THE TKEE ITS SHOOT-SYSTEM 103 



phloem. Each winter it pauses in this process, and 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, &c., 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 

 protoplasm sufficient food materials, and has accom- 

 plished 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 processes, 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 secondary xylem, and the phloem secondary 

 phloem, and so on, to distinguish them from the pri- 

 mary structures found in the early stage. 



