Trees grow vertically because of two things: gravity and light.
Geotropism, in which microscopic particles in plant cells react to gravity, tends to make the roots grow straight down, which means that the stems grow straight up.
Phototropism tends to make plants grow vertically as well, following the direction from which light comes. Phototropism was originally called heliotropism, or bending toward the sun, until scientists found out that plants would bend toward light in general, not just sunlight.
The phenomenon was studied by Charles Darwin and his son Francis, who recognized that the bending started just below the tip. But they did not discover the mechanism that caused the bending.
Subsequent researchers found that a class of plant hormone called auxins can regulate the growth of plant cells, interacting with other plant substances to direct and control the plant’s final shape, both above and below the ground.
In a growing tree, auxins, produced at the growth tip, promote the elongation of plant cells. Auxins are present in greater concentrations on the darker side of the plant shaft, so those cells grow longer than the cells exposed to light.
Thus, the tree bends toward the light. inner needles senesce, or grow old, turning yellow or brown, and drop from the tree after one to several years, depending on the species. By November of most years, for example, white pines may have only a year’s worth of needles attached to the tree.
Austrian and Scotch pines usually retain their needles for three years. Spruce and fir needles also yellow and drop with age, but these trees retain their needles for several years, so needle drop is often not noticeable. Arborvitae and some cedar needles usually turn brown rather than yellow with age.
These general patterns of needle drop may vary from tree to tree and year to year; adverse conditions in the summer and fall may lead to a more pronounced needle drop.