You wouldn’t expect to come across a plant with strep throat, or pneumonia.   But have you ever seen a tree with a big, twisted knot growing out of it?   You know what that is?   Plant cancer.    It’s not super easy to define what cancer is. But in humans, cancer is often described as uncontrolled cell division that can spread into surrounding tissues, forming a tumor.   And plants can get their own version of a tumor. It’s just much harder for it to spread, because of how plants are structured. Still, plant cancer can be harmful, sometimes even fatal.   Cancer often -- but not always -- starts in humans or other animals because of mutations in a cell’s DNA that mess with the normal cellular growth cycle.    A healthy cell will stop dividing when the area around it starts getting crowded, but cancer cells just keep dividing over and over, no matter how little room there is.   In plants, the cell can keep dividing, too, not because of genetics, but because of other factors, like an infection or some kind of structural damage.   The main advantage that plants have here, though, is that it’s a lot harder for this uncontrolled cell division to spread to other areas.   In animals, cancerous cells invade other types of tissue by circulating around in the blood.   But plants don’t have true circulatory systems, and the fluids they use for nutrient transport, like sap, don’t have cells in them.    And there’s another factor keeping cancerous plant cells in place: the cell wall, a rigid structure that surrounds the outside of the cell and helps the plant stay upright.   Those firm cell walls prevent cells from moving around within the plant. So even if it wanted to, a cancerous cell couldn’t break off from a tumor and go spread to other tissues. It’s trapped.   Instead, these cells keep dividing in place, over and over again.   That’s what forms growths like those funky knots on trees -- which are called galls or burls.    Since the out-of-control cells are kept so firmly in place, the plant is usually able to live out its life with these growths just fine. But sometimes, they will overwhelm it, and the plant can eventually die.   So, sadly, plants do get cancer, too, in a way. It’s just different because they’re plants.   Thanks for asking! And did you know that SciShow is now on Patreon? If you want to keep getting answers to questions like this, you can go to Patreon.com/SciShow to learn how you can support us by becoming a SciShow Patron!