214 PRACTICAL BOTANY. 



several small resin ducts. The narrow dark lines running 

 in the radial direction through the xylem, and even ex- 

 tending out through the cambium layer into the phloem, 

 are the beginnings of medullary rays. In the central part 

 of the stem is the pith parenchyma, K. The projections 

 of the pith parenchyma into the xylem are the places of 

 origin of branches which developed just above the plane 

 of this section. 



Cut transverse sections of a second year stem that has 

 lain in alcohol to dissolve out the pitch arid has afterward 

 been softened by soaking in equal parts of alcohol and 

 glycerine for twenty-four hours. Do not try to get com- 

 plete sections, but get thin cuttings, showing all the parts 

 from the epidermis to the pith. Mount in Schultze's solu- 

 tion, and identify under low power all the above-mentioned 

 parts. Draw enough of the section to locate everything 

 observed. 



Cut similar transverse sections of first year shoots. 

 Compare with the preceding section. Draw a portion as 

 before. Cut similar sections of a piece of the stem that 

 is four or five years old. Compare with the other two, 

 drawing a portion as before. 



Under high power examine the cambium layer in a 

 delicate section. In this layer the growth of the stem in 

 thickness originates. Find in this layer cells with very 

 thin walls. Draw a few. What is their shape ? During 

 the period of growth the cells undergo rapid fission in the 

 tangential direction : the outer halves developing into 

 the various tissues of the bark ; the inner halves into 

 the wood. From this layer follow out into the phloem. 

 What changes do the cells undergo as they increase in 

 age ? Are the changes abrupt or gradual ? Starting 

 again with the thinnest cells of the cambium layer, 

 trace the development of the cells into the xylem, stating 

 what you observe. Compare with the changes on the 



