60 



STRATIFICATION. 



[CH. IV 



When a longitudinal section of the xylem is examined 

 with a high power (fig. 24) it will appear that the vessels 

 are of various kinds. Near the inner margin they are 

 very narrow in diameter and are marked by a spiral line, 

 further towards the circumference they are wider and 

 the walls are covered with dots instead of being spirally 

 marked. These two kinds of vessels are known as spiral 

 and dotted vessels. To understand the meaning of this it 

 is necessary to consider the way in which cell walls are 

 thickened. When a transverse section of a cell wall is 

 examined under a high power of the microscope it can be 

 seen to be delicately striped by numerous parallel lines, so 

 that it seems to be made up of concentric layers or shells 

 as described above in starch-grains. This appearance is 

 known as stratification and has been the subject of much 



FIG. 25. 

 MODEL REPRESENTING THE STRUCTURE OP A PITTED CELL-WALL: 



b represents a square sheet of paper pierced by a circular hole ; in the 

 upper figure a number of sheets like 6 are shown in section pasted 

 one over the other, the lowest being pasted to an unperforated sheet a. 



