ClilCULATION OF SAP. 



91 



of trees, as it will be observed that the absorption of 

 carbon, and the giving out of the oxygen becomes 

 less active as the light is diminished, while during 

 the night the contrary process occurs, the oxygen be- 

 ing absorbed, and the carbon released. 



Thus it is by the assistance of 

 these vessels that the little oak- 

 plant becomes a perfect tree in 

 miniature, and continues to increase 

 in size and strength from year to 

 year, until the woodman levels with 

 his axe the sturdy trunk that has 

 defied the storms of a century. Let 

 us here examine the stump that re- 

 mains, and we will observe a num- 

 ber of concentric rings commencing 

 at the bark, and running around 

 the tree, one inside of the other, until they reach the 

 centre. (See figure of Exogenous Wood on a pre- 

 vious page.) Each ring indicates one year's growth, 

 a new layer of wood being depo- 

 sited every season immediately 

 under the bark. The age of 

 any tree may' be determined 

 with considerable accuracy by 

 counting these rings. 



Some species of Oaks retain 

 their foliage during winter, as 

 the Live-oak. This tree in- 

 habits the Southern States, 

 where it occasionally grows to Spanish Oak 



Chestnut-Oak. 



