354 THE BIOLOGY OF THE PLAKT 



ments of the xylem. Do you find that those with 

 large cavities, or vessels, occur in definite places'? 

 Do you find any elements which run in radial 

 lines, the medullary rays, from the pith to the 

 cambium ? Can you trace the rays beyond the 

 cambium ? What is the shape of the cells forming 

 the medullary rays ? Why are these rows of cells 

 called " medullary rays" ? 

 Draw a portion of the section showing all of the ele- 

 ments with their contents. 



Study the shape, arrangement, and contents of 

 the cells of the pith. Stain sections in Schulze's 

 solution and in phloroglucin. Note particularly 

 the color assumed by the elements of each tissue. 

 Cut other sections two or three internodes below, 

 and compare with those just examined. What 

 gives the xylem the appearance of being composed 

 of concentric rings, the annual rings ? Do these 

 rings vary in thickness as compared with one 

 another? If so, to what is the difference due? 

 Do difi'erent parts of the same ring vary in thick- 

 ness ? Why ? Make drawings showing the cause 

 of the ringed appearance. Cut other cross-sec- 

 tions through the upper and the lower end of 

 the green stem of the current year, and com- 

 pare with the others. How many fibro-vascular 

 bundles can you find in the young stem ? What 

 changes take place in their number and position 

 in the older stems ? Using the proper reagents, 

 especially phloroglucin, try to trace the change of 

 the cellulose into the lignifled elements of the 

 xylem as the stem gets older. Make longitudinal 

 sections and compare with the transverse sections. 

 Draw. 



