HORTICULTURE A GREAT GREEN CARPET 429 



has become standard practice with both apples and pears. The action is 

 apparently to delay the development of the abscission zone in the pedicel 

 of the fruit. Naphthaleneacetic acid and naphthaleneacetamide are the ma- 

 terials most frequently employed. Proper timing with relation to temperature, 

 rainfall, and fruit development is critical. 2,4-Dichlorophenoxyacetic acid has 

 been found to be effective and specific in the prevention of pre-harvest drop 

 of the Stayman and the Winesap varieties of apple. Curiously, this material 

 is ineffective with a number of other apple varieties. 2,4,5-Trichlorophen- 

 oxypropionic acid causes apples not only to ripen early but also to color 

 early. But success has been uncertain. Fortunately, this material is also 

 effective for reducing pre-harvest drop. 



Blossom induction and photoperiod. One of the features of Horticulture 

 is that it frequently removes plants from their natural habitat and places 

 them in an environment where one or more climatic variables is markedly 

 altered. Thus, a glasshouse may reproduce the temperature at which a plant 

 grows in its native habitat, but the characteristic long day length of summer 

 may be replaced by a short day length of winter with resultant alteration 

 of flowering habit. Further, the horticultural industry variously demands 

 flowers, fruits, seeds, and special vegetative parts from different kinds of 

 plants. Accordingly, there is great concern about the factors which both 

 induce and reduce blossom formation, reduce and induce seed development, 

 and promote and retard various vegetative structures. This is a major field 

 for horticultural research which offers additional great promise. 



Light, day length, temperature, nutrition, and various chemical treatments 

 have been shown to induce blossom formation. Thus, in the case of the tomato, 

 a cool temperature early in the life of the plant induces the formation of 

 flower clusters, a fact which explains why Northern tomato plants grown in 

 the field directly from seed may in a cool season produce fruit earlier than 

 Southern-grown transplanted plants. 



An undesirable effect of early cool temperatures is found with early celery. 

 When exposed to cool temperatures, blossom formation is induced and seed- 

 stalks are produced (bolting) which makes such celery unsalable. Lettuce 

 tends to form blossoms and seedstalks with high summer temperatures and 

 long days. Breeding programs for these crops are aimed at selecting plants 

 with "non-bolting" characters. 



Some plants, as the chrysanthemum, respond to short day length. By ex- 

 tending the day length with artificial light, plants may be prevented from 

 forming flowers. By reducing day length, they may be induced to flower. By 

 proper attention, chrysanthemums can be brought into flower at any month 

 of the year. 



Other plants, as the cineraria, respond to long day length and may be 

 similarly controlled in flower formation by adjusting the day length. 



Plant regulators may likewise exert a profound effect upon blossom forma- 



