KEN MUDGE: The asexual propagation methods that we're talking about include propagation by division. Here we can take any plant that has branched out to form a clump, and it includes many unit plants basically. We can simply pull apart. And each chunk includes both leaf, stem, and root tissue. So it's basically an entire plant unit, and now we've created two of them. And that's really the most basic form of plant propagation, the one that was involved in the domestication of the very earliest cultivated plants.
Next would be propagation by introducing adventitious roots. Adventitious roots are those roots that occur on stems and leaves when the adventitious roots occur on a plant that is intact, where a stem trunk is attached to its own root system. This vanilla orchid vine is an example of adventitious root formation. Here on the intact plant, adventitious roots are forming at each one of the nodes. Here's a very long one coming down from up there. And when adventitious root formation occurs on the stem of an intact plant, the process is called layering.
Here you can see not only the layers, in fact, the way that this orchid would be propagated from here on would be just to cut it in sections, stick those in the ground. And by the way, up here you can see the pod of the vanilla orchid flower. That pod is what's dried and extracted for vanilla that's used in cooking.
That's basically natural layering. That process has been adapted to plants that don't form natural adventitious roots, such as this tree here. In order to get this to successfully layer, in other words to form adventitious roots while the stem is still attached to its own root system, we can wound the stem, wrap it with wet Sphagnum moss, cover it with plastic, and allow roots to form around the wound site. Once it's rooted, then we can detach it right below the layer, take off the plastic bag, pot it up, and we have a new plant propagated by layering.
Now cutting propagation is very similar, except that in the case of cutting propagation, we induce those adventitious roots to form after we have separated the stem, leaf, or shoot from the parent plant. For example, you're all familiar with the simple practice of taking a coleus cutting and sticking it in a glass of water like this. It's very quick to form roots.
Now most plants that we're dealing with horticulturally don't form roots on a cutting quite so easily. So we can do a variety of different things to expedite the process, manipulation of the environment and manipulation of rooting hormones and so forth. But there are certain trees in which it occurs rather easily, too. This, for example, is a willow post that I cut from a downed tree near my house last spring. I just cut it off and took my ax and sharpened the base of the thing, stuck it in the ground in my backyard, and pounded it into the ground, walked away. And came back about two months later, dug it up, and you can see here that it has struck adventitious roots, basically a very large cutting. And the thing begins to grow a new shoot system, and we have a new plant. In fact, this technique of stake cutting propagation is actually used in a number of tropical countries, places like Uganda, to establish natural hedgerows. They just pound in a bunch of stakes and come back and they have trees growing there.
The next of the vegetative or asexual propagation techniques is grafting. Now grafting is a process that occurs naturally. It was observed and presumably modified to bring about the grafting techniques that allowed us to domesticate crops like apples. Here, for example, is a naturally grafted trunk of two red pine trees growing close together in a hedgerow. A branch of one of them got caught in the crutch of another, and they have fused together. And they are in complete anatomical contact.
Natural grafting also occurs with root systems. This rather ornate sculpture happens to be a natural grafted root system of a pine tree. So it's really happening all around us. All these root grafts are not necessarily so obvious, but they are quite common. And so it probably took a while for early agriculturalists to realize that they could do grafting, too.
Here's an example of a deliberate graft on an apple tree. It's called a cleft graft, where two pieces from a particular valuable tree called scion pieces, the upper portion of a grafted plant, were grafted into an under stock or root stock. And then the scions have fused with the under stock, and these will grow into the new tree, if we hadn't cut it down.
And one last method of clonal propagation that I haven't talked about yet is what we call micropropagation. And basically, that involves plant tissue culture, growing plants under aseptic conditions, artificial nutrients, with various plant hormones to take advantage of some of these same developmental processes but in this plant tissue culture environment.
So for example, we can put a shoot in culture on the artificial nutrient medium, and it may surprise you to learn that within a year's time, with appropriate subcultures, we could multiply this single shoot from one plant to a million plants. There is no other propagation technique where we can get such fantastic rates of increase. So this technique of micropropagation has made a tremendous difference in our ability to multiply plants rapidly.
Now, this shoot formation involves the process of adventitious organ formation. At least when these shoots are ready to be rooted, that's basically the process of cutting propagation except they're micro cuttings.
On the other hand, it's possible to take advantage of the phenomenon of asexual embryogenesis, induce asexual embryos to form in a tissue culture situation like this, and it's usually called somatic embryogenesis. But both somatic embryogenesis and micropropagation are both types of plant tissue culture.
We'll be talking more about micropropagation in the final section where we talk about micropropagation and its contribution to genetic engineering.
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In this Room I share with you my fascination with plant reproductive biology and its application to horticulture and related disciplines.
I begin by dispelling the widely held oversimplification that "plants grow from seeds" - indeed many of them do, but quite a few have evolved the capacity for asexual (clonal) reproduction. Even before the origins of agriculture, about 12,000 years ago, mankind has been observing wild plants performing feats of asexual reproduction.
From this increasingly sophisticated understanding of the natural history of cloning, early agriculturists domesticated a number of fruit, nut and other food crops and eventually a host of ornamentals as well. The Room includes hands-on demonstrations of clonal propagation by layering, cuttings, grafting and micropropagation.
This video is part 2 of 7 in the Natural and Human History of Plant Cloning series.