The scanty productiveness of many Indian mangos has been attributed by several writers to defective pollination. A. C. Hartless, superintendent of the Government Botanical Gardens at Saharanpur, India, discussed the matter at some length in the Agricultural Journal of India, April, 1914. The writer has personally investigated the subject in Florida, and the results have been published in Bulletin 542 of the United States Department of Agriculture. Burns and Prayag have written on the structure and development of the mango flower in the Agricultural College Magazine, Poona, India, March, 1911. The mango is polygamous and produces its flowers on terminal panicles varying in length from a few inches up to two feet. Each panicle carries from 200 or 300 up to more than 4000 flowers, of which only 2 or 3 per cent are perfect in some varieties, or as many as 60 to 75 per cent in others. The character of the panicle and the number of flowers produced upon it differs according to the variety.
The individual flower (Fig. 13) is subsessile, 6 to 8 millimeters in diameter when the corolla is outspread; the calyx composed of five ovate-lanceolate, finely pubescent, concave sepals; and corolla of five elliptic-lanceolate to obovate-lance-olate petals, 3 to 4 millimeters long, whitish, with three or four fleshy orange ridges toward the base, and inserted at the base of a fleshy, almost hemispherical disk, obscurely 5-lobed and usually about 2 millimeters in diameter. In the perfect flower the disk is surmounted by a globose-oblique ovary 1 millimeter broad, with a slender lateral style about 2 millimeters high. To one side and inserted upon the disk is the single fertile stamen, composed of a slender subulate filament about 1.5 millimeters long, surmounted by an oval purplish red anther 0.5 millimeter long, which dehisces longitudinally. Occasionally two such stamens are produced. The whorl is completed by staminodes of varying prominence, short and subulate in some varieties, larger and capitate in. others, some even becoming fertile and producing a few pollen-grains. In the staminate flower the ovary is wanting.
Fig. 13. A bisexual mango flower. (X 4)
Several writers have affirmed that the mango is largely if not solely wind-pollinated. It seems evident, however, that it has none of the characteristics of an anemophilous plant, but, on the other hand, presents well-developed adaptations to insect pollination. In anemophilous or wind-pollinated flowers, the pollen is usually abundant in order to compensate for the enormous loss in transport; the pollen-grains are dry and incoherent, so that they may easily be carried by the wind; and the stigmas are commonly bushy and freely exposed, so as to have every chance of catching the floating grains. The mango shows none of these adaptations. It produces comparatively few pollen-grains, often not more than 200 or 300 to an anther. These grains show a decided tendency to cling together, especially in damp weather; and even in dry sunny weather it is difficult to dislodge them with a strong draft of air. The stigma is small and not provided with projections of any sort to assist in catching pollen.
The production of nectar for the attraction of insects also indicates that the mango is entomophilous. Observations have shown that the flowers are visited by numerous insects of the orders Diptera, Hymenoptera, Lepidoptera, and Coleop-tera, ranking in the order given as to the number of visits. Pollen-grains have been observed adhering to the bodies of many species belonging to these orders.
In spite of numerous insect visits, however, a large number of the stigmas are never pollinated, and it seems probable that very little pollen is transferred from one flower to another. Most of the stigmas receive their pollen from the anther (rarely is more than one fertile) of the same flower. Cross-pollination is in all probability uncommon. In damp cloudy weather the pollen-grains swell and are much more difficult to dislodge than when the weather is dry and sunny. After a heavy dew they will be found in this swollen condition, but when the sun comes out they return to their normal dry form. Protection of the flowers from dew and rain by means of a canvas shelter did not increase the production of fruit in the case of an experiment carried out in Florida.
Sometimes there is considerable differentiation in the size of the pollen-grains. In most varieties the larger number, however, are uniform in shape and size, plump and apparently perfect. They can be germinated in sugar solution of the proper density, and there is nothing to suggest that impotency is common.
From the fact that pollination ordinarily is scanty, it might be assumed that productiveness could be increased by making it more abundant. This has not, however, been found to be the case, except when the pollen was obtained from a tree of a different variety (cross-pollination); under these conditions there was a somewhat better yield. The total number of flowers produced is so enormous that it is of little importance whether all are pollinated or not. Seedling mangos, which are not pollinated more abundantly than budded varieties, nor furnished with a greater number of anthers, nor, so far as can be ascertained, with pollen of greater potency, often set many more fruits than they can carry to maturity. This has been noted also with several grafted kinds, such as Bennett and Cambodiana.
Sometimes the entire tree comes into bloom at one time, covering itself with flowers; again, one side of the tree may flower, while the other shows no buds; or the flowering may be confined to a small section of the tree, probably the branchlets arising from one large limb. This behavior of the mango corresponds to the growth habit of the tree which is mentioned but not explained by A. F. W. Schimper.1 When one side of the tree flowers independently, it might be expected that the remainder would flower at another time, but this is not always the case.
Some varieties develop all their flowers within ten days after the first buds open; others, such as Sandersha and Julie, push out flower-panicles during a period of several weeks, or even months; thus, in 1915 there was not a single day between the middle of January and the latter part of May on which flowers could not be found on the old Sandersha tree in the Plant Introduction Garden at Miami, Florida. This feature is of importance in that it gives the tree a greater opportunity to set fruit. Often the attacks of the anthracnose fungus are severe when the tree is in bloom, and the entire crop of flowers is destroyed. In some varieties this means a crop failure, since the tree will not produce any more flowers that season; but in the Sandersha (if early in the season) it need mean only the loss of the flowers which were present at that particular time. Those developed later might enjoy more favorable weather, with consequent freedom from the anthracnose peril, and a crop of fruit would result. Anthracnose, one of the greatest enemies of the mango, is discussed under the heading pests and diseases.
Some varieties which fruit heavily are characterized by a high percentage of perfect flowers. Others which are known to be unusually regular in fruiting, although they may not produce such heavy crops, have relatively few perfect flowers. The Philippine race of seedlings, which sometimes bears heavily, commonly has more perfect than staminate flowers. Most of the Indian varieties have fewer perfect flowers than the seedling races.
1 Plant Geography.
The experiments conducted in Florida indicate that the scanty fruiting of many varieties is not due to any morphological defect in the pollen or to defects in the mechanism of pollination. While such factors as lack of pollinating insects and loss of pollen through rains or moist weather probably lessen the production of fruit in some seasons, from a practical standpoint the question of pollination seems relatively unimportant. The problem is more probably a physiological one, connected with nutritional conditions as influenced by changes in soil-moisture and food-supply, principally the former. Suggestions are given under the heading culture for encouraging the formation of fruit-buds on soils or under climatic conditions which normally tend to produce vegetative growth to the detriment of reproduction.