n8 TEXTBOOK OP BOTANY 



when it becomes active in the spring, it again forms large, 

 thin-walled cells. This new spring wood is next to the 

 summer wood of the year before ; and the sharp contrast be- 

 tween the dark summer wood and the lighter-colored spring 

 wood causes the familiar appearance of rings in a section of a 

 tree trunk. It is because these rings are formed as just de- 

 scribed that one can tell about how old a tree is by counting 

 the rings in a section near its base. The number of rings is 

 not a perfectly accurate measure of a tree's age, because the 

 development of new wood may have been changed or pre- 

 vented in places by pressure or by wounds, and because occa- 

 sionally in a year of exceptional weather, as when a long 

 summer drought is followed by heavy rains, two rings are pro- 

 duced instead of one. It is only in the outermost part of the 

 wood much more than the present year's growth, however 

 that the sap flows upward ; this outer wood the sap- 

 wood is soft and moist. In the cells and cell walls of the 

 older heartwood changes have taken place which prevent the 

 passage of water. The heartwood is therefore dryer and 

 harder than the sap wood. 



143. Cork and Bark. Another kind of thickening occurs 

 in the cortex of the trunk. This results from the division of 

 the cells of a layer a short distance within the epidermis, 

 called the cork cambium. The new cells formed on the outer 

 side of the cork cambium develop thick walls that contain 

 a fat-like substance called cork. Water will not pass through 

 the walls of these cork cells. Now, since the whole water 

 supply of the trunk comes from the roots and is carried up 

 inside the trunk in the sapwood, the cork cells and the cells 

 outside them are cut off by the corky walls from a supply 

 of water ; these cells therefore become dry and dead, cracks 

 appear in their walls, and so a hard, dry bark is formed. 

 Later (in some species of pine only ;n parts of the tree that 

 are eight to ten years old), a new layer of cork cambium ap- 

 pears a little way beneath the first layer, new layers of cork 



