The papaya is usually propagated by seeds, which in Florida should be sown as early in the year as possible, preferably in January, in order to have the plants in bearing by the following winter. If seeds are washed and dried after removal from the fruit, and stored in glass bottles, they will retain their viability for several years. Higgins and Holt say:
"It is best to plant the seeds in a well-drained, porous soil in flats or boxes, covering them about half an inch deep. In from two to six weeks the seedlings should appear, germination being hastened by heat. In the open in cool weather the time will not be less than a month, but in a warm greenhouse it may be shortened to two weeks. In about a month after germination the seedlings should be large enough to be transferred to pots, in which they should remain for another month before being placed in the orchard or garden."
Wester advocates planting in seed-beds and transferring the young seedlings directly into the open ground. He writes :
"The seed-bed should be prepared by thoroughly pulverizing the soil by spading or hoeing the ground well, and the clearing away of all weeds and trash. Sow the seed thinly, about 1 to 2 centimeters apart, and cover the seed not more than 1 centimeter with soil, then water the bed thoroughly. In the dry season it is well to make the seed-bed where it is shaded from the hot midday rays of the sun, under a tree; or it may be shaded by the erection of a small bamboo frame on the top of which is placed grass or palm leaves. If the seed is planted during the rainy season a shed of palm leaves should always be put up over the seed-bed to protect the seed from being washed out and the plants from being beaten down by the heavy rains."
Vegetative propagation of the papaya by two means has been shown to be possible, but it is not yet demonstrated that either of these methods produces satisfactory plants. Cuttings are readily grown, but they develop more slowly than seedlings. Grafted plants are more rapid in growth and come into fruit early, and it was thought at one time that this method offered great possibilities; but later experience has shown that when propagated by this means in Florida, a given variety degenerates rapidly, and in the third or fourth generation from the parent seedling the grafted plants make very little growth and their fruits are small and practically worthless. The explanation of this behavior has not been found, nor is it known whether it will occur in other regions; but its effect in Florida has been to do away with grafting and cause all growers to return to seed-propagation.
In order that those who are interested in the subject may experiment for themselves, a brief extract is given here from "The Grafted Papaya as an Annual Fruit Tree," by Fairchild and Simmonds. 1 These investigators found that seeds of the papaya, when planted in the greenhouse in February, produce young seedlings large enough to graft some time in March; that these grafted trees, which can be grown in pots, when set out in the open ground in May or the latter part of April, make an astonishing growth and come into bearing (in Florida) in November or December; that they continue bearing throughout the following spring and summer, and if it is advisable, can be left to bear fruit into the following autumn.
"After a seedling begins to fruit, it does not normally produce side-shoots which can be used for grafting. It has been observed for some time, however, that if the top of a bearing tree is cut or broken off accidentally, a large number of shoots begin to form, one from the upper part of each leaf scar; that is, the axil of the leaf. This takes place three or four weeks after the tree is decapitated. It is these small shoots, of which as many as 50 or more may be produced by a single tree, that are used in grafting the papaya. One of these shoots is taken when a few inches long and about the diameter of a lead pencil, is sharpened to a wedge point, the leaf surface reduced, and inserted in a cleft in a young seedling papaya plant which has been decapitated when 6 to 10 inches high and split with an unusually sharp, thin grafting knife. At this age the trunk of the young seedling has not yet formed the hollow space in the center. It is not necessary for the stock and the cion to be of equal size; the cion should not, however, be larger than the stock. After inserting the cion, the stock is tied firmly, but not tightly, with a short piece of soft twine. The grafted plant should be shaded for a few days after the grafting has been done and the twine should be removed on the sixth or seventh day. The best success has been secured in these experiments by grafting potted seedlings in the greenhouse, or under the shade of a lath-house, presumably because the stock can be kept in good growing condition under these circumstances."
1 Circ. 119, Bur. Plant Industry.
One of the most remarkable features of the papaya is the irregularity which it presents in the distribution of the sexes. Normally it is dioecious, with staminate and pistillate (male and female) flowers produced on different plants. Cross-pollination is necessary to enable the pistillate flowers to develop fruits. This is effected by insects. Among seedling plants the number of staminates is usually greater than that of pistillates. Only a few of the former being necessary as pol-linizers (certainly not more than one in ten), this excess of staminates is, from the grower's standpoint, an objectionable feature.
In addition to the staminate and pistillate forms, many intermediates have been observed in which both sexes are combined in one plant. Staminate flowers may occur with rudimentary stigmas and ovaries which give rise to small worthless fruits; and there is a hermaphrodite type which regularly produces perfect flowers, is self-pollinated, and yields excellent fruits. Numerous other forms have been described (see the bulletin by Higgins and Holt), but the importance of these is lessened by the fact that during the lifetime of a plant it may change from one form to another.
In general, it may be said that plants which develop from the seed as pure pistillates will retain their sex without modification, but plants which commence life as pure staminates may undergo a change of sex. It has been asserted that a change of sex may be induced by topping the male tree or breaking its roots. M. J. Iorns, who studied this question in Porto Rico, reached the conclusion that other conditions than the loss of the terminal bud must be present to induce a change of sex, and he suggested that the trees may pass through definitely recurring cycles of development, and be subject to the change only at certain periods. L. B. Kulkarni, 1 who investigated the matter in India, came to the belief that change of sex is not in any way connected with the removal of the terminal bud. He found that male plants, in the course of their development, may present a number of different sex-combinations, as follows:
First stage: Staminate flowers only. Second: Staminate, with a few hermaphrodite flowers. Third: A few staminate, with many hermaphrodite flowers. Fourth: A few staminate, with many hermaphrodite, and a few pistillate flowers.
Fifth: Hermaphrodite flowers only. Sixth: Hermaphrodite, with a few pistillate flowers. Seventh: A few hermaphrodite, with many pistillate flowers. Eighth: Pistillate flowers only.
Thus the plant in the course of its life history may change from a staminate to a hermaphrodite and then to a pure pistillate.
At the Hawaii Experiment Station, much attention has been devoted to breeding papayas. Some of the objects in view have been hermaphroditism (in order to eliminate the necessity of male trees to act as pollinizers), fruit of suitable size and shape for market purposes, uniformity in ripening, good keeping qualities, and good color and flavor of flesh. The dioecious type has not been satisfactory in breeding, principally because the staminates do not show the characters which are inherent in them and which will appear in the fruits of their progeny. "The hope, therefore," says J. E. Higgins, 1 "must lie in the use of a hermaphrodite type. Here it is possible to select an individual of known qualities. This may be used as the sole parent stock or may be combined with another parent of known qualities. What mixtures there may be in the individual at the start may not be known; but through repeated selections and elimination of undesirable characters, it should be possible to produce a reasonably pure strain, provided, of course, that the stock is kept pure by constantly avoiding cross-pollination with plants of different characters."
1 Poona Agrl. College Magazine, 1, 1915.
Some excellent hermaphrodite forms have already been produced, and, although they do not breed true, a sufficient number of the seedlings are hermaphrodites and produce fruit of good quality for it to be felt that a definite advance has been made. Breeding work should be continued until a strain has been purified to a point that it will breed true and retain its fruit characteristics as closely as do cultivated varieties of eggplant, tomato, and other vegetables.