Bamboo is a tall grass that can be used both as a lumber and as a food. Some forms of bamboo can reach 70 feet tall. Bamboo "stalks," more properly called "culms," are the main growth supports for the plant's leaves. Although the culms can differ in size, strength, thickness, and color, the culms of different species of bamboo are very similar in structure.
Bamboo culms grow in sections. The sections are defined by thin areas called nodes. The parts of the culm between these nodes are called internodal areas. The length of the internodal areas, and the thickness of their walls, will vary greatly, depending on the species of bamboo.
Unlike trees, which grow solid, the centers of the internodal parts of bamboo culms are hollow. However, the nodes are solid. This solid node is what allows a dried bamboo culm to be used as a glass, cup, or scoop. The hollow culm makes the side of the vessel, with the node making the sealed bottom.
Wood has a coarse, open-cell structure that is not found in bamboo. Although bamboo is used as lumber, bamboo is made up of bundles of parallel microscopic fibers. These bundles of fibers work in a way similar to the cables on a bridge. They create strength, but don't weigh much. These bundles of fibers are filled with lignin, which makes the bamboo flexible. This unique structure makes bamboo both very strong and very flexible. If you look at a bamboo wall cross section under a microscope, you can see dark bundles of fibers with light areas of lignin between.
Because bamboo is hollow, many people think it is less dense than wood. However, this perception is not accurate. The actual woody part of a bamboo culm is actually two or three times as dense as pine, and can often be more dense than oak. It has about the same density as hickory. Fibers can contribute between 60 and 70 percent of the weight of the bamboo, with lignin making up the balance.
Bamboo culms transfer moisture to the leaves to allow for photosynthesis to help the plant grow. As a result, culms can actually contain as much as 170 percent of the dry weight of of the culm as water. For example, a length of culm that weighs 1 pound dry can sometimes have contained as much as 1.7 pounds of water when green.