So there two primary purposes for plant tissue culture (outside of genetic modifications).
The first is to remove items that are considered 'pests'. This includes, but is not limited to, bugs, molds, mildews, and viruses. Different techniques are used to accomplish these, as noted above.
The second is for "micropropagation" which is exactly as it sounds, to clone tiny plants. Lots and lots and lots of tiny plants.
First, I will cover the two primary techniques used to culture tissue, nodal and meristem. There are other ways to create callus, use bioreactors to grow cells in solution but this is all very complicated for folks without a strong grasp on agricultural biotech lab expertise so we will use the K.I.S.S. method.
Nodal tissue culture is very similar to what one would do to take a traditional clone, which for all intents and purposes, is still a tissue culture. When you take a traditional cutting you take a larger part of a branch tip and you propagate the stem tissue
ex vitro. In nodal TC you cut a tiny little piece of a node and put that into a culture,
in vitro. The idea here is that with a little bit of patience you can take the 50 normal size traditional clones from a mother plant or you can use that same amount of starting material and turn it into 100s of tissue culture "nodal explants". In short, using plant tissue culture you can make exponentially more starts that you can with traditional cuttings. Instead of propagating you are micropropagating.
But wait, it gets better. Let's say that now instead of 50 clones from that mom you have 300 explants. In 30 days you will have 50 plants from those 50 traditional cuttings but from the 300 explants, you will now have 1500 new explants. Under ideal conditions, you should end up with 4x-8x multiplication from each of you initiated nodal explants. Sometimes more, sometimes less, etc.
Nodal explants are a primary way to clone genetics on an exponential factor. You do not need to be a mathematician to see how it works. 4x-9 and each x multiply each time. This is why they call Stage 2 Multiplication. And let's face it, how many of you have the space for 5000 plants? Well, if you have some skeleton racks in your garage you can fit about 1000 in the same place that you might keep one normal size mother plant.
Meristematic tissue culture is a totally different beast with a different desired outcome. It is a similar concept as nodal but instead of putting a piece that includes an axillary node you are taking the very outermost growing tip of the plant, the apical meristem. This is part of the plant that is undergoing to most transformation.
When you excise the meristematic dome you get a cluster of undifferentiated cells that are undergoing such a high rate of cell division that the plant's primary permanent tissue is not tied to it yet. While challenging to perform the benefits are enormous.
While plants can be micropropagated using stem cells it is laborious and time-consuming. The primary advantage of meristem cultures is to remove diseases like viruses. I took this screenshot during a lecture the other day and it really needs NO captioning at all. You can see the stained virus in the vascular system and you can see the meristem dome. Remove the dome and culture it away from the virus and you get a clean mom, period. (By the way, this is a meristem from a grape plant)
Hopefully, this continues to outline the what's and why's of cannabis plant tissue culture.