Like many other fruit-trees, the mango has been propagated in the tropics principally by seed. In some instances seedling trees produce good fruits; this is particularly true of certain races, such as the Manila or Philippine. But in order to insure early bearing, productiveness, and uniformity of fruit, it is necessary to use vegetative means of propagation. Inarching, budding, and grafting are the methods most successfully employed.
The seedling races of the tropics are, so far as has been observed, polyembryonic in character. Three to ten plants commonly grow from a single seed. Since these develop vegetatively from the seed tissues, they are not the product of sexual reproduction, but may be compared to buds or cions from the parent tree. Most of the grafted Indian varieties, on the other hand, have lost this characteristic. When their seeds are planted a single young tree develops, and this is found to differ from its parent much as does a seedling avocado or a seedling peach. Usually the fruit is inferior, and the tree may be quite different in its bearing habits.
Dr. Bonavia, a medical officer in British India who did much to stimulate interest in mango culture, at one time took up the question of seedling mangos and wrote several articles advocating their wholesale planting. He argued that not only would many new varieties, some of them superior in quality, be obtained in this way, but also earlier and later fruiting kinds, and perhaps some suited to colder climates.
Just what percentage of seedling mangos will produce good fruit depends largely on their parentage. Seedlings of the fibrous mangos of the West Indies are invariably poor, while those from budded trees of such varieties as Alphonse and Pairi, although in most instances inferior or rarely equal or superior to the parent, are practically never so poor as the West Indian seedlings. At the Saharanpur Botanic Gardens, in northern India, some experiments were conducted between 1881 and 1893 to determine the average character of seedlings from standard grafted varieties. The results led to the conclusion that seedlings of the Bombay mango were fairly certain to produce fruit of good quality. An experimenter in Queensland, at about the same time, reported having grown seedlings of Alphonse to the fourth generation, all of which came true to the parent type.
Experience in the United States has shown, however, that degeneration is common. A number of seedlings of Mulgoba have been grown in Florida, but very few have proved of good quality. There is a tendency for the fruits to be more fibrous than those of the parent. The whole question is probably one of embryogeny. When monoembryonic seeds are planted, the fruit is likely to be inferior to that of the parent, if the latter was a choice variety; with polyembryonic seeds, even though of fine sorts like the Manila, the trees produce fruit closely resembling that of the parent.
The embryogeny of the mango cannot be discussed at great length here. It is not yet thoroughly understood, although it has been studied by several investigators. The most recent account and the only one which has been undertaken with the horticultural problems in mind, is that of John Belling, published in the Report of the Florida Agricultural Experiment Station for 1908. Belling says:
"In the immature seed of the sweet orange E. Strasburger has shown by the microscope, and Webber and Swingle have proved by their hybridizing experiments that besides the ordinary embryo which is the product of fertilization, the other embryos present in the young or mature seeds arise by the outgrowth of nucellar cells into the apical part of the embryo-sac. The first-mentioned embryo, when present, is liable to any variation which is connected with sexual multiplication, - the vicinism of H. De Vries. The remaining embryos, on the other hand, presumably resemble buds from the tree which bears the orange in whose seed they grow, in that they inherit its qualities with only a minor degree of variation."
The behavior of the mango has suggested a similar state of affairs. Belling goes on to quote Strasburger's account of the embryogeny of the mango, and describes his own investigations :
"Even in the unopened flower bud the nucellar cells at the apex of the embryo sac which are separated from the sac only by a layer of flattened cells, are swollen with protoplasm. In older fruits it may be noticed that the cells around the apical region of the sac except on the side near the raphe are also swollen. The adventitious embryos arise from these swollen cells, which in fruits 7 mm. long with ovules 3 mm. long divide up, sometimes forming the rudiments of a dozen or more embryos, but often fewer. The nucleated protoplasm on the embryo-sac wall is undivided into cells, and is thick opposite the places where embryo formation is going on."
Belling worked with fruits of the No. 11 mango, seedling race of Florida identical with the common mango of the West Indies, Mexico, and Central America. He was not able to determine whether the egg-cell develops into an embryo, or whether all of the embryos are adventitious, - the egg-cell being crowded out or destroyed in some other way. If the fertilized egg-cell develops and is represented in the mature seed, the plant arising from it should exhibit variation; but the seedling races are so constant that it seems probable that the egg-cell is lost at some stage in the development of the fruit, and that all of the embryos are normally adventitious. There is as yet no proof, however, that fruits will develop on this or other mangos unless the flowers are pollinated. The subject is an important one and will repay further investigation.
It has been observed in Florida that monoembryonic grafted varieties, such as Mulgoba, will, when grown from seed, sometimes revert to polyembryony in the first generation (Fig. 10).
Fig. 10. Seedlings of grafted Indian mangos usually do not produce fruit exactly like the parent. Each of the fruits here shown represents a tree grown from a seed of the Mulgoba mango. The variations in size and shape of fruit, and in the amount of fiber around the seed, are noteworthy. (X 1/6)
Plate VI. Left, the Sandersha mango; right, the ambarella.