Taking cuttings from plants and placing them in a cup of water to grow roots is something that most of us have encountered in the past. You can root cannabis cuttings the very same way if you keep the water from getting anaerobic or swampy. It really is that simple. The fact that many folks spend a fortune to accomplish such a simple task is amazing. Even if you plan to use another method, this is a great way to temporarily store cuttings before rooting.
Uncle Pete usually uses the bottom branches cut off during regular maintenance of larger plants and stores them in labeled nursery pots or Solo cups for an hour or two prior to putting them in the cloner. A dedicated "mother plant" is unnecessary - any plant in vegetative growth can become a mother plant. See the intermediate topic Caring For A Mother Plant for more detail.
Most of the time, gardeners make cuttings from the growing tips of branches, but any cutting from a growing branch will grow roots. However, make sure the cutting has at least three nodes for best results. Depending on your growing and pruning style, you can get a small head start by using these "topped" cuttings.
Cannabis plants grow roots naturally after being cut from the plant - there is only one thing you need to understand about the science behind putting roots on a plant: environment. As always, the importance of environment in growing plants cannot be overstated. For rooting cannabis cuttings, air temperature, water temperature, and light are the parameters to be controlled. The air and water temperature should be kept at 68-75 degrees Fahrenheit, and low-intensity fluorescent light (T8 or T12) is all that is needed.
There are two main methods Uncle Pete uses to root cannabis cuttings: