THE TREE— ITS SHOOT-SYSTEM.
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 phloëm, 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 transverse 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 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 processes, and so on. At last the older ones can no longer grow and divide, but become changed into elements of the xylem or phloëm, according to their position. All the xylem thus produced by the cambium is called secondary xylem, and the phloem secondary phloëm, and so on, to distinguish them from the primary structures found in the early stage.