessels or sirce tubes. 

 Externally it appears in cross section to be 

 composed of rounded cells with thickened walls; 

 the bast fibres or sclerenchyma. Draw the section. 



//. The coTiex, consisting of several layers of large 

 rounded cells containing chlorophyll. Note that 

 at the four corners of the stem the walls of these 



