Bamboo vs. Wood
In talking to people over the years, I have discovered that many newcomers think that because it has woody stems, bamboo grows like other woody shrubs and trees. In this article you will learn a little bit about the vascular system of wood, the process of secondary growth, and how it differs from that of bamboo. Click on the links below to see an image, and click in the image to close it.
Woody shrubs and trees increase in height/length by cell division at the tips of branches, and in girth by cell division in the vascular cambium, the layer of life between the wood and the bark. Over the growing season many layers of cells are laid down to the inside of the vascular cambium, increasing the girth of the trunk (branches grow this way too). The walls of these new cells thicken and harden with a substance called lignin. Over the growing season, the cell walls become impervious to water and oxygen, eventually killing the protoplasm in the nucleus of the cell. These hollow, dead cells are called secondary xylem, better known as wood. The wood closest to the bark is called sapwood, and it is the water-conducting tissue of the tree. The annual process of laying new cells on top of dead cells is called secondary growth, and can measured by the thickness of the growth rings in a cross section of a tree trunk.
It took me a few year to figure this out, but bamboos are grasses, so they don't have bark, secondary xylem, (wood) or a vascular cambium. Instead, water, nutrient and food-conducting tissues are arranged in vascular bundles which are scattered throughout the stem. The vascular bundles are surrounded by lignified fiber cells, which give the stem its woody characteristics. There is no secondary growth, so you won't find annual growth rings in a bamboo stem! In fact, a bamboo stem emerges from the ground at its full girth, and grows to its full height in a single growing season. Pictured below are the new shoots of Phyllostachys aureosulcata, well on their way. Click on the picture for a larger image.
New stems do all their growing in a single season
So, those stems on that little bamboo plant you just dug up or bought, will never get any bigger, but the new canes in years to come will come up fatter and grow up taller from year to year. You can learn more about how bamboo grows by reading my article on The Bamboo Growth Cycle