|The stem (ok, trunk) of our hackberry tree with its great hackberry bark.|
|Close-up of the edge of a ridge in hackberry bark showing the layers unique to hackberry bark.|
|Our giant hackberry tree with very rough bark for a hackberry.|
First: protection. Bark seals off the tree from the environment. It prevents the tree from drying out in the heat or getting soggy in the rain, just like our skin protects us. Bark also keeps out insects and diseases, also like skin. The stuff in bark that forms a seal against the world is called cork. Cork is a spongy, softer material found in most types of bark, and it is waterproof due to the presence of a wax called suberin. Some trees make more cork than others, and humans harvest cork for sealing bottles from the corkiest tree - the cork oak. In most trees, the cork is interspersed with harder material in the outermost part of the bark. The ridgy bumps as well as the smooth parts of hackberry bark both contain enough cork to protect the hackberry tree.
The second function of bark is food transport. Trees and plants use the sun to make their energy in a process called photosynthesis. Photosynthesis is the name for the chemical reaction that plants do to make sugar, and that chemical reaction is powered by sunlight. Plants' basic food is sugar, which they can use for energy (just like you do) or for building other necessary plant parts. Trees do photosynthesis in their leaves, but they need food in all parts of the plant. The sugars from photosynthesis combine with water in the tree to form a liquid called sap, and liquid sugar is easy for trees to move around. Tiny tubes in the bark transport dissolved sugars in the form of tree sap from the leaves to the rest of the plant, which is very similar to how the tiny tubes called blood vessels transport blood (which contains dissolved sugars too!) all around your body.
If you've ever tasted maple syrup, you have tasted the concentrated tree sap taken from maple bark. Maple syrup is sweet because maple trees' leaves did photosynthesis using the sun to make sugar. Unfortunately the way I've explained this makes me think of maple syrup as tree blood, but that's really not quite true. Blood is way more complex than sap, and blood has many more functions in our bodies than sap has in trees, but that's a story for another day.