16 Trees, Stars, and Birds 



protoplasm. After the cell reaches a certain size, a 

 partition is built across it which divides it into two cells. 

 In like manner the two may become four, and so on 

 until a large number of cells have been formed. This 

 multiplication of cells and the increase in size of the 

 cells as they become old cause growth. 1 The trunk 

 of a tree is built upward, and the branches and roots 

 increase in length in this way. New material is built 

 on the top of the trunk, and added to the tips of the 

 branches and roots by the multiplication of the cells 

 in these parts. 



Proof of where the trunk lengthens. It is said that 

 if two nails are driven into a living tree trunk, one 6 

 feet above the other, they will remain 6 feet apart, no 

 matter how long the tree grows. Driving nails into a 

 trunk is not good for the tree or for the saw that may 

 sometime cut it into lumber. Without driving in nails 

 or waiting for years to see the result, you can decide 

 about the lengthening of a trunk by examining trees 

 that have been used as posts for a wire fence. You 

 may find some in which the wire and staples have been 

 covered by years of growth. Are the wires farther 

 apart and higher above the ground than where they were 

 fastened to dead posts? How do you account for the 

 fact that on old trees the distance from the ground to 



1 In older plant cells there is usually in the center of the cell a large 

 amount of water ("cell sap") in which mineral matter and certain food 

 materials are dissolved. The taking in of this water stretches the cell 

 and greatly enlarges it, sometimes to several hundred times the size 

 it has before the absorbing of the water begins. It is this pumping of 

 water into the young shoots that causes the young branches of trees to 

 push out so rapidly in the spring. 



