Plant Crosses, Hybrids and Cross Pollination

In our study of flowers and their pollination we have seen that the seed is usually the descendant of two parents, or at least of two organs—one the ovary, producing the seed; the other the pollen, which is necessary to fertilize the ovary.

It happens that sometimes the pollen of one blossom fertilizes the ovary of its own flower, but more often the pollen from one plant fertilizes the ovary of another plant. This latter method is called cross-pollination. As a rule cross-pollination makes seed that will produce a better plant than simple pollination would. Cross-pollination by hand is often used by plant-breeders when, for purposes of seed-selection, a specially strong plant is desired. The steps in hand pollination are as follows: (1) remove the anthers before they open, to prevent them from pollinating the stigma (the steps in this process are illustrated in Figs. 37, 38-39); (2) cover the flower thus treated with a paper bag to prevent stray pollen from getting on it (see Fig. 40); (3) when the ovary is sufficiently developed, carry pollen to the stigma by hand from the anthers of another plant which you have selected to furnish it, and rebag to keep out any stray pollen which might accidentally get in; (4) collect the seeds when they are mature and label them properly.

Hand pollination has this advantage—you know both parents of your seed. If pollination occur naturally you know the maternal but have no means of judging the paternal parent. You can readily see, therefore, how hand pollination enables you to secure seed derived from two well-behaved parents.

Sometimes we can breed one kind of plant on another. The result of such cross-breeding is known as a hybrid. In the animal kingdom the mule is a common example of this cross-breeding. Plant hybrids were formerly called mules also, but this suggestive term is almost out of use.

It is only when plants of two distinct kinds are crossed that the result is called a hybrid; for example, a blackjack oak on a white oak, an apple on a pear. If the parent plants are closely related, for example, two kinds of apples, the resulting plant is known simply as a cross.

Hybrids and crosses are valuable in that they usually differ from both parents and yet combine some qualities of each.

They often leave off some of the qualities of the parent plants and at other times have such qualities more markedly than did their parents. Thus they often produce an interesting new kind of plant. Sometimes we are able by hybridization to combine in one plant the good qualities of two other plants and thus make a great advance in agriculture. The new forms brought about by hybridization may be fixed, or made permanent, by such selection as is mentioned in Section XVIII. Hybridization is of great aid in originating new plants.

It often happens that a plant will be more fruitful when pollinated by one variety than by some other variety. This is well illustrated in Fig. 41. A fruit-grower or farmer should know much about these subjects before selecting varieties for his orchard, vineyard, etc.

EXERCISE

Do a little experiment and  try to cross some plants. Such an experiment will take time, but will be most interesting. You must remember that many crosses must be attempted in order to gain success with even a few.

Propagation By Buds