KEN MUDGE: At this point, I'd like to talk about the various different asexual propagation techniques that are used in modern horticulture. Let's begin that just by refreshing our memory of the diagram I showed earlier. We're considering asexual propagation methods through regeneration of adventitious organs and will begin with the process called layering. Later on, we'll consider cuttings, grafting, and finally micropropagation.
All right now, let's consider layering. You saw last time I showed an example of natural layering and an example of deliberate horticultural layering called air layering. This is an example of air layering, or marcottage, as the French put it. This is a layer I put on a hibiscus plant a couple of weeks ago, hoping it would be rooted by now but unfortunately it's not.
Now layering is a technique that's been practiced for thousands of years. And on your right, you can see an example of modern layering that's being done in Madagascar in the hill country. It's probably exactly as it was done hundreds and hundreds of years ago. This man is air layering or performing marcottage on a lychee tree, and he's already wounded the stem. And what he's done is wrapped the wounded area, not with the sphagnum moss that I've shown you with hibiscus but rather with a ball of mud and cow dung. And then he takes the fiber of Raffia palm and wraps it off.
This thing is supposed to stay moist for a while, but in fact, the raffia palm is not a particularly good barrier to water loss. And so when the dry season comes, this air layer dries out. The whole process stops for a while until the rains come again, get the mud ball wet, and finally, adventitious root formation proceeds.
Now that practice takes about a whole year. When we introduced the use of moss gathered off the side of a tree, wrapped with polyethylene available from the local market, it accelerated the process from one year to about three months.
Air layering is only one of a number of different layering techniques. Another one that's very important in modern horticulture is mound layering or stooling. It's very, very important in apple production. Now you might think, well, everyone knows that apples are propagated by grafting, right? What does layering have to do with apple production? The answer is it's the method by which root stocks are cloned. Nowadays, modern fruit trees are often propagated on clonal root stocks rather than seedling root stocks.
As you can see on the right, the production cycle for apple nursery stock begins with the process of clonal reproduction of the root stock. In other words, mound layering of the plants that are eventually going to be the root system for the grafted apple tree. What is involved there is cutting back the stem of a clonal plant in the ground, cutting it all the way back to the ground, and it begins to throw up lateral shoots from the point below the cut.
It's typical of any plant. You cut it back, and that encourages lateral branching. In this case, you get many four, five, six rapidly growing shoots. As they elongate, the nurseryman piles them up, creates a mound with sawdust-- moist sawdust-- enough of a mound to cover most of the stem, but the tips of the shoots are still protruding so there is some leaf area exposed to the sun. By the end of that growing season, then those shoots in that dark, moist environment underneath the mound will produce adventitious roots. And by fall, you cut those rooted layers off below the new adventitious root system, and you have five or six rooted layers. Those are put in cold storage and they're ready for planting out in the field and subsequent grafting later in the year.
Let's move on now to a consideration of another major asexual propagation technique, and that is propagation by cuttings. You'll recall the difference between layering and cutting propagation isn't that great. Both involve adventitious root formation on stems. In the case of the layering, the stem is still attached to its original root system. In the case of cutting propagation, it's cut away. The shoot is cut away from the original plant before rooting is induced.
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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 3 of 7 in the Natural and Human History of Plant Cloning series.