72 



ELEMENTARY BOTANY. 



tissue. The pith and cortex belong to this, as do also the 

 cells in the medullary ray a. The latter are seen as radiating 

 lines in the woody stems (Fig. 111). Some of them extend 

 from the pith. These had their origin in the compressed tis- 



MeduUary rays 



Fig. 110. 



Fig. 111. 



sue between the encroaching woody strands. The cells of the 

 medullary rays are longer in the radial direction, whereas the 

 other lignifled cells of the trunk have their vertical diameter 



Fig. 112. 



rowmg pomt 

 ^O" Soot-cap 



Fig. 113. 



much longer than their other dimensions. In lumber where 

 they are especially conspicuous, as in Oak, etc., the medullary 

 rays are called the '' silver grain." The milky juice (latex) is 



