There are two ways to propagate plants from leaves. Short-stemmed leaves of African-violets, some begonias, and various other plants are rooted somewhat the same as stem cuttings. Large leaves, or parts of them, of Rex and other begonias, are laid flat on the rooting medium.
To propagate African-violets, select firm leaves of medium size but not from the oldest, mature ones in the outside ring of growth. Cut sharply with about 1-inch petioles (leaf stems), insert halfway in the rooting medium and firm this well around them. Even so, you may have to prop each leaf up with a plant label until it forms supporting roots. If you are rooting just a few leaves, you can place each in a 2 1/2 inch pot and cover with a fruit-juice glass or plastic bag, propped on sticks, to insure humidity. The pot-in-pan device is also convenient for African-violets.
Tiny new plants appear at the base of a parent leaf in a few weeks but, unless the parent leaf deteriorates, do not cut it off until there is a sizable cluster of plantlets. When these are about a third the size of the old leaf, you can dispense with it and divide the colony into separate plants. These will usually flower in eight to nine months, sometimes in six.
African-violet leaves also root readily in water. Place a piece waxed paper or plastic over the top of a glass of water and secure it with a rubber band. Insert one or more leaf stems through small holes in the covering. Suspend them so that only the ends of the stem reach into the water. Add more water as it is needed to keep the water level constant. Roots will soon form and plantlets develop above them. When they are large enough to handle, transfer each leaf with its cluster of plantlets to a small pot. Let the plants become well established before cutting off the parent leaf.
Plants with large, prominently veined leaves, like many of the begonias, are easily propagated from a healthy, well-developed leaf, again middle-aged rather than old or young. Make two or three slashes across the main veins and peg the leaf to the sand or soil mixture with hairpins or toothpicks, or weight it with pebbles. The cut edges must be in contact with the moist mixture. As plantlets develop along the cuts, give light liquid feedings. When the young plants are large enough to handle, cut them apart, each with a bit of root, and transfer to separate small pots or space out in a flat where there is room for development.
Begonias and some other plants can be propagated by V-shaped pieces of leaves, each section having a large vein. Insert the point of the V deep enough in the rooting medium to hold the leaf upright. Small plants will develop around the point.