EXPERIMENTAL EMBRYOLOGY 403 



subject, Gustafson (1942) says: "Some plants under some condi- 

 tions produce enough growth hormone so that with or without 

 pollination, as the case may be, they are able to prevent the absciss 

 layer from being formed in the pedicel; and that under favorable 

 nutritive conditions and with a minimum of competition, they are 

 further able to transport the necessary food and bring about en- 

 largement of the cells in the ovary to produce mature fruits with- 

 out seeds, whereas other plants are unable to do this." 



Although the hormonal method of producing seedless fruit is an 

 invention of only the last 10 to 15 years' research, it has already 

 begun to be applied on a commercial scale in some countries. In 

 the United States there are several states where tomatoes are grown 

 in greenhouses during the winter (January to March), but owing 

 to the short days and low light intensity, the pollen is frequently 

 defective, pollination inadequate, and pollen tube growth restricted. 

 Many flowers, therefore, fall away without producing any fruits. 

 Even the fruits that are set are frequently small in size or are not 

 well filled with the gelatinous pulp characteristic of fruits of good 

 quality. The economic losses resulting from these difficulties 

 prompted the use of hormones in stimulating fruit growth. 



As the chief objective in tomato culture is mainly to make up 

 for the deficiency of good pollen, rather than to obtain fruits which 

 are wholly seedless, castration is unnecessary. The procedure is 

 merely to spray the chemical on the flowers in a suitable manner. 

 Several substances have been tried, viz., indolebutyric acid, indole- 

 acetic acid, naphthaleneacetic acid, methylindolebutyrate, a-naph- 

 thylthioacetamide, potassium naphthaleneacetate, 2,4-dichloro- 

 phenoxyacetic acid, etc. Of all these, indolebutyric acid has been 

 found to be one of the most effective, and the fruits resulting from 

 its application are as large as those produced after natural pollina- 

 tion, if not larger (Fig. 213). The most marked improvement in 

 size is seen during the period from January to February, when 

 pollination is especially deficient and much of the pollen is non- 

 viable. Indeed, the success achieved is so significant that treat- 

 ment of greenhouse tomatoes with synthetic hormones bids fair 

 to become a standard practice (Howlett, 1943, 1944; Mitchell and 

 Marth, 1947). "■ 15 



14 Some very spectacular results have also been achieved in pineapples (Van 

 Overbeek, 1946). By using 2,4-dichlorophenoxyacetic acid and a-naphthalene- 



