The Living Trunk and Branches 141 



several months old, and hence somewhat tough. 

 But when we strip a spring branch we break tender 

 new bark, and soft young wood, just forming, or 

 newly formed, and the sap which fills their cells 

 escapes, wetting our fingers. This newly made 

 substance is a green, moist layer, just within the 

 inner bark. Here the cambium is doing its work. 



Perhaps we can try a little experiment, and 

 catch the trees in the act of building. 



" Early in the spring," says an expert in the 

 United States Forest Service, " select a young, 

 vigorously growing tree, from three to four inches 

 in diameter, with a thin bark which peels easily — 

 for example, a willow or a box-elder. With a 

 sharp knife, make a horizontal cut, about one inch 

 long, through the bark. From each end of this 

 cut make a vertical slit extending upward for 

 about an inch and a half. Carefully peel back 

 the flap of bark thus loosened, being sure to expose 

 the young wood, and put a thin sheet of tinfoil 

 beneath the bark, on the exposed surface of the 

 wood. Turn the bark back into its place, and seal 

 up the cuts with grafting wax. Then at the close 

 of the growing season turn the flap back and look 

 at the tinfoil. 



" We notice that the new wood is on the outside 

 of the tinfoil. The cambium layer has built it." 



In spring, when growth goes actively forward, 

 large vessels and large tubes are a-building just 

 under the bark of all the trees. Cambium is at 



