TUB TREE ITS SHOOT-SYSTEM. 1Q3 



each spring it renews its activity. Further peculiarities 

 will be noticed as we proceed. 



Now let us see what the cambium cells are, and how 

 they change into new elements of the xylem and phloem, 

 etc., respectively. 



Each cell of the cambium is a thin-walled prism, 

 many times longer than broad or thick, and with its 

 ends brought to an edge like that of a thick chisel, and 

 so arranged that these edges run radially and fit in 

 between those of cambium cells at higher and lower 

 levels. As we have seen, the prism is oblong in trans- 

 verse section. Each of these cells contains protoplasm 

 and a nucleus, surrounding a sap-cavity, and they are 

 nourished like other cells by the substances brought 

 down from the leaves and up from the roots, taking 

 what they need from the sap. 



"When a given cambium cell has taken into its pro- 

 toplasm sufficient food materials, and has accomplished 

 other life-processes under the action of oxygen, which it 

 absorbs dissolved in the water of the sap, it grows larger, 

 especially in the radial direction, and then it divides 

 into two cells ; then each of these may repeat these pro- 

 cesses, and so on. At last the older ones can no longer 

 grow and divide, but become changed into elements of 

 the xylem or phloem, according to their position. All 

 the xylem thus produced by the cambium is called sec- 

 ondary xylem, and the phloem secondary phloem, and 

 so on, to distinguish them from the primary structures 

 found in the early stage. 



