Fig. 3 Callus formation and Root Differentiation
Callus Formation. Plant parts such as leaves, when forcibly detached from the mother plant, undergo stress. The natural response for such plant parts (just like in other life forms) is to start healing. The first step in the healing process is the formation of soft protective tissue, known as callus, to cover the cut or wound. Callus is characterized by a thickened outer tissue which is brought about by the rapid formation of undifferentiated mass of cells. In Fig. 3, the brown colored end of the petioles is the callus.
Root Differentiation. The initial purpose of the callus is to form a protective surface over the wound to prevent further damage to the leaf. As long as the conditions are favorable, cells continue to grow even after the wound has healed. The leaf that was once a part of a plant will now start its own. Cells begin to respond to chemical, hormonal, and physical factors which trigger differentiation. Differentiation is the process wherein new cells take on new and identifiable form and role. When the wound that triggered callus formation is completely healed new events begin to happen. Instead of continuing to grow and clump with the callus, new cells begin to be different. Fig. 3 and Fig. 4 show roots forming from the callus. The first sign of new life from the leaf has begun. However, an old leaf with roots does not make a plant. Something still has to happen.
Fig. 5 Shoot differentiation above the roots.
Shoot differentiation. Cells continue to multiply as shown by the increased root mass (Fig. 5). The leaf (that was detached from a mother-plant) advances further to regenerate into a complete functioning plant. The wound and the area around it is now the center of various physiological activities. Not only have cells differentiated into roots but some begin to differentiate into shoot around the girth of the petiole just above the roots (Fig.5). The shoots which are the part of the plant that will eventually bear the leaves and flowers have emerged.
Fig. 6. Trichome-covered shoots emerging around the girth of the petiole.
Note that differentiated tissues immediately assume the appearance of the organ. The shoot buds contrast with the roots by the following characteristics: presence of trichomes; green coloring which indicates the presence of chlorophyll; the form; and location and direction of growth (Fig. 6). Cells that differentiate into roots tend to grow towards the ground and those that differentiate into shoots grow upwards. Differentiation can be paralleled to the sorting of citizens into their political inclinations - some will be Republicans while others will be Democrats. Regardless of the direction and position of growth, all plant parts play a key role in the overall functioning of the plant.
Steps in Cloning African Violets through Leaf Cuttings1. Take leaves from the outer ring of the foliage. Make sure to include part the petiole. Leaf blades can be used - new plants grow from the midrib, however, the petiole, when cut diagonally, provides a greater surface are for more shoots to grow.
2. Fill up a pots with sand, perlite or potting soil. Potting soil and recycled plastic containers can also be used as seen in Fig. 2. Water until just moist and not soggy.
3. Lay the leaves with the cut surface touching the media (Fig. 2). Cover the pot to prevent fast water loss.
4. Maintain soil moisture.
5. Wait and observe till plants develop and ready to be transplanted.