246 EXPERIMENTS WITH PLANTS 



wood, and new sap-wood must be formed to take its 

 place. If we examine sections of the Oak (Fig. 135) 

 or Pine (Fig. 138) , we can see how the new wood is 

 formed. At the outer edge of the wood is a tissue 

 called the cambium, composed of very small, rapidly 

 growing cells. We can see how these cells grow larger 

 to form wood on one side and bast on the other (this 

 is well shown in the Squash, Fig. 133) . In the middle 

 of the cambium layer the cells divide frequently into 

 two, thus forming more cells, which ultimately 

 develop into wood or bast. If we make sections of the 

 extreme tip of the stem, where the bundles are still 

 separate, we find that the cambium layer grows out 

 from the bundles and unites to form a riug, which 

 soon forms wood and soft bast in a complete circle 

 around the stem. 



When we separate the bark of a tree from the wood, 

 we find between the two a white, glistening, juicy 

 layer; this is the cambium. If we carefully make an 

 incision in the bark of a tree (preferably a young tree 

 on which the bark is thin), and slip a thin piece of 

 metal (a ten -cent piece hammered thin is very good 

 for this purpose) between the cambium and the wood, 

 it will be found in the course of a year to be covered 

 with a thin veneer of wood as the result of the activity 

 of the cambium. You may easily try this experiment 

 for yourselves. Axe-heads, iron bolts, etc., have been 

 taken from the heart of trees which were covered by 



