Sweet-potato plants are produced from shoots, or growing buds, taken from the potato itself, so that in their case too the piece that we use in propagating is a part of the original plant, and will therefore be like it under similar conditions. Just as with the Irish potato, it is important to know how good a yielder you are planting. You should watch during harvest and select for propagation for the next year only such plants as yield best.
We should exercise fully as much care in selecting proper individuals from which to make a cutting or a layer as we do in selecting a proper animal to breed from. Just as we select the finest Jersey in the herd for breeding purposes, so we should choose first the variety of plant we desire and then the finest individual plant of that variety.
If the variety of the potato that we desire to raise be Early Rose, it is not enough to select any Early Rose plants, but the very best Early Rose plants, to furnish our seed.
Fig. 47. Layering
It is not enough to select large, fine potatoes for cuttings. A large potato may not produce a bountifully yielding plant. It will produce a plant like the one that produced it. It may be that this one large potato was the only one produced by the original plant. If so, the plant that grows from it will tend to be similarly unproductive. Thus you see the importance of selecting in the field a plant that has exactly the qualities desired in the new plant.
Fig. 48. Currant Cutting
One of the main reasons why gardeners raise plants from buds instead of from seeds is that the seed of many plants will not produce plants like the parent. This failure to "come true," as it is called, is sometimes of value, for it occasionally leads to improvement. For example, suppose that a thousand apple or other fruit or flower seeds from plants usually propagated by cuttings be planted; it may be that one out of a thousand or a million will be a very valuable plant. If a valuable plant be so produced, it should be most carefully guarded, multiplied by cuttings or grafts, and introduced far and wide. It is in this way that new varieties of fruits and flowers are produced from time to time.
Sometimes, too, a single bud on a tree will differ from the other buds and will produce a branch different from the other branches. This is known as bud variation. When there is thus developed a branch which happens to be of a superior kind, it should be propagated by cuttings just as you would propagate it if it had originated from a seed.