292 



STRUCTURE OF STEMS AND ROOTS 



ring itself to enlarge its circumference and to move out- 

 ward by this growth. 



496. To study secondary thickening: Cut thin cross- 

 sections of basswood stems of different ages (one to three 

 years old) . Stain and mount. Examine with low power and 



r.d 



D 



462. Microphotograph of cross-section showing secondary growth in larch, June 

 13; a, cambium; a-b, new phloem; a-c, new wood. D. Tagential section of wood 

 of pine, showing transverse section of medullary rays; rd, transverse resin-duct. 



sketch the arrangement of bundles in the oldest and young- 

 est. Note the effect of growth on the medullary rays. Test 

 them with iodin for starch. Now with the high power 

 study the peculiar character of the bast tissue. Note the 

 abundance of fibrous tissue all through it. Draw a 

 single bundle from the stem one year old, carefully show- 

 ing the location of the cambium and the different tissues 

 in the xylem and phloem strands. (Fig. 464). It may 

 be thought best to precede this experiment with a similar 

 study of two-year-old stem of moonseed, ivy or other 

 vines. 



