34 Plants and their Ways in Soiith Africa 



and the bark may then be readily removed. All the growth 

 outside of this ring of cells is bark or cortex. Every boy 

 who makes willow whistles takes advantage of the active 

 season of the cambium, for it is when the cells are young and 

 tender that careful and judicious pounding with the handle 

 of a jack-knife will bring the bark off entire. Later in the 

 season, when the cambium is not so active (for the whistle- 

 making season is limited, " as every schoolboy knows "), the 

 cork gets to be a rather tight fit, growth is less active, conse- 

 quently the cells of the wood are not so large. The large and 

 small cells alternating make the annual rings in the wood by 

 which the age of a tree may be determined. Rings are formed 

 in the bark, but they are not so well marked as those in the 

 wood. 



