144 SCOTCH PINE. 



\. Shape and arrangement. 

 ii. Their emptiness. 



iii. Their thick walls, showing in thin parts of the 

 section, a middle lamella. 



iv. In the thinnest part of the section, search for 

 places where the radial walls 7 of contiguous 

 cells bow away from each other like two watch 

 glasses placed with concavities together. 

 They are most readily found in the youngest 

 part of the xylem. In the most favorable 

 sections these bowed walls may be seen to be 

 interrupted at their points of greatest diverg- 

 ence thus -O-. These are sections of the 

 bordered pits (further described at A. 18. b. 



iii.). 



v. Compare the tracheides of the outer growth 

 ring with adjacent ones of the inner one. 



vi. Wide one-sided bordered pits where the 

 tracheides adjoin the cells of the medullary 

 rays. 



10. The cambium. Note 



a. The radial rows of rectangular, 8 very thin-walled 

 cells, passing abruptly on the one hand into the 

 xylem, but shading almost imperceptibly on the 

 other into 



11. The phloem. Note the two elements which compose 



it : 



a. Angular thick-walled cells with a whitish luster 

 and constituting the greater part of the phloem, 

 the sieve cells. In favorable sections the radial 



7 7. e., those lying along a radius of the stem. 

 8 Very apt to be distorted in cutting. 



