Many plants have the ability to produce roots along their stems and at budding sites. This is because the basic building block cells (meristem cells) of plants are able to develop into a variety of cells as they mature. The meristem cells found in the leaves, stems and roots all start out the same, and it is only as they develop that they grow into specialized leaf cells, stem cells and root cells. This is important because which type of cell a particular meristem cell will turn into depends in part on the environment it is in.
Some plants make use of this to extend their range. Vining plants in particular are often able to make roots along their stem (especially at budding sites with their high concentration of undeclared meristem cells) where the stem comes in contact with moist soil. This gives not only additional anchoring points for the plant, but the roots can help supply moisture and nutrients along the length. The same plant may have a central main root system, and then satellite root systems at various places.
Since this creates sections of the plant that have both root systems and shoots, these sections— once established—don’t need to be connected to the original plant anymore. If for some reason the vine is damaged or cut, the satellite section can continue to grow independently of the parent plant.
Gardeners can use this property to propagate such plants. A technique called simple layering involves intentionally burying sections of stem to encourage rooting. Once the roots are established, the sections are cut apart. It is important to note that up until the connection is severed, it is still all the same plant, and once cut while technically they could be called separate plants, they will share the same genetic code and gender since they were originally the same plant.
There are variations on the layering theme. Sphagnum moss or similar can be tied to a branch and kept moist to root above ground branches (known as air layering), growing tips can be bent to come into contact with soil (tip layering), stems can be bent horizontally and either partially buried at several points (serpentine layering) or buried straight across in stages (trench layering). All of these techniques involve getting roots to develop, and then cutting off the rooted sections of stem to create “new” plants, even if they are also in a way still the same plant).
Layering methods all basically involve putting a section of stem into a growth medium, then keeping it moist enough to encourage roots, and finally cutting the rooted section away from the parent plant.
However, there is another way to vegetatively (non-sexually) propagate plants, which is very similar to layering, but changes the order. Instead of separating the new plant after it is rooted, it is separated first, then placed into a grow medium, and then roots are encouraged to develop. This method is commonly referred to as rooting cuttings, or cloning, in tribute to the matching DNA of both plants.
Since the cutting is removed from the parent plant before it has developed a root system, the stem must be kept moist so it can supply moisture to the rest of the plant. Eventually the meristem cells in the moistened stem will take their cue and mature into root cells, forming the start of a new root system.
At its most basic, cutting the end of a branch and placing the stem in water on a windowsill until it roots is a way to root cuttings. Ways to improve over this method include aerating the water (adding an air stone), supplying stable lighting with an artificial light, and regulating environmental factors such as heat and humidity.
Care must be taken not to allow the stems to dry out, or the plant will go into terminal wilt and die, but care must also be taken not to keep the stem too wet for too long, or it can encourage lethal fungal growth commonly known as root rot. Up to a point, the better aerated the water, the longer contact with the stem is allowable. There are rooting solutions on the market that contain hormones to help convince the meristem cells into developing into root cells.
While not all plants can be started from cuttings—some woody plants are easier to layer than keep cuttings alive for the length of time needed—for those that will (tomatoes in particular are easy to root cuttings from) taking cuttings tends to be the method of choice for starting new plants from a mother plant.
Rooting cuttings has some benefits over layering: it tends to be less labor intensive, requires less space, can root plants quicker and more plants can be made from a single parent plant easily.
Trying down branches to layer takes both effort and time. Well-cared for cuttings can start to root days or weeks before their layered equivalent. Any section with a growing tip (apical meristem) and stem section can be attempted to be used, allowing for several cuttings to be made from each donor plant.
An advanced form of rooting cuttings, known as tissue culture, involves using only small sections of a mother plant, treating them and then placing them in a special sterile grow medium to allow the meristem cells from any part of the plant to grow roots (and shoots).
Even for gardeners who specialize in plants that are usually propagated by either layering or cloning, there is value in understanding the similarities and differences between the two methods so they have a better overall understanding of the mechanisms involved.