How plants respond also depends upon what kind of chemical is applied. For instance, many growth regulators ( indoleacetic acid, naphthaleneacetic acid, and others) will cause the stems to bend, but others ( 2,3,5–triiodobenzoic acid and related benzene derivatives) do not cause bending. Instead, they greatly alter the shape and rate of growth of the leaves located some distance from the part of the stem to which they are applied. The reactions of the treated plants, then, might include bending, rooting, increased respiration and growth, chemical changes and movement of food materials within the plant, and the production of new cells.

Already we have put the discoveries to many practical uses in agriculture; in fact, few other fields of plant research have progressed so fast or affect so many branches of farming and gardening as this one.

Most of the practical uses have come from experiments that you might consider to be of minor importance. An instance : A scientist experimented with holly flowers, which generally fall off unless they are pollinated. When sprayed with a growth-regulating substance, even the unpollinated flowers remained on the plant and produced berries. From this beginning, a practical application of considerable importance to fruit growers has developed—the use of growth regulators to prevent apples and pears from dropping to the ground before they are ready to pick.

As most kinds of fruits ripen, they generally become less firmly attached to the branches. That is because a small section of the stem of the fruit close to the branch serves a special purpose and, as the fruit ripens, it gradually becomes a weak link, finally breaks, and allows the fruit to fall. Difficulty arises when this weakening process proceeds too rapidly in a crop such as apples; the stems no longer can support the weight of the fruits and the apples fall before they are ripe. Many of the fruits are injured as they hit the ground, their quality is lowered, and the grower loses money.

Reasonably good assurance against such loss to apple and pear crops can be had by spraying the trees with one of several growth-regulating substances. You can buy them under fully descriptive trade-marked names at hardware or seed stores in a form ready for applying in a water spray. They should be applied according to the manufacturer's directions. After spraying, the section of stem that ordinarily would weaken and allow the fruit to fall remains surprisingly strong for a week or two. Few fruits do fall; the rest remain on the trees until ripe enough to be picked.

The harvest sprays generally remain effective long enough to prevent much loss from dropping, but some orchardists find it hard to judge the exact time at which the substance should be applied, because if the trees are sprayed too early, the effect of the chemical runs out before picking.

We are testing new growth regulators, however, to find the compound that gives the longest effective period during which the sprayed fruit will remain on the tree. One compound, 2,4–D (2,4-dichlorophenoxyacetic acid) shows promise. Trials made in 1944 by L. P. Batjer and P. C. Marth near Beltsville indicate that Winesap apples sprayed with 2,4–D (a 0.001 percent water mixture) stayed on the trees about twice as long as did comparable fruit sprayed with naphthaleneacetic acid, the first substance widely used to prevent fruit drop.

In experiments by C. P. Harley and others, Stayman Winesap apples sprayed with 2,4–D a month before harvest time also remained on the branches well past the commercial picking date. At the end of the experiments, 19 days after the commercial harvest, the stems were still firmly attached, although the fleshy parts of many apples had rotted and fallen away. The experiments lead one to believe that the effective period of harvest-drop sprays can be extended through the use of 2,4–D, in place of the earlier naphthaleneacetic compound on Winesap and Stayman Winesap trees, and perhaps other varieties.

The work by Harley and his colleagues illustrates a characteristic of plants that, in the case of fruit drop, somewhat limits the usefulness of certain of these chemicals. That is that different varieties behave in different ways when growth regulators like 2,4–D are applied. Delicious, Golden Delicious, and York Imperial apples respond favorably to harvest sprays of the naphthalene type (naphthaleneacetic acid and naphthaleneacetamide). When sprayed with 2,4–D, however, their fruit fell as though the trees had not been sprayed at all.

In fruit-drop sprays, as in insect sprays, a spreading agent is often added to the mixture. The choice of the spreader is important. The addition of a water-miscible waxy substance known as Carbowax 1500 to the sprays containing naphthaleneacetic acid has significantly extended their effective period.