Taxodium distichum, baldcypress
Scientists aren't sure what function the knees have. One theory is that the knees are a mechanism to supply oxygen to the roots. Another theory is that the knees help stabilize the tree when it's growing in mucky conditions.
The possibility of knees should be considered when planting a bald cypress in your landscape. If you are planting the tree in a boggy area, knees will almost certainly develop. This could be a problem if you want to mow around the tree. If you are planting the tree in a moist, well-drained area, it will probably never develop knees.
Baldcypress roots don't send up knees unless they are submerged in water at least part of the time. However, bald cypress roots are attracted to moisture and when they find a boggy place, they will send up knees there, even though it may be far from the trunk.
Here's an example of distant knees. I saw a baldcypress tree growing on a riverbank, probably 30 feet above the water. Down at the bottom of the river's little canyon, in the shallow water at the river's edge, the bald cypress roots had grown a little forest of knees.
Wherever you plant your baldcypress tree, keep the soil moist during dry weather. They won't tolerate hard dried-out soil. This is one tree that it will be hard to overwater!
Check out this beautiful, massive old baldcypress, growing in an Arkansas swamp. (Scroll down to the second photo on the page.) Its knees are huge!