THE LIFE OF THE TREES 11 



observation proves. Fence wires stapled to growing trees 

 are not spread apart nor carried upward, though the trees 

 may serve as posts for years, and the growth in diameter 

 may swallow up staple and wire in a short time. Normal 

 wood fibres are inert and do not lengthen. Only the 

 season's rootlets and leafy shoots are soft and alive and 

 capable of lengthening by cell division. 



The work of the leaves has already been described. The 

 return current, bearing starch in soluble form, flows freely 

 among the cells of the cambium. Oxygen is there also. 

 The cambium cell in the growing season fulfils its life mis- 

 sion by absorbing food and dividing. This is growth — 

 and the power to grow comes only to the cell attacked by 

 oxygen. The rebuilding of its tissues multiplies the sub- 

 stance of the qambium at a rapid rate. A cell divides, 

 producing two "daughter cells." Each is soon as large as 

 its parent, and ready to divide in the same way. A cam- 

 bium cell is a microscopic object, but in a tree there are 

 milKons upon millions of them. Consider how large an 

 area of cambium a large tree has. It is exactly equivalent 

 to the total area of its bark. Two cells by dividing make 

 four. The next division produces eight, then sixteen, 

 thirty-two, sixty-four, in geometric proportion. The 

 cell's power and disposition to divide seems limited only by 

 the food and oxygen supply. The cambium layer itself 

 remains a very narrow zone of the newest, most active 

 cells. The margins of the cambium are crowded with cells 

 whose walls are thickened and whose protoplasm is no 

 longer active. The accumulation of these worn-out cells 

 forms the total of the season's growth, the annual ring of 

 wood on one side of the cambium and the annual layer of 

 \ark on the other. 



