THE SPERMAPHYTES. 



217 



to the x}4em cells with the transition to the phloem cells. 

 Which is the more abrupt ? 



Study the cells of the xylem. They are called trache- 

 ides. What is their shape? Note the discoid markings 

 on their walls. These are the bordered pits ; recall their 

 appearance as seen in the transverse section. Do you find 

 any cell contents in the trache'ides? Notice the medul- 

 lary rays which 

 run horizontally 

 across the tra- 

 cheides. Can 

 you trace them 

 through the 

 cambium into 

 the phloem? 

 Count the num- 

 ber of cells in 

 height. Is the 

 number con- 

 stant in a given 

 specimen ? Ex- 

 amine radial sec- 

 tions of differ- 

 ent species of 

 Pine. Is the 

 number of cells 

 in the height of 

 the medullary rays the same in different species ? Follow 

 across the tracheides from the inner side of one of the 

 annual layers into the next outside layer. What changes 

 do you notice in the size and character of the tracheides ? 

 Account for the difference. Do you find resin ducts in 

 the xylem ? On the border between the tracheides and 

 the pith parenchyma, find tracheides with spiral or annu- 

 lar markings. Draw. Do you also find some that have 



FIG. 136. Radial section of Georgia Hard Pine, 

 showing two of the annual layers. A is the bound- 

 ary between the growth of the two years; B, B 

 are medullary rays ; C, C, pitted ducts. (X 35.) 



