THE TREE—ITS SHOOT-SYSTEM 103 — 
phloém. 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 
phloém, &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. Hach of these cells contains protoplasm 
and a nucleus, surrounding a sap-cayity, 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 soon. At last the older ones can 
no longer grow and divide, but become changed into 
elements of the xylem or phloém, according to their 
position. All the xylem thus produced by the cambium 
is called secondary xylem, and the phloém secondary 
phloém, and so on, to distinguish them from the pri- 
mary structures found in the early stage. 
