Many plants reproduce themselves by either seeds or spores. However, there is another means of plant reproduction that does not involve either of these methods. In some cases, plants that break apart can grow whole new plants out of the broken fragments. This kind of asexual reproduction is called fragmentation. The resultant offspring are clones of the parent plant, as they have all the same DNA.
Mosses can reproduce themselves by fragmentation. When the tops of liverworts such as Marchantia are broken off, each fragmented top grows into a new plant. Potato tubers, when broken apart and planted, form a whole new plant from each fragment. Many weeds can regenerate completely just from fragments of stems left in the ground, which is why weeds need to be pulled up instead of cut down.
Gardeners and farmers use fragmentation to their benefit when they start new plants from the cuttings of old plants. They also use it when they graft parts of one plant onto another. Scientists can even grow whole plants out of individual cells taken from other plants. This allows them to rapidly produce thousands of clones of the original plant in a short amount of time.
One of the chief advantages of reproduction by fragmentation is that it can rapidly produce many new plants without the waiting and uncertainty of pollination and sexual reproduction. Marchantia liverworts fragment and regrow so easily that the process can be initiated simply by having raindrops strike the tops of the plants. It can also help plants reproduce in times of drought. They need water to allow fertilization to occur. During drought, sexual reproduction becomes more difficult. Fragmentation also allows gardeners to make many identical copies of plants that have particularly desirable looks or other characteristics.
Reproduction by fragmentation carries its disadvantages as well, such as a limiting of genetic variability. If a mutation occurs at any point during the process, then all subsequent generations will carry the mutation. This leaves them much more vulnerable to rapid widespread die-offs. If the parent plant is particularly vulnerable to a disease or predator, then all of the offspring of that plant will carry the same vulnerability, and the entire population may be quickly killed by the same problem.