HOW TREES GROW

Wairarapa Times-Age, 14 July 1941, Page 6

HOW TREES GROW

SOME LITTLE-KNOWN FACT'S. The heart-wood of trees is inert and does not grow. The bark of trees is likewise inactive. And, contrary to an opinion held by many, trees do not grow in height by the gradual lengthening of trunk and limbs. The position has been discussed many times by recognised authorities, and it can be said confidently that tree trunks do not stretch upward. This is plainly indicated by the fact that fence wires stapled to growing trees are not spread apart nor carried .upward as the tree adds yearly to its height, though wire and staple may soon be engulfed .by the tree’s growth in girth. The true explanation of the growth of the tree lies in the fact that it is covered completely, from root to topmost branch, with a sort of cloak of youthfulness called the cambium. This cambium is a moist, colourless' layer of living cells found between the wood and the bark. The millions of cells in the cambium, during the growing season, go about their business of enlarging and dividing into two parts. Thus, one cell becomes two; then they become four; next 16; then 32; and so on. The cambium layer never adds to its own thickness, remaining only a thin layer of dividing cells, but adds a new layer of wood and one of bark each year, increasing the tree’s girth by just that much.

A tree grows in height only by the lengthening of the tips of its topmost branches. Very rapid cell division takes place in the tender branch tips, which are made up mostly of pith and cambium cells. And these cambium cells not only divide quickly, but they also have a type of growth not found in the cambium cell's of the trunk. The cells in the tips and green shoots can lengthen as well as thicken. As the terminal cambium cells lengthen and thicken, the tree branches stretch ever outward and upward each growing season, adding to the height of . the tree as well as to the spread of its branches. A tree’s growing season, in north temperate climes, usually begins with the first mild days of spring and ends soon after midsummer.