Ornamental trees sometimes produce fruit that is objectionable when it matures and falls to the lawn or street. Chemical sprays remove the blossoms and prevent fruit set without injury to the trees. A spray of naphthaleneacetic acid, 20 p.p.m., applied to the flowers of horse-chestnut prevented production of burs, which litter the lawn and damage mowers. The same treatment applied to mulberry eliminated the purple staining of clothes and streets from the ripe fruits otherwise produced in abundance.

Horace V. Wester, of the Department of the Interior, and Paul C. Marth, of the Department of Agriculture, prevented growth of the vile-smelling fruits of ginkgo trees in Washington by spraying them with 250 p.p.m. of 3-chloro-isopropyl N-phenylcarbamate (Chloro IPC) at blossom-time.

THE GROWTH-REGULATING chemicals sometimes are used to stimulate the development of parts of the fruit other than seeds. In other instances the production of viable seeds has been increased by their use.

Scientists have worked with a number of economically important varieties of fruit such as the Marsh grapefruit, Washington navel orange, and Thompson seedless grape in order to develop good fruits that are free of seeds. Fruits so produced are called parthenocarpic.

Some plants apparently produce enough natural hormones so that they do not require the stimulus of developing seeds to prevent the falling off, or abscission, of flowers and young fruits.

Synthetic plant regulators have been discovered that, applied externally, simulate the effects of natural hormones so that now we can produce seedless or partly seedless fruits of tomatoes, blackberries, eggplants, peppers, cucumbers, green beans, and others by a simple spraying procedure.

The production of greenhouse-grown tomato fruits has been increased by the use of a suitable plant regulator treatment. Many of the fruits produced in this way are seedless. The greatest benefit is that tomato crops can be grown during periods of cloudy weather and low-light intensity, as during the short days of winter and early spring, when the fruit-setting stimulus of pollination reaches a low ebb, flower drop is excessive, and yields are low.

Sprays containing beta-naphthoxyacetic acid, alpha-ortho-chlorophenoxyacetic acid, parachlorophenoxyacetic acid, and others have been used effectively by commercial growers of greenhouse tomatoes. If the flower clusters are sprayed just before pollen is shed, most of the fruits will be entirely or almost seedless.

The yield of field-grown tomatoes, particularly in the first pickings, has been increased by the use of these regulators, notably when crops bloom under conditions of low-night temperatures that are unfavorable for fruit set. Therefore, perhaps, the greatest and most consistent increases in yield of field-grown tomatoes have been reported in England, where the crop may be almost entirely lost because of excessive flower drop during cool growing seasons.

R. J. Weaver, of the California Agricultural Experiment Station at Davis, has reported that the Black Corinth grape normally produces fruit that contains only thin, papery seedcoats. The variety is generally considered seedless. Experimentally, the application of parachlorophenoxyacetic acid sprays of 10-p.p.m. concentration at full bloom induced numerous hard seeds (seed-coats) to develop. When the same spray treatment was delayed until 3 days after full bloom, the fruits produced were seedless and larger than those produced on untreated vines or on those that were mechanically girdled to increase the size of berries.

Dr. Weaver also discovered that gibberellic acid is superior to other growth regulators he tested as an aid in increasing the size of the grapes, particularly the Thompson seedless variety.

The size of apricot fruits has been increased as much as 40 percent through the application of 2,4,5-trichlorophenoxyacetic acid at 75 p.p.m. to the trees when the pits were beginning to harden. These experiments were conducted by J. C. Crane and his coworkers at Davis.

In attempting to explain the cause for this increase in the size of the fruit because of the treatment, they studied the rate of growth of all parts of the fruits both on the sprayed and unsprayed trees.

They concluded that the growth of the seed about midseason competed with the fleshy parts of the fruits for naturally occurring hormones and thereby greatly retarded growth of the flesh for a rather long period until the seed stopped enlarging. Applications of the 2,4,5-T spray just before the period of rapid seed development eliminated this competition, so that the flesh did not slow down in rate of growth and at harvest was much larger than that of the untreated fruits. The final size of the seeds was about the same in both instances.

PLANT-REGULATING chemicals have been used as an aid in obtaining viable seed in the breeding of new plants that are of good quality and resistant to diseases.

In the breeding of new resistant types of cantaloups, for instance, C. P. Burell and T. W. Whitaker, of the Horticultural Field Station of the Department of Agriculture in La Jolla, Calif., found that the young fruit fell off without maturing seeds, even though an abundance of viable pollen was used in their hand crosses. Often the hybridizations were between varieties where there should be no question of compatibility between the pollen of one plant and ovules of another.

They learned that the application of an infinitesimal amount of indoleacetic acid, applied to the female flowers soon after pollination, reduced fruit abscission and so increased production of seed by more than 40 percent.

Plant breeders sometimes wish to obtain hybrid seeds by crossing species that differ considerably in growth characteristics. The growth rate of the pollen of one plant may be so slow that the ovules of the female plant wither and die before the pollen germinates.