174 



STEMS 



are not simple cells. They might be regarded as compound 

 cells. They have been formed by rows of cells whose end 

 walls disappear and which thus become united into a 

 single vessel or canal. As you have noted in the picture 

 {Figure 60), the walls of the tracheary vessels are peculiar. 

 They are thick, and the thickenings are not evenly arranged 



over the surface. Thus 

 at A in Figure 60 we 

 have what are called 

 spiral vessels ; the thick- 

 ened portion of the wall 

 takes the form of a spiral. 

 The advantage of this ar- 

 rangement is not clear, 

 but it has been found 

 that the first tracheary 

 vessels to be formed are 

 ^i.^^^^ of this type. At B in 

 m^^^^^ FigMre 60 we have what 

 " are called pitted vessels. 



In these the thickening 

 covers nearly all of the 

 surface, but it is marked 

 This is the type of vessel of which the 

 (By secondary wood 



A B 



Fig. 00. — Tracheary vessels of the xylem 

 viewed lengthwise, very highly magnified ; 

 A, spiral vessels; B, pitted vessels. 



by frequent pits. 



secondary wood is chiefly composed. 



is meant, as you will recall, wood which is produced 



by the cambium and not by the original growth of the 



stem.) 



The tracheary vessels are of more service to the plant 

 when they are dead than when they are alive, and they 

 die soon after they are formed. This death, however, is 

 not accompanied by decay. Completely surrounded as 



