Many plants can be artificially cloned by leaf cuttings; species that work well include African violet (Saintpaulia), Peperomia, bryophyllum (Kalanchoe) and jade plant (Crassula). In this process a leaf (blade + petiole) is removed from the donor plant. The leaf is placed in moist soil or, perhaps, in water and placed in the light.
In some plants, the callus sends up multiple shoots. In those cases, the gardener has to separate the individual plantlets before potting up the individual plants.
Obviously this process can be accelerated if the last centimeter of the petiole is dusted with a mixture of talcum powder and synthetic auxin (indole butyric acid) before the leaf is planted in the moist soil at the beginning. This mixture is often called "rooting hormone" and two leading brands of this are Rootone&174 and Hormodin&174.
Many species cannot be propagated from leaves alone. The leaf may root, but shoots do not form, or form so slowly that you do not dare wait for it. For such species, it may be possible to instead make a stem cutting. The process is very much the same, but is much faster due to the contributions of auxin from several leaves, and the contributions of photosynthesis in several leaves.
Generally the gardener cuts a short piece of stem (three or four internodes maximum). It does not have to include an apical meristem, but often it does. The leaves from the lowest node are torn from the stem. The basal end of the cutting may be dusted with rooting hormone (if needed or desired).
The several leaves remaining on the stem cutting all send IAA (auxin) down from the blade. This accumulates at the basal end of the cutting (along with any applied rooting hormone). The accumulated auxin stimulates formation of callus and ultimately roots.
Once we have a stem with roots, our plant has been cloned and the process is complete.
A tip layer is a natural method of propagation in such species as black raspberry. A simple layer is fundamentally an artificially-forced tip layer. In many shrubs that cannot be cloned by leaf or stem cuttings, a simple layer may work well. Rather than making a single leaf or a small stem cutting provide for its own needs and enough extra to make a new plant, a simple layer can be used to allow contributions from the whole donor plant to drive the cloning process.
1. A stem, low on the shrub, is arched mechanically to the ground and held in place with some small wire hoops.
2. The lower surface of the held stem is wounded (with a small notch or some grooves) and the wound is dusted with rooting hormone.
3. The wounded, dusted, section of the stem is buried in the moist soil.
4. With time, the hormone powder and contributions of IAA from the rest of the leaves on the donor plant cause roots to form from the wounded area.
5. The rooted stem can then be separated from the donor plant.
If a gardener is trying to propagate a tree instead of a shrub, there is no way to bring a branch down to the ground for a tip layer. If you cannot bring Mohammed to the Mountain, then you bring the Mountain to Mohammed! This is the idea of the air layer.
1. Somewhere along a young stem in the tree canopy, the lower surface of a stem is wounded and the wound dusted with rooting hormone.
2. A few hundred cubic centimeters of moist sphagnum moss or even moist soil is packed around the wound.
3. The sphagnum/soil is held in place with plastic film, and the film is secured around the stem with twist-ties above and below the wound. The soil has to be kept continually moist!!
4. With time, roots develop in the small wad of sphagnum. The rooted branch can then be cut from the donor tree.
When one is trying to make many thousands of individuals of one clone, then a different method is needed. Tissue culture to the rescue!
1. First you need some donor tissue. This is called the explant. The tissue has to be at least parenchyma, but is better if it is meristematic. The tissue has to be surface-sterilized. This is done by immersing the tissue in a solution of sodium hypochlorite (laundry bleach), rinsing in sterile distilled water, then perhaps slicing off the killed surface layers.
2. The explant is placed in a sterilized liquid medium on a shaking machine. This medium contains water, minerals, vitamins, sugar (energy for growth by respiration), and dilute auxin (usually indole-butyric acid). The auxin stimulates the cells of the explant to divide rapidly to form callus. The callus is broken up by the shaking action. Thus the liquid medium becomes loaded with small callus pieces.
3. The callus pieces are now spread across a sterile solid medium. The medium contains water, minerals, vitamins, sugar, and cytokinins. The medium is solidified into a gel by adding a small amount of agar. The container is kept under light. The combination of cytokinin and light causes shoots to be initiated on each callus piece.
4. The shoots are moved onto a solid medium that contains water, minerals, and agar. Depending on the species, there may also be some auxin in the medium. The auxin from the leaves on the shoots (or in combination with auxin from the medium) cause root initiation.
5. The plants can be moved to soil and slowly eased from humid conditions to successively drier conditions. Ultimately the plants can be moved to field conditions.
Because each explant can initiate indefinitely many callus pieces, and each callus piece can make a new plant, the number of plants produced can be extremely high. One company in Lebanon, CT (Pride's Corner Farm) produces about 80,000 Rhododendron plants for nursery sale in just this way!
This page © Ross E. Koning 1994.
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